EntranceElectrochemical analysis methods are process based. Electrochemical methods of analysis
Electrochemical methods analysis are based on the study and use of processes occurring on the surface of the electrode or in the near-electrode space. Any electrical parameter (potential, current, resistance, etc.), functionally related to the concentration of the analyzed solution and amenable to correct measurement, can serve as an analytical signal.
There are direct and indirect electrochemical methods. Direct methods use the dependence of the current strength (potential, etc.) on the concentration of the component being determined. In indirect methods, the current strength (potential, etc.) is measured in order to find the concentration.
Potentiometry, conductometry and voltammetry have found application in food analysis.
Potentiometric method of analysis
The potentiometric method is based on measuring the electromotive forces of reversible galvanic elements and is used to determine the concentration of ions in a solution. This method actively uses the Nernst equation:
E = E° + R*T/(n*F) ln (a oxide/a reduction)
Where E° is the standard potential of the redox system; R - universal gas constant; T - absolute temperature; F is Faraday's constant; n is the number of electrons taking part in the electrode reaction; and oxide, and reduce - the activity of the oxidized and reduced forms of the redox system, respectively.
The main advantages of the potentiometric method are its high accuracy, high sensitivity and the ability to carry out titrations in more dilute solutions than visual indicator methods allow. It should also be noted that this method can determine several substances in one solution without prior separation and titration in turbid and colored media.
This method makes it possible to analyze food products for:
· presence of nitrites and nitrates in meat products;
· determination of the acidity of dairy products, beer, barley and other grain crops;
· measurement of pH of syrups;
· determination of potassium in milk;
· determination of starch in sausages.
Conductometric method of analysis
The conductometric method is based on changes in the electrical conductivity of solutions depending on the concentration of charged particles present.
The objects of such analysis are electrolyte solutions.
The main advantages of conductometry:
high sensitivity (lower limit of detectable concentrations ~10 -4 -10 -5 M), fairly high accuracy (relative error of determination 0.1-2%), simplicity of methods, availability of equipment, the ability to study colored and turbid solutions, and also automation of analysis.
The conductometric method of analysis makes it possible to determine:
· sulfates in solution,
· determination of citric acid in fruit and berry raw materials;
· ash in sugar and molasses.
Amperometric method of analysis (Voltammetry)
Voltammetry is a group of methods based on the processes of electrochemical oxidation or reduction of the analyte, occurring on a microelectrode and causing the occurrence of a diffuse current. The methods are based on the study of current-voltage curves, which reflect the dependence of the current on the applied voltage. Voltammograms make it possible to simultaneously obtain information about the qualitative and quantitative composition of the analyzed solution, as well as about the nature of the electrode process.
To carry out current-voltage analysis, a voltage from an external source is applied to the electrode system. By changing the voltage, we study the dependence of the diffusion current on the applied potential difference, which is described by a voltammogram.
The graph has a wave shape and consists of 3 sections. Section I - from the beginning of registration analytical signal Before the electrochemical reaction begins, current passes through the cell. Section II - a sharp increase in current due to an electrochemical reaction. Section III - diffusion current, having reached the limiting value, remains almost constant, electrical chemical reaction completed.
Using this method, the following food analyzes can be carried out, which will determine:
· amylose in starch;
· heavy metals in dairy products;
· ascorbic acid in drinks and juices.
Electrochemical methods– the most dynamically developing in terms of their application in environmental monitoring. The most common methods used in MOS systems are voltammetry (including polarography), potentiometry (including ionometry), coulometry and conductometry.
Electrochemical methods of analysis use the dependence of various electrical properties of the medium on the quantitative content and qualitative composition of the substances analyzed in it:
· change potential electrode depending on physical and chemical processes, flowing in the substance ( potentiometric method), incl. selective reactions of ion-selective electrodes, individually sensitive to a large number of cations and anions ( ionometric method);
· change electrical conductivity (current) And dielectric constant substances depending on the nature of the medium and the concentration of its components ( conductometric And amperometric methods);
· changes amount of electricity when the analyte gets into the electrochemical cell ( coulometric method);
· recovery of the analyzed compound on a mercury dripping or rotating electrode, as a rule, when analyzing trace amounts of substances in different states of aggregation ( polarographic or voltammetric method).
Polarographs of all devices in this group have the highest sensitivity, equal to 0.005–1 μg/ml of sample.
Voltammetry includes a group of electrochemical analysis methods based on the study of polarization curves. These methods are polarography And amperometric titration – have many varieties and modifications. Most common constant current polarography.
A polarographic installation consists of a direct current source, a voltage divider, a dropping (usually mercury) or rotating electrode and an auxiliary (usually also mercury or other) electrode. To measure the current, a microammeter is connected to the system. The electrodes are placed together with the test solution in an electrolyzer (cell).
Voltage applied to an electrolytic cell causes polarization of the anode and cathode E= f a– f k +iR, Where i– current strength; TO - solution resistance; f a and f k– potentials of the anode and cathode.
If you reduce the resistance of the solution by adding a strong electrolyte (background), then the value iR(potential drop in solution) can be neglected.
The anode potential remains virtually constant during cell operation, since the current density is low and the relatively large surface of the anode is not polarized. Then the potential of a dripping polarizing cathode with a small surface will be equal to: E= -f k. Often in polarographic measurements, instead of a layer of mercury at the bottom of the vessel, a non-polarizing saturated calomel electrode is used, the potential of which is taken equal to zero.
Polarographic data is obtained by measuring the current passing through an electrolytic cell as a function of the potential applied to the electrodes. The graphical dependence of current on potential is called a polarographic wave ( rice. 2).
At the beginning of electrolysis at small values of the imposed EMF force The current will be almost constant and only increase very slowly. This is the so-called residual current, which remains throughout the electrolysis.
Rice. 2. Polarogram of a 10–3 M solution of zinc chloride and a 1 M solution of potassium chloride (curve 1) and a 1 M solution of potassium chloride (curve 2)
As soon as the ion reduction potential is reached (for example, for the determined zinc ions it is equal to -1.0 V), their discharge begins on a drop of mercury:
Zn 2+ + 2 +Hg ® Zn (Hg).
A dilute zinc amalgam Zn (Hg) is formed at the cathode, which decomposes into its constituents as soon as the falling drop comes into contact with the anode:
Zn (Hg) – 2 ® Zn 2+ +Hg.
At the reduction potential of zinc ions, the current strength increases sharply ( rice. 2), but after reaching a certain value, despite the increase in the applied EMF, it remains almost constant. This current is called limiting or diffusion; its value is usually proportional to the concentration of the substance being determined.
When taking polarograms, an indifferent electrolyte with cations that are reduced much more difficult than the analyzed cation is added to the electrolyte under study, for example, KCl, KNO 3, NH 4 Cl; at a concentration 100–1000 times higher than the concentration of the substance being determined. This electrolyte is called “background”. It is created in the test solution to increase electrical conductivity and for shielding electric field indicator electrode (cathode). Therefore, the cations of the analyte are not attracted by the electric field of the cathode, but move towards it due to diffusion.
The most important characteristic of a polarogram is the half-wave potential E 1/2 and polarographic wave height h(limit diffusion current). The half-wave potential is used in quality polarographic analysis. The half-wave potentials of various substances, arranged in order of increasing negative value, constitute the so-called “polarographic spectrum”. Since the half-wave potential significantly depends on the composition of the solution (the medium being analyzed), the background is always indicated in polarographic tables.
IN quantitative In polarographic analysis, the methods of calibration graph, additives, comparisons and calculation methods are used to measure concentration.
Among the various options for polarography, the method differential pulse polarography (DIP) ) is most effective for solving environmental monitoring problems, mainly due to its high sensitivity. The DIP method allows you to evaluate the content of all substances determined by classical polarography. Among other polarographic methods, it is especially convenient for trace analysis square wave polarography, which provides a detection limit close to that of DIP, but only in the case of reversible electrode processes, and therefore this method is often used for the determination of traces of heavy metals. The DIP method can also be used to determine surfactants that change the capacitance of the double electrical layer of the electrode.
Methods can be used to determine microcontents of heavy metal ions inversion electrochemical analysis (IEA) or in another way, stripping voltammetric analysis (IVA ), in which the metals to be determined are pre-deposited on the electrode and then dissolved during polarographic control. This option, in combination with DIP, is one of the most sensitive methods of electrochemical analysis. The hardware design of the IEA (IVA) is relatively simple, which makes it possible to carry out analyzes in the field, and automated continuous control (monitoring) stations can also work on this principle.
IEA (IVA) methods provide the determination of Cu, Pb, Bi, Sb, As, Sn In, Ga, Ag, Tl, Cd, Zn, Hg, Au, Ge, Te, Ni, Co ions and many anions. An important advantage of IEA (IEA) methods is (in contrast to other methods, for example, such as atomic absorption spectrometry) ability to distinguish free ions from their bound chemical forms, which is also important for assessing physical and chemical properties analyzed substances from the point of view of eco-analytical control (for example, when assessing water quality). Many organic matter can also be determined by IEA (IVA) methods after their adsorption accumulation on the electrode surface.
Polarographic methods can also be used to determine aerosols of various metals in the atmosphere and air of industrial premises after they are captured on appropriate filters, followed by transferring the concentrates into solution. Organic compounds present in the form of gases and vapors in the atmosphere can be determined polarographically after they are absorbed by specially selected solutions. Metals and various compounds in biological materials are usually determined polarographically after their extraction. All polarographic measurements, including IEA (IVA), can be fully automated, which is essential when performing serial analyses.
One of the most important areas of application of polarography is the determination of oxygen in water. For this purpose, amperometric detectors are used, generating a current proportional to the oxygen concentration in the solution.
By applying an enzyme to the surface of the detector membrane, it is possible to obtain various enzyme amperometric sensors convenient for biochemical and clinical analyses. Such sensors are also used in environmental monitoring systems.
Electrodes operating on the electrocatalytic principle are suitable for monitoring various gases (SO 2, H 2 S, CO, NO x) in the air of industrial premises. Electrochemical reactions of these gases (playing the role of a catalyst) occurring on the surface of the electrode generate a current in the electrode system that is functionally related to the concentration of gases in the air.
The use of polarography is not limited to the analysis of discrete samples, and the method is gradually moving to the principles of continuous analysis of gases and liquids.
Voltammetric polarographic detectors have been successfully used in high-performance liquid chromatography (HPLC). In this case, the combination of a highly selective separation method with a sensitive detection method leads to a noticeable expansion of the range of substances determined by the chromatographic method (traces of highly toxic substances, herbicides, drugs, growth stimulants, etc.).
Details of the method can be clarified in the specialized literature.
Potentiometry– a method for determining the concentration of substances, based on measuring the emf of reversible galvanic cells.
In practice, two are used analytical method: direct potentiometry to determine the particle activity, which can be calculated using the Nernst equation from the emf of the galvanic cell, and potentiometric titration , in which the change in activities chemical substances during the titration process leads to a change in the EMF of the galvanic cell.
The equipment for carrying out potentiometric titrations and for direct potentiometry is the same. The potentiometric measurement circuit includes an indicator electrode and a reference electrode with a stable constant potential, as well as a secondary device. The principle diagram of the method is shown in rice. 3.
1 – indicator electrode; 2 - reference electrode
Rice. 3. Potentiometric cell
The potential of a pair of electrodes is constant. Changing the concentration of the analyte in the solution changes the EMF of the circuit. Indicator electrodes usually come in four types, depending on the membrane used, which separates the electrode solution from the test solution: 1) electrodes with a homogeneous membrane made of powdery or crystalline material; 2) electrodes with a heterogeneous membrane, in which the electrode active substance distributed, for example, in silicone rubber; 3) electrodes with a liquid membrane, in which the membrane is a solution applied to a neutral substance, for example, porous glass; 4) glass electrodes with different chemical composition glass
Indicator electrodes acquire the potential of the solution in which they are placed. There are two kind indicator electrodes:
1) indifferent electrodes (non-destructible during electrolysis);
2) electrodes that change (oxidize or reduce) during measurements.
Role indifferent electrodes(these are sometimes called electrodes third kind) is to give or gain electrons, i.e. be conductors of electricity. Such electrodes can be made of gold, polished platinum, graphite and other materials. Examples of variable electrodes (sometimes called electrodes) first kind) may be plates of copper, zinc and other metals, as well as quinhydrone and hydrogen indicator electrodes. In addition, indicator electrodes can be ion selective membrane electrodes for the determination of numerous cations: Li +, Pb +, Cs +, Tl +, NH +, Na +, K +, Ag +, etc. As reference electrodes ( standard electrodes), the potential of which remains constant throughout the measurement, the most commonly used are, for example, normal and decinormal calomel (calomel) electrodes with potentials of +0.282 V and +0.334 V, respectively, as well as a saturated silver chloride electrode with a potential of +0.201 V.
In an ideal case, direct potentiometric measurement of the EMF of a galvanic cell can be related through the Nernst equation to the activity of the particle being determined, or to the concentration, if the corresponding activity coefficients are known:
Where E 0 – standard electrode potential, V; R– gas constant; T– absolute temperature; F – Faraday number; n– number of electrons lost or gained; , [reduced] – equilibrium concentrations of oxidized and reduced forms, respectively, mol/dm 3 .
If we substitute the reference values of the constants and move from the natural logarithm to the decimal one, then for a temperature of 25°C we get:
The most important indicator in characterizing the state of the environment is the pH value of this environment, the determination of which ( pH-metry ) is currently usually carried out using glass indicator (measuring) electrodes. For long-term measurements, special designs of glass electrodes with additional devices have been developed to ensure cleaning of the glass membrane. Glass electrodes covered with a semi-permeable membrane with an electrolyte film also serve as the basis for various types of probes ( sensors ), used in the analysis of water and air under production conditions for a number of pollutants (NH 3, CO 2, NO x, SO 2, H 2 S, etc.).
The process in the field of creating ion selective electrodes (ISE) allows for monitoring the ions F – , I – , Br – , Cl – , CN – , SCN – , NO 3 – , NO 2 – , ClO 4 – , S 2 – , Na + , K + Ca 2+ , Ag + , Cu 2+ , Cd 2+ , Pb 2+ in concentration ranges from 10 –2 to 10 –7 mol/l (approximately 1–10 –5 mg/ml). Monitoring using ISE is characterized by rapidity, simplicity and greater possibilities for carrying out continuous measurements. ISEs have been developed that are selective to a wide class of organic substances, as well as isomers in their mass, surfactants and detergents found in the air of a production area and the water management regime of industrial enterprises.
Potentiometry is also used in measuring the redox potentials of various redox (O/R) systems in water. As a rule, the measurement results correspond to a mixed potential, since several O/W systems usually coexist simultaneously in water.
It should be noted that the use of sensors based on semiconductor metal oxide chemically selective and ion-selective field-effect transistors (HSFT, ISFT) is promising. Selectivity in these systems is achieved by choosing the composition of the membrane and the layer deposited on the transistor gate. The system is immersed in the solution being analyzed, and the potential difference between the reference electrode and the gate of the transistor modulates the current flowing between its source and drain. Due to the selectivity of the membrane or deposited layer, the modulated current becomes a function of the activity of the corresponding component of the solution. Semiconductor sensors form the basis of monitors and analyzers of various gases and vapors. The small size of such sensors allows them to be combined in the form of a mosaic on a single substrate, so that an analyzer is obtained that can monitor a whole set harmful substances. Signals from individual sensors included in the mosaic can be sequentially and periodically recorded by the measuring center of the analytical system.
The development of microelectronics makes it possible to design compact probe-type analyzers using modern ISEs. In this case, a circuit that processes the response from the environmental control object, and even a display, can be mounted in the probe handle.
In the specialized literature you can find out the details of the method, , , .
Coulometric the analysis method is a measurement of the current of the electrode reaction into which the substance under study enters the coulometric cell with the analyzed flow. The schematic diagram of a coulometric cell is shown in rice. 4.
1 – cathode chamber; 2 – anode chamber; 3 – microammeter
Rice. 4. Schematic of a coulometric cell
Coulometric analysis is based on measuring the amount of electricity spent on quantitative conduct of a given electrochemical process in a given sample, i.e. provided that the current efficiency is 100%. This is the amount of electricity with the help of a current-time integrator connected in series with the measuring cell, or a coulometer-electrolyzer, in which an electrochemical process is carried out with one hundred percent current efficiency, accompanied by the release of a substance, the amount of which can be easily and accurately restored.
In accordance with Faraday's law:
m( x)/M(x) = m(k)/M(k),
Where m(x), m(k) – mass of the substance being determined X and the substance released in the coulometer, respectively; M(x), M(k) – molar mass substance equivalents X and substance released in the coulometer, g/mol.
The calculation can also be made using the equation describing Faraday's law:
if current strength is measured during analysis i, A and time t, s, spent on carrying out the electrochemical process.
In another modification of this method, called
coulometric titration
, the titrant is generated electrolytically in the analyzed solution at a given current. The consumption of the titrant in the analytical reaction is replaced by the charge flowing through the solution when the titrant is generated until the equivalence point is reached.
One of advantages of coulometric methods is that the titrant standardization process is often not necessary, since calculations are based on Faraday's constant, i.e. the method is absolute and allows you to estimate the amount of the substance being determined, and not its concentration. The disadvantage of coulometry with a given potential is the duration of the analysis procedure, associated with the need for complete completion of electrolysis. Computer technology makes it possible to reduce this time by predicting the end of electrolysis by mathematically processing the current-time curve for the initial stages of electrolysis and by calculating the amount of electricity or the concentration of a substance in solution. When analyzing multicomponent samples, it can be used scanning coulometry , in which the electrolysis potential is changed continuously or stepwise. For such systems, coulometric titration is preferable to direct coulometry, since 100% current efficiency in titrant generation can be easily achieved by the correct choice of titrant reagent and composition of the working medium. Coulometric titration is applicable to the determination of substances from 0.01 to 100 mg (sometimes below 1 μg). The working sample volume is usually from 10 to 50 ml. The method is characterized by high accuracy, the relative error does not exceed several tenths of a percent even with coulometric titration of microgram contents. Under optimal conditions, titrations can be performed with very low overall errors of 0.01% (rel.). Various acid-base, redox; Precipitation and complexometric titration options can be carried out coulometrically.
Coulometric gas analyzers and aqua analyzers (“coulometers”) have been developed and produced for the determination of sulfur dioxide and hydrogen sulfide (sulfates and sulfides), ozone (and hydrogen peroxide), chlorine in the air (and active chlorine in water), carbon monoxide and nitrogen dioxide in air (nitrates and nitrites in water). Coulometry is also used as an electrochemical detection tool in liquid chromatography.
Details of the method can be found in specialized literature.
Conductometric method analysis is based on measuring the electrical conductivity of the solution. The conductometric method of analysis consists of measuring the change in the resistance of an electrolyte solution when a component of the mixture is absorbed. Conductometric installations are used, for example, to determine carbon monoxide and dioxide, gasoline vapor, ammonia and others.
Electrical conductivity is the reciprocal of resistance R, its dimension is cm (Siemens) i.e. æ = 1/ R.
The electrical conductivity of a solution depends on the number of ions per unit volume of the solution, i.e. on concentration WITH, on the mobility of these ions – V. Based on known relationships
Where Z– distance between electrodes; S – electrode area; k– proportionality coefficient.
For a specific pair of electrodes with a constant distance between them S/Z= const. Then
,
Where k 1 = k(S/Z).
When making calculations in conductometry, the concept of “electrical conductivity” æ 0 is used:
In calculations it is convenient to use the equivalent electrical conductivity, which is equal to:
Where P - number of moles equivalent in 1 cm 3 of solution. The equivalent electrical conductivity l ¥ at infinite dilution is equal to the sum of the cation mobilities U and anion V.
The ratio of the equivalent electrical conductivity of a weak electrolyte solution to the equivalent electrical conductivity of this electrolyte at infinite dilution is equal to the degree of dissociation a of this electrolyte:
Despite its non-specificity, this method is quite often used in environmental monitoring systems compared to other electrochemical methods. This is explained by the fact that when assessing pollution, for example, water and atmosphere, not stage-by-stage, but output (final) control of industrial processes is possible. Due to the extremely low electrical conductivity of water, it is often enough to estimate the total content of contaminants, which is what conductometry provides. Typical examples of the use of conductometric methods in control environment are analyzers of detergents in wastewater, concentration of synthetic components in irrigation systems, quality (salinity) drinking water. Conductometric analyzers are used for continuous monitoring of air and precipitation pollutants such as SO 2 and H 2 SO 4 . In addition to direct conductometry can be used to determine certain types of pollution indirect methods, which provide very effective estimates of the content of the substances listed above, which interact before measurement with specially selected reagents and the recorded change in electrical conductivity is caused only by the presence of the corresponding products in the reaction. This way you can determine nitrogen oxides after their catalytic reduction of pre-ammonia, as well as HCl, HBr and CO 2 after a preliminary reaction with Ba(OH) 2 or NaOH. The described principle for determining CO 2 can also be used for the indirect determination of organic substances in water.
In addition to classical conductometry, there is also a high-frequency version ( oscillometry ), in which the indicator electrode system does not contact the sample. This principle is often implemented in continuous conductivity analyzers.
Electrochemical methods of analysis are also described in a number of educational and special publications.
LITERATURE
1. Drugov Yu.S., Rodin A.A.Environmental analytical chemistry.
St. Petersburg: 2002. – 464 p.
2. Pashkevich M.A., Shuisky V.F. Environmental monitoring. Tutorial. St. Petersburg State University. – St. Petersburg, 2002. – 90 p.
3. Cattrall Robert W. Chemical sensors. M.: Scientific world, 2000. – 144 p.
4. Turyan Ya.I., Ruvinsky O.E., Zaitsev P.M.Polarographic catalymetry. M.: Chemistry, 1998. – 272 p.
5. Budnikov G.K., Maistrenko V.N., Murinov Yu.I. Voltammetry with modified and ultramicroelectrodes. M.: Nauka, 1994. – 239 p.
6. Brainina Kh.Z., Neiman E.Ya., Slepushkin V.V. Inversion electroanalytical methods. M.: 1988. – 240 p.
7. Salikhdzhanova R.F. and etc. Polarographs and their use in practical analysis and research. M.: Chemistry, 1988. – 192 p.
8. Kaplan B.Ya., Pats R.G., Salikhdzhanova R.F. AC voltammetry. M.: Chemistry, 1985. – 264.
9. Bond A.M. Polarographic methods in analytical chemistry. M.: Chemistry, 1983.
10. Efremenko O.A. Potentiometric analysis. M.: MMA im. THEM. Sechenova, 1998.
11. Reference Guide to the Application of Ion Selective Electrodes. M.: Mir, 1986.
12. Koryta I. Ions, electrodes, membranes. M.: Mir, 1983.
13. Nikolsky B.V., Materova E.A. Ion selective electrodes. L.: Chemistry, 1980.
14. Efremenko O.A.Coulometric titration. M.: MMA im. THEM. Sechenova, 1990.
15. Khudyakova T.A., Koreshkov A.P. Conductometric method of analysis. Textbook for universities. M.: graduate School, 1975. – 207 p.
16. Budnikov G.K., Maistrenko V.N., Vyaselev M.R. Fundamentals of modern electrical analysis. M.: Chemistry, 2000.
17. Prokhorova G.V. Introduction to electrochemical methods of analysis. M.: Moscow State University Publishing House, 1991. – 97 p.
18. Electroanalytical methods in environmental monitoring. /Ed. R. Kalvoda, R. Zyka, K. Shtulik and others. M.: Chemistry, 1990. – 240 p.
19. Plambeck J.Electrochemical methods of analysis. Fundamentals of theory and application./Trans. from English M.: Mir, 1986.
“Electrochemical methods of analysis and their modern hardware design: a review of WEB sites of companies selling chemical analytical equipment”
Introduction
Chapter 1. Classification of electrochemical methods
1.1 Voltammetry
1.2 Conductometry
1.3 Potentiometry
1.4 Amperometry
1.5 Coulometry
1.6 Other electrochemical phenomena and methods
1.7 Applied electrochemistry
Chapter 2. Electrochemical methods of analysis and their role in environmental protection
Chapter 3. Devices based on electrochemical methods of analysis
Chapter 4. Review of WEB sites of companies selling chemical analytical equipment
Literature
INTRODUCTION
Electrochemical methods of analysis (electroanalysis), which are based on electrochemical processes, occupy a worthy place among methods for monitoring the state of the environment, as they are capable of determining a huge number of both inorganic and organic environmentally hazardous substances. They are characterized by high sensitivity and selectivity, rapid response to changes in the composition of the analyzed object, ease of automation and the possibility of remote control. And finally, they do not require expensive analytical equipment and can be used in laboratory, industrial and field conditions. Three electroanalytical methods are directly related to the problem under consideration: voltammetry, coulometry and potentiometry.
CHAPTER 1. CLASSIFICATION OF ELECTROCHEMICAL METHODS
Electrochemical methods of analysis (EMA) are based on the study of processes occurring on the surface of the electrode or in the near-electrode space. The analytical signal is an electrical parameter (potential, current, resistance, etc.), functionally related to the concentration of the solution component being determined and amenable to correct measurement.
The EMA classification proposed by IUPAC has undergone certain changes over the past decades; clarifications (explanations) and additions have been made to it.
Significant attention is paid to electrochemical cells and analytical signal sensors (electrode systems, various electrochemical sensors); it is these primary electrochemical converters that determine the analytical capabilities of any method. Currently, the most advanced and fastest processing of the signal from the sensor, calculation of statistical characteristics of both the original signal and the results of the entire analysis as a whole are not a problem. This is why it is important to obtain a reliable initial signal in order to calibrate it in concentration units.
According to general classification, proposed
IUPAC, EMA are divided into methods in which the excited electrical signal is constant or equal to zero and into methods in which the excited signal varies over time. These methods are classified as follows:
voltammetric - voltammetry,I ≠ 0; E = f(t);
potentiometric – potentiometry, (I = 0);
amperometric – amperometry (I ≠ 0; E=const);
chronopotentiometric,E = f(t); I =const;
impedance, or conductometric- measurements using the application of low amplitude alternating voltage; other, combined(for example, spectroelectrochemical).
1.1 VOLTAMPEROMETRY
VOLTAMPEROMETRY- a set of electrochemical methods of research and analysis based on studying the dependence of the current in an electrolytic cell on the potential of an indicator microelectrode immersed in the analyzed solution, on which the electrochemically active (electroactive) substance under study reacts. In addition to the indicator electrode, an auxiliary electrode with a much larger surface is placed in the cell so that when current passes, its potential practically does not change (non-polarizing electrode). The potential difference between the indicator and auxiliary electrodes E is described by the equation E = U - IR, where U is the polarizing voltage, R is the resistance of the solution. An indifferent electrolyte (background) is introduced into the analyzed solution in high concentration in order, firstly, to reduce the value of R and, secondly, to eliminate the migration current caused by the action of the electric field on electroactive substances (obsolete - depolarizers). At low concentrations of these substances, the ohmic voltage drop IR in solution is very small. To fully compensate for the ohmic voltage drop, potentiostating and three-electrode cells are used, which additionally contain a reference electrode. In these conditions
Stationary and rotating ones are used as indicator microelectrodes - made of metal (mercury, silver, gold, platinum), carbon materials (for example, graphite), as well as dripping electrodes (made of mercury, amalgam, gallium). The latter are capillaries from which liquid metal flows drop by drop. Voltammetry using dropping electrodes, the potential of which changes slowly and linearly, is called. polarography (method proposed by J. Heyrovsky in 1922). Electrodes of the second type are usually used as reference electrodes, for example. calomel or silver chloride (see Reference electrodes). Dependence curves I =f(E) or I =f(U) (voltammograms) are recorded with special devices - polarographs of various designs.
Voltammograms obtained using a rotating or dripping electrode with a monotonic change (linear sweep) of voltage have the form shown schematically in the figure. The current increasing section is called wave. Waves m.b. anodic, if the electroactive substance is oxidized, or cathodic, if it is reduced. When the solution contains oxidized (Ox) and reduced (Red) forms of a substance that react quickly (reversibly) on the microelectrode, a continuous cathode-anode wave is observed on the voltammogram, crossing the x-axis at a potential corresponding to the redox potential of the Ox/Red system in this environment. If the electrochemical reaction on the microelectrode is slow (irreversible), an anodic wave of oxidation of the reduced form of the substance and a cathodic wave of reduction of the oxidized form (at a more negative potential) are observed on the voltammogram. The formation of the limiting current area on the voltammogram is associated either with a limited rate of mass transfer of the electroactive substance to the electrode surface by convective diffusion (limiting diffusion current, I d), or with a limited rate of formation of the electroactive substance from the analyte component in solution. This current is called the limiting kinetic current, and its strength is proportional to the concentration of this component.
The waveform for a reversible electrochemical reaction is described by the equation:
where R is the gas constant, T is the absolute temperature, E 1/2 is the half-wave potential, i.e. potential corresponding to half the wave height (I d /2;). The E 1/2 value is characteristic of a given electroactive substance and is used for its identification. When electrochemical reactions are preceded by the adsorption of the analyte on the electrode surface, peaks rather than waves are observed in the voltammograms, which is associated with the extreme dependence of adsorption on the electrode potential. In voltammograms recorded during a linear change (sweep) of potential with a stationary electrode or on one drop of a dripping electrode (obsolete - oscillographic polarogram), peaks are also observed, the descending branch of which is determined by the depletion of the near-electrode layer of the solution in the electroactive substance. The height of the peak is proportional to the concentration of the electroactive substance. In polarography, the limiting diffusion current (in μA), averaged over the lifetime of a drop, is described by the Ilkovich equation:
where n is the number of electrons participating in the electrochemical reaction, C is the concentration of the electroactive substance (mM), D is the diffusion coefficient (cm 2 / s), the lifetime of a mercury drop (s), m is the flow rate of mercury (mg/s) .
With a rotating disk electrode, the limiting diffusion current is calculated using the equation:
where S is the surface area of the electrode (cm 2), is the circular frequency of rotation of the electrode (rad/s), v is the kinematic viscosity of the solution (cm 2 / s), F is the Faraday number (C/mol).
Cyclic voltammetry (voltammetry with a relatively fast triangular potential scan) allows you to study the kinetics and mechanism of electrode processes by observing on the screen of an oscilloscope tube with an afterglow simultaneously voltammograms with anodic and cathodic potential scans, reflecting, in particular, the electrochemical reactions of electrolysis products.
The lower limit of the determined concentrations of Cn in V. methods with linear potential sweep is 10 -5 -10 -6 M. To reduce it to 10-7 -10 -8 M, improved instrumental options are used - alternating current and differential pulse voltammetry.
In the first of these options, a small amplitude alternating component of a sinusoidal, rectangular (square wave voltammetry), trapezoidal or triangular shape with a frequency usually in the range of 20-225 Hz is superimposed on the constant component of the polarization voltage. In the second option, voltage pulses of the same magnitude (2-100 mV) with a duration of 4-80 ms are applied to the constant component of the polarization voltage with a frequency equal to the frequency of the mercury dripping electrode, or with a frequency of 0.3-1.0 Hz when using stationary electrodes. In both options, the dependence of the alternating current component on U or E is recorded with phase or time selection. In this case, voltammograms have the form of the first derivative of a conventional voltammetric wave. The peak height on them is proportional to the concentration of the electroactive substance, and the peak potential serves to identify this substance from reference data.
The peaks of various electroactive substances are usually better resolved than the corresponding voltammetric waves, and the peak height in the case of an irreversible electrochemical reaction is 5-20 times less than the peak height in the case of a reversible reaction, which also determines the increased resolution of these voltammetric options. For example, irreversibly reduced oxygen practically does not interfere with the determination of electroactive substances by alternating current voltammetry. The peaks in alternating current voltammograms reflect not only the electrochemical reactions of electroactive substances, but also the processes of adsorption and desorption of non-electroactive substances on the electrode surface (non-Faraday admittance peaks, obsolete - tensammetric peaks).
For all variants of voltammetry, a method of reducing Cn is used, based on preliminary electrochemical, adsorption or chemical accumulation of the determined component of the solution on the surface or in the volume of a stationary microelectrode, followed by registration of a voltammogram reflecting the electrochemical reaction of the accumulation product. This type of voltammetry is called inversion voltammetry (the old name for inversion voltammetry with accumulation on a stationary mercury microelectrode - amalgam polarography with accumulation). In stripping voltammetry with preliminary accumulation, Cn reaches 10 -9 -10 -11 M. The minimum values of Cn are obtained using thin-film mercury indicator electrodes, incl. mercury-graphite, consisting of tiny droplets of mercury, electrolytically separated onto a substrate of specially treated graphite.
For phase and elemental analysis solids, stripping voltammetry with electroactive carbon electrodes (so-called mineral-carbon paste electrodes) is used. They are prepared from a mixture of coal powder, the powdered substance being studied and an inert binder, for example. Vaseline oil. A variant of this method has been developed, which makes it possible to analyze and determine the thickness of metal coatings. In this case, a special device (clamping cell) is used, which makes it possible to record a voltammogram using a drop of background electrolyte applied to the surface under study.
Application
Voltammetry is used: for the quantitative analysis of inorganic and organic substances in a very wide range of contents - from 10 -10% to tens of%; to study the kinetics and mechanism of electrode processes, including the stage of electron transfer, preceding and subsequent chemical reactions, adsorption of initial products and products of electrochemical reactions, etc.; to study the structure of the electrical double layer, the equilibrium of complexation in solution, the formation and dissociation of intermetallic compounds in mercury and on the surface of solid electrodes; to select amperometric titration conditions, etc.
1.2 Conductometry
Conductometry - based on measuring the electrical conductivity of a solution and is used to determine the concentration of salts, acids, bases, etc. In conductometric determinations, electrodes made of identical materials are usually used, and the conditions for their conduct are selected in such a way as to minimize the contribution of potential jumps at both electrode/electrolyte interfaces (for example, high-frequency alternating current is used). In this case, the main contribution to the measured cell potential is made by the ohmic voltage drop IR, where R is the solution resistance. The electrical conductivity of a one-component solution can be related to its concentration, and measuring the electrical conductivity of electrolytes of complex composition allows one to estimate the total ion content in the solution and is used, for example, in monitoring the quality of distilled or deionized water. In another type of conductometry - conductometric titration - a known reagent is added in portions to the analyzed solution and the change in electrical conductivity is monitored. The equivalence point, at which a sharp change in electrical conductivity is noted, is determined from a graph of the dependence of this value on the volume of added reagent.
1.3 Potentiometry
Potentiometry - used to determine various physical and chemical parameters based on data on the potential of a galvanic cell. The electrode potential in the absence of current in the electrochemical circuit, measured relative to the reference electrode, is related to the concentration of the solution by the Nernst equation. In potentiometric measurements, ion-selective electrodes are widely used, sensitive primarily to one ion in solution: a glass electrode for measuring pH and electrodes for the selective determination of sodium, ammonium, fluorine, calcium, magnesium, etc. ions. surface layer enzymes can be incorporated into the ion-selective electrode, resulting in a system sensitive to the appropriate substrate. Note that the potential of an ion-selective electrode is determined not by the transfer of electrons, as in the case of substances with electronic conductivity, but mainly by the transfer or exchange of ions. However, the Nernst equation, which relates the electrode potential to the logarithm of the concentration (or activity) of a substance in solution, is also applicable to such an electrode. In potentiometric titration, the reagent is added to the solution being analyzed in portions and the change in potential is monitored. The S-shaped curves characteristic of this type of titration allow one to determine the equivalence point and find thermodynamic parameters such as the equilibrium constant and standard potential.
1.4 Amperometry
The method is based on measuring the limiting diffusion current passing through a solution at a fixed voltage between the indicator electrode and the reference electrode. In amperometric titration, the equivalence point is determined by the break in the current curve - the volume of the added working solution. Chronoamperometric methods are based on measuring the dependence of current on time and are mainly used to determine diffusion coefficients and rate constants. Miniature electrochemical cells that serve as sensors at the output of liquid chromatograph columns operate on the principle of amperometry (as well as voltammetry). Galvanostatic methods are similar to amperometric ones, but they measure the potential when a certain amount of current passes through the cell. Thus, in chronopotentiometry, the change in potential over time is controlled. These methods are used mainly to study the kinetics of electrode reactions.
1.5 Coulometry.
In coulometry, at a controlled potential, complete electrolysis of a solution is carried out by intensively mixing it in an electrolyzer with a relatively large working electrode (bottom mercury or platinum mesh). The total amount of electricity (Q, C) required for electrolysis is related to the amount of the forming substance (A, g) by Faraday’s law:
where M – mol. mass (g/mol), F – Faraday number. Coulometric titration involves using a constant current to electrolytically generate a reagent that reacts with the substance being determined. The progress of the titration is controlled potentiometrically or amperometrically. Coulometric methods are convenient because they are absolute in nature (i.e., they allow you to calculate the amount of the analyte without resorting to calibration curves) and are insensitive to changes in electrolysis conditions and electrolyzer parameters (electrode surface area or stirring intensity). In coulogravimetry, the amount of substance that has undergone electrolysis is determined by weighing the electrode before and after electrolysis.
There are other electroanalytical methods. In alternating current polarography, a low amplitude sinusoidal voltage is applied to a linearly varying potential over a wide frequency range and either the amplitude and phase shift of the resulting alternating current or the impedance is determined. From these data, information is obtained about the nature of substances in solution and about the mechanism and kinetics of electrode reactions. Thin-layer methods use electrochemical cells with an electrolyte layer 10–100 µm thick. In such cells, electrolysis proceeds faster than in conventional electrolyzers. To study electrode processes, spectrochemical methods with spectrophotometric registration are used. To analyze substances formed on the surface of the electrode, their absorption of light in the visible, UV and IR regions is measured. Changes in the properties of the electrode surface and the medium are monitored using electrical reflection and ellipsometry methods, which are based on measuring the reflection of radiation from the electrode surface. These include methods of specular reflection and Raman scattering of light (Raman spectroscopy), second harmonic spectroscopy (Fourier spectroscopy).
1.6 Other electrochemical phenomena and methods
With the relative movement of the electrolyte and charged particles or surfaces, electrokinetic effects occur. An important example of this kind is electrophoresis, in which the separation of charged particles (for example, protein molecules or colloidal particles) moving in an electric field occurs. Electrophoretic methods are widely used to separate proteins or deoxyribonucleic acids (DNA) in gels. Electrical phenomena play big role in the functioning of living organisms: they are responsible for the generation and distribution nerve impulses, the emergence of transmembrane potentials, etc. Various electrochemical methods are used to study biological systems and their components. It is also of interest to study the effect of light on electrochemical processes. Thus, the subject of photoelectrochemical research is the generation of electrical energy and the initiation of chemical reactions under the influence of light, which is very important for increasing the efficiency of converting solar energy into electrical energy. Semiconductor electrodes made of titanium dioxide, cadmium sulfide, gallium arsenide and silicon are commonly used here. Another interesting phenomenon is electrochemiluminescence, i.e. generation of light in an electrochemical cell. It is observed when high-energy products are formed on the electrodes. Often the process is carried out in a cyclic mode to obtain both oxidized and reduced forms of this connection. Their interaction with each other leads to the formation of excited molecules, which pass to the ground state with the emission of light.
1.7 Applied electrochemistry
Electrochemistry has many practical applications. With the help of primary galvanic cells (disposable elements) connected to batteries, chemical energy is converted into electrical energy. Secondary current sources - batteries - store electrical energy. Fuel cells are primary power sources that generate electricity through a continuous supply of reactants (such as hydrogen and oxygen). These principles underlie portable power sources and batteries used on space stations, electric vehicles and electronic devices.
Large-scale production of many substances is based on electrochemical synthesis. The electrolysis of brine in the chlor-alkali process produces chlorine and alkali, which are then used to produce organic compounds and polymers, as well as in the pulp and paper industry. The products of electrolysis are compounds such as sodium chlorate, persulfate, sodium permanganate; Industrially important metals are obtained by electroextraction: aluminum, magnesium, lithium, sodium and titanium. It is better to use molten salts as electrolytes, since in this case, unlike aqueous solutions, the reduction of metals is not complicated by the release of hydrogen. Fluorine is produced by electrolysis in molten salt. Electrochemical processes serve as the basis for the synthesis of some organic compounds; for example, adiponitrile (an intermediate in the synthesis of nylon) is obtained by hydrodimerization of acrylonitrile.
Electroplating of silver, gold, chromium, brass, bronze and other metals and alloys is widely practiced on various objects in order to protect steel products from corrosion, for decorative purposes, for the manufacture of electrical connectors and printed circuit boards in the electronics industry. Electrochemical methods are used for high-precision dimensional processing of workpieces made of metals and alloys, especially those that cannot be processed by conventional mechanical methods, as well as for the manufacture of parts with complex profiles. When the surface of metals such as aluminum and titanium is anodized, protective oxide films are formed. Such films are created on the surface of workpieces made of aluminum, tantalum and niobium in the manufacture of electrolytic capacitors, and sometimes for decorative purposes.
In addition, studies of corrosion processes and the selection of materials that slow down these processes are often based on electrochemical methods. Corrosion of metal structures can be prevented using cathodic protection, for which an external source is connected to the structure being protected and the anode and the structure is maintained at a potential such that its oxidation is excluded. Possibilities are being explored practical application other electrochemical processes. So, electrolysis can be used to purify water. A very promising direction is the conversion of solar energy using photochemical methods. Electrochemical monitors are being developed, the operating principle of which is based on electrochemiluminescence.
Electrochemical methods of analysis (electroanalysis), which are based on electrochemical processes, occupy a worthy place among methods for monitoring the state of the environment, as they are capable of determining a huge number of both inorganic and organic environmentally hazardous substances. They are characterized by high sensitivity and selectivity, rapid response to changes in the composition of the analyzed object, ease of automation and the possibility of remote control. Finally, they do not require expensive analytical equipment and can be used in laboratory, industrial and field conditions. Three electroanalytical methods are directly related to the problem under consideration: voltammetry, coulometry and potentiometry.
Brief historical background. The beginning of the development of electroanalysis is associated with the emergence of the classical electrogravimetric method (around 1864, W. Gibbs). The discovery of the laws of electrolysis by M. Faraday in 1834 formed the basis of the coulometry method, but the use of this method began in the 30s of the twentieth century. A real turning point in the development of electroanalysis occurred after the discovery of the polarography method in 1922 by J. Heyrovsky. Polarography can be defined as electrolysis with a dropping mercury electrode. This method remains one of the main methods of analytical chemistry. In the late 50s and early 60s, the problem of environmental protection stimulated the rapid development of analytical chemistry, and in particular electroanalytical chemistry, including polarography. As a result, improved polarographic methods were developed: alternating current (Mr. Barker, B. Breuer) and pulsed polarography (Mr. Barksr, A. Gardnsr), which significantly surpassed in their characteristics the classic version of polarography proposed by J. Heyrovsky. When using solid electrodes made of various materials instead of mercury (used in polarography), the corresponding methods began to be called voltammetric. At the end of the 50s, the work of V. Kemuli and Z. Kublik laid the foundation for the method of stripping voltammetry. Along with the methods of coulometry and voltammetry, methods based on measuring electrode potentials and electromotive forces of galvanic cells are being developed - methods of potentiometry and ionometry (see).
Voltammetry. This is a group of methods based on studying the dependence of the current in an electrolytic cell on the potential applied to an indicator microelectrode immersed in the analyzed solution. These methods are based on the principles of electrolysis; the analytes present in the solution are oxidized or reduced at the indicator electrode. In addition to the indicator electrode, a reference electrode with a much larger surface is placed in the cell so that when current passes, its potential practically does not change. The most commonly used indicator microelectrodes are stationary and rotating electrodes made of platinum or graphite, as well as a mercury dripping electrode, which is a long narrow capillary, at the end of which small mercury drops with a diameter of 1-2 mm are periodically formed and separated (Fig. 1). The qualitative and quantitative compositions of a solution can be determined from voltammograms.
Rice. 4. Electrochemical cell with a dropping mercury electrode: 1 - solution to be analyzed, 2 - dropping mercury electrode, 3 - reservoir with mercury, 4 - reference electrode
Voltammetric methods, especially sensitive variants such as differential pulse polarography and stripping voltammetry, are routinely used in all areas of chemical analysis and are most useful in solving environmental problems. These methods are applicable for the determination of both organic and inorganic substances, for example, for the determination of most chemical elements. Using the stripping voltammetry method, the problem of determining traces of heavy metals in waters and biological materials is most often solved. For example, voltammetric methods for the simultaneous determination of Cu, Cd and Pb, as well as Zn and Pb or Ti in drinking water are included in the standard Germany. An important advantage of voltammetry is the ability to identify the forms of metal ions in waters. This makes it possible to assess water quality, since different chemical forms of metals have different degrees of toxicity. From organic substances, it is possible to determine compounds that have groups capable of reduction (aldehydes, ketones, nitro-, nitroso compounds, unsaturated compounds, halogen-containing compounds, azo compounds) or oxidation ( aromatic hydrocarbons, amines, phenols, aliphatic acids, alcohols, sulfur-containing compounds). The possibilities for determining organic substances by stripping voltammetry are significantly expanded when using chemically modified electrodes. By modifying the electrode surface with polymer and inorganic films, including reagents with specific functional groups, including biomolecules, it is possible to create conditions for the component being determined where the analytical signal will be practically specific. The use of modified electrodes provides selective determination of compounds with similar redox properties (for example, pesticides and their metabolites) or electrochemically inactive on conventional electrodes. Voltammetry is used to analyze solutions, but it can also be used to analyze gases. Many simple voltammetric analyzers have been designed for use in the field.
Coulometry. An analysis method based on measuring the amount of electricity (Q) passed through an electrolyzer during electrochemical oxidation or reduction of a substance at the working electrode. According to Faraday's law, the mass of an electrochemically converted substance (P) is related to Q by the ratio:
P =
QM/
Fn,
where M is the molecular or atomic mass of the substance, n is the number of electrons involved in the electrochemical transformation of one molecule (atom) of the substance, p is Faraday’s constant.
A distinction is made between direct coulometry and coulometric titration. In the first case, an electrochemically active substance is determined, which is deposited (or transferred to a new oxidation state) on the electrode at a given electrolysis potential, while the amount of electricity expended is proportional to the amount of the reacted substance. In the second case, an electrochemically active auxiliary reagent is introduced into the analyzed solution, from which a titrant (coulometric titrant) is electrolytically generated, and it quantitatively chemically interacts with the substance being determined. The content of the component being determined is assessed by the amount of electricity passed through the solution during the generation of the titrant until the completion of the chemical reaction, which is determined, for example, using color indicators. It is important that when carrying out coulometric analysis, there are no foreign substances in the test solution that can enter into electrochemical or chemical reactions under the same conditions, that is, no side electrochemical and chemical processes occur.
Coulometry is used to determine both trace (at the level of 109-10 R mol/l) and very large quantities of substances with high accuracy. Coulometrically, it is possible to determine many inorganic (almost all metals, including heavy metals, halogens, S, NO3, NO2) and organic substances (aromatic amines, nitro- and nitroso compounds, phenols, azo dyes). Automatic coulometric analyzers for determining very low levels (up to 104%) of gaseous pollutants (S02"Oz, H2S, NO, N02) in the atmosphere have successfully proven themselves in field conditions.
Potentiometry. An analysis method based on the dependence of the equilibrium electrode potential E on the activity of the a components of the electrochemical reaction: aA + bB + ne = mM + pP.
In potentiometric measurements, a galvanic element is formed from an indicator electrode, the potential of which depends on the activity of one of the components of the solution, and a reference electrode, and the electromotive force of this element is measured.
There are direct potentiometry and potentiometric titration. Direct potentiometry is used to directly determine the activity of ions from the potential value (E) of the corresponding indicator electrode. In the potentiometric titration method, the change in E during the reaction of the analyte with a suitable titrant is recorded.
When solving problems of environmental protection, the most important method is direct potentiometry using membrane ion-selective electrodes (ISE) - ionometry. Unlike many other methods of analysis, which allow one to estimate only the total concentration of substances, ionometry allows one to estimate the activity of free ions and therefore plays a large role in studying the distribution of ions between their different chemical forms. Automated monitoring methods are especially important for monitoring environmental objects, and the use of ISE is very convenient for this purpose.
One of the main indicators in characterizing the state of the environment is the pH value of the environment, which is usually determined using glass electrodes. Glass electrodes coated with a semi-permeable membrane with a film of the appropriate electrolyte are used in the analysis of water and atmosphere to control pollutants (NH3, SO 2 NO, NO 2, CO 2, H 2 S). ISE is usually used to monitor the content of anions, for which there are traditionally much fewer determination methods than for cations. To date, ISEs have been developed and are widely used for the determination of F, CI, Br, I, C1O4, CN, S2, NO] and NO2, allowing the determination of the listed ions in the concentration range from 10 -6 to 10 -1 mol/l .
One of the important areas of application of ionometry is hydrochemical studies and determination of the concentration of anions and cations in different types of water (surface, sea, rain). Another area of application of ISE is food analysis. An example is the determination of NO – 3 and NO 2 – in vegetables, meat and dairy products, and baby food products. A miniature needle-shaped ISE has been created to determine NO - 3 directly in the pulp of fruits and vegetables.
Ionometry is also widely used to determine various biologically active compounds and drugs. At present, we can already say that there are carriers that are selective to almost any type of organic compounds, which means that it is possible to create an unlimited number of corresponding ISEs. A promising direction is the use of enzyme electrodes, the membrane of which contains immobilized enzymes. These electrodes have the high specificity inherent in enzymatic reactions. With their help, for example, it will be possible to determine cholinesterase-inhibiting insecticides (organophosphorus compounds, carbamates) at concentrations of -1 ng/ml. The future of the method is associated with the creation of compact, specific sensors, which are modern electronic devices in combination with ion-selective membranes, which will make it possible to avoid the separation of sample components and will significantly speed up analyzes in the field.
Wastewater analysis
Electroanalytical methods, which are usually used in water analysis to determine inorganic components, are often inferior in sensitivity to gas and liquid chromatography and atomic adsorption spectrometry. However, cheaper equipment is used here, sometimes even in the field. The main electroanalytical methods used in water analysis are voltammetry, potentiometry and conductometry. The most effective voltammetric methods are differential pulse polarography (DIP) and stripping electrochemical analysis (IEA). The combination of these two methods makes it possible to carry out determinations with very high sensitivity - approximately 10 -9 mol/l, while the instrumentation is simple, which makes it possible to carry out analyzes in the field. Fully automated monitoring stations operate on the principle of using the IEA method or a combination of IEA with DIP. The DIP and IEA methods, in a direct version, as well as in combination with each other, are used to analyze water contamination with heavy metal ions and various organic substances. At the same time, sample preparation methods are often much simpler than in spectrometry or gas chromatography. The advantage of the IEA method is (in contrast to other methods, for example, atomic adsorption spectrometry) the ability to “distinguish” free ions from their bound chemical forms, which is important both for assessing the physicochemical properties of the analyzed substances and from the point of view of biological control ( for example, when assessing the toxicity of water). Analysis time is sometimes reduced to several seconds by increasing the polarizing voltage sweep speed.
Potentiometry using various ion-selective electrodes is used in water analysis to determine large number inorganic cations and anions. The concentrations that can be determined in this way are 10 0 -10 -7 mol/l. Monitoring using ion-selective electrodes is simple, fast and allows for continuous measurements. Currently, ion-selective electrodes have been created that are sensitive to certain organic substances (for example, alkaloids), surfactants and detergents. Water analysis uses compact probe-type analyzers using modern ion-selective electrodes. In this case, a circuit that processes the response and a display are mounted in the probe handle.
Conductometry used in the operation of detergent analyzers in wastewater, in determining the concentrations of synthetic fertilizers in irrigation systems, and in assessing the quality of drinking water. In addition to direct conductometry, indirect methods can be used to determine some types of pollutants, in which the substances being determined are reacted with specially selected reagents before measurement and the recorded change in electrical conductivity is caused only by the presence of the corresponding reaction products. In addition to the classical variants of conductometry, its high-frequency version (oscilloscopy) is also used, in which the indicator electrode system is implemented in continuous conductometric analyzers.
Chapter 3. Devices based on electrochemical methods of analysis
The voltammetric method of analysis today is considered one of the most promising among electrochemical methods, due to its wide capabilities and good performance characteristics.
Modern stripping voltammetry, which has replaced classical polarography, is a highly sensitive and rapid method for determining a wide range of inorganic and organic substances with redox properties.
This is one of the most universal methods for determining trace amounts of substances, which is successfully used for the analysis of natural geo- and biological, as well as medical, pharmaceutical and other objects.
Voltammetric analyzers make it possible to simultaneously determine several components (up to 4 - 5) in one sample with a fairly high sensitivity of 10 -8 - 10 -2 M (and stripping voltammetry - up to 10-10 - 10 -9 M).
Adsorption stripping voltammetry, based on preliminary adsorption concentration of the element being determined on the surface of the electrode and subsequent recording of the voltammogram of the resulting product, is considered the most promising in analytical chemistry today. In this way, it is possible to concentrate many organic substances, as well as metal ions in the form of complexes with organic ligands (especially nitrogen- and sulfur-containing ones). With a sequential accumulation time of 60 s and using a differential pulse mode for recording a voltammogram, it is possible to achieve detection limits at the level of 10 -10 - 10 -11 mol/l (10 -8 - 10 -9 g/l or 0.01 - 0.001 μg/dm 3 ).
Voltammetric complex for metal analysis "IVA - 400MK" (NPKF "Akvilon", Moscow) designed for the analysis of 30 elements (Cu, Zn, Pb, Cd, As, Co, Ni, Cr, and other metals), sensitivity 0.1 - 10 -3 μg/dm 3.
Voltammetric analyzer with UV irradiation of samples - TA-1M (Tomsk), which, in addition to metal ions, allows you to determine a number of organic compounds. The device is characterized by the following features:
· simultaneous analysis in three electrochemical cells,
· small sample (0.1 - 1.0 g),
· low cost of sample preparation and analysis.
In St. Pereburg NFT “Volta” produces the ABC-1 voltammetric complex with a rotating disk glassy carbon electrode, which allows for the analysis of toxic elements in waters, food products and various materials. The detection limit without sample concentration is: 0.1 mg/l for Pb, 0.5 mg/l for Cd, 1.0 μg/l for Cu. The sample volume is 20 ml, the time to obtain a current-voltage curve is no more than 3 minutes.
"AZHE - 12" (Vladikavkaz) is intended for express analysis of the ionic composition of waste and circulating waters. The analyzer uses a traditional mercury electrode. Controlled components - Cu, Zn, Pb, Cd, In, Bi, Tl, Sb, As, Co, Ni, Cr, CN -, Cl -, S 2-. The analyzer allows you to carry out measurements without sample preparation.
"Ecotest-VA" ("Ekoniks", Moscow) - portable voltammetric analyzer. Made on a modern microprocessor element base and equipped with a whole complex of electrodes - graphite, glassy carbon, microelectrodes from noble metals and mercury dripping electrode.
Instruments of this series are intended for the determination of metals Cu, Zn, Pb, Cd, As, Bi, Mn, Co, Ni, Cr, as well as acetaldehyde, furfural, caprolactam and other substances in samples of drinking, natural, waste water, soil, and after appropriate sample preparation - in food and feed.
The capabilities of many analytical methods for water analysis can be significantly expanded when using flow-injection concentrating attachments operating in automatic mode - for example, the BPI-M and BPI-N types - in the sample preparation process.
BPI-M - designed for automated sample preparation, it includes microcolumns with highly efficient sorbents. The unit's productivity is 30-60 analyzes per day with full automation of the process. The use of the block allows you to increase sensitivity by 20 times per minute of concentration. The unit works best in combination with atomic absorption detection, as well as X-ray fluorescence, atomic absorption and electrochemical methods.
BPI-N- designed for concentrating metal ions on selective sorbents simultaneously in four microcolumns with DETATE sorbent or on 4 thin-layer sorption DETATE filters. It can be used with X-ray fluorescence, atomic absorption, atomic emission, and electrochemical methods.
Analyzers based on voltammetry
Instruments based on the principle of inverse voltammetry are used in Lately special demand. They combine selectivity and high sensitivity with ease of analysis.
With regard to the determination of elemental composition (for example, for heavy metals), these devices successfully compete with atomic absorption spectrophotometers, since they are not inferior to them in sensitivity, but are much more compact and cheaper (about 5 - 10 times). They do not require additional consumables, and also provide the opportunity for simultaneous rapid determination of several elements.
Polarograph ABC - 1.1 (NTF "Volta" St. Petersburg).
The detection limits for metals without sample concentration are (mg/l): Cd, Pb, Bi - 0.0001, Hg - 0.00015, Cu - 0.0005, Zn, Ni - 0.01. Cost $1700.
Analyzers based on the conductometric principle are designed to quantitatively determine the total salt content in water. "EKA-2M" (St. Petersburg) measures salt content in a wide range of values from 0.05 to 1000 µS/cm ($900). “ANION”, “MARK”, KSL (from 330 to 900 $), COD - analyzers ($750).
Gas analyzers for harmful substances
An automatic gas analyzer is a device in which air sampling, determination of the amount of the controlled component, issuance and recording of analysis results are carried out automatically according to a given program without operator participation. To control the air environment, gas analyzers are used, the operation of which is based on various principles.
Thermoconductometric gas analyzers.
The operating principle is based on the dependence of the thermal conductivity of the gas mixture on its composition. The sensitive element of this type of analyzers is thin platinum filaments. Depending on the composition of the gas, the temperature of the sensitive element changes, and a current arises, the strength of which is proportional to the concentration of the controlled component.
Coulometric gas analyzers.
The operating principle is based on measuring the limiting electric current that occurs during the electrolysis of a solution that contains the substance being determined, which is an electrochemical depolarizer. The mixture to be analyzed, containing, for example, sulfur dioxide, is fed into an electrochemical cell. It reacts with iodine to form hydrogen sulfide, which is then electrooxidized at the measuring electrode. Electric current is a measure of the concentration of the component being determined.
CHAPTER 4. OVERVIEWWEB– SITES OF COMPANIES SELLING CHEMICAL ANALYTICAL EQUIPMENT
"AGILENT.RU"
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"ANALITPRIBOR"
Offers gas analyzers
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"WATSON", JSC, Mytishchi, Moscow region.
Instruments and measuring instruments;
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"DIPOL", NPF, St. Petersburg
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"EuroLab SPb", LLC, St. Petersburg
Spectral analysis devices, chromatographs.
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"IZME.RU"
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"INSOVT", JSC
Development and production of gas analyzers
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"Institute of Information Technologies", Minsk, Belarus
Specializes in development and production measuring instruments for fiber optics...
"KIPARIS", LLC, St. Petersburg
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"CONTINENT", Gomel
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"Instrumentation and equipment", Volgograd
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"Kontur", ITC, LLC, Novosibirsk
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"Christmas+", JSC, St. Petersburg
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"KURS", LLC, St. Petersburg
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"METTEC"
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"MONITORING", STC, St. Petersburg
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"Scientific Instruments", JSC, St. Petersburg
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"NevaLab", JSC, St. Petersburg
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"OVEN", PO, Moscow
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"OCTAVA+", Moscow
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"OPTEK", CJSC, St. Petersburg
Develops and produces gas analyzers and analytical systems for various purposes for use in ecology, industry and scientific research...
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"POLITECHFORM", Moscow
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"Praktik-NTs", JSC, Moscow, Zelenograd
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"INSTRUMENTS AND ANALYTICAL TECHNOLOGY"
Instruments for chemical analysis.
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"Sartogosm", JSC, St. Petersburg
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"Special", JSC, Moscow
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"TKA"
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"TST", CJSC, St. Petersburg
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"ECOPRIBOR", NPO, Moscow
Offers gas analyzers and gas analysis systems...
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HIMMED
Analytical Instruments and Chromatography
e-mail:[email protected]
LITERATURE
1. Geyrovsky J., Kuta J., Fundamentals of polarography, trans. from Czech., M., 1965;
2. Gal yus 3., Theoretical foundations of electrochemical analysis, trans. from Polish, M., 1974;
3. Kaplan B. Ya., Pulse polarography, M., 1978;
4. Brainina X. Z., Neiman E. Ya., Solid-phase reactions in electroanalytical chemistry, M., 1982;
5. Kaplan B. Ya., Pats R. G., Salikhdzhanova R. M.-F., AC voltammetry, M., 1985.
6. Plambeck J. Electrochemical methods of analysis. / Per. from English M.: Mir, 1985. 496 p.
7. Brief chemical encyclopedia. M.: Soviet encyclopedia, 1964. Volume 1. A–E. 758 p.
8. Classification and nomenclature of electrochemical methods // Journal. analyte chemistry. 1978. T. 33, no. 8. pp. 1647–1665.
9. Recommended Terms, Symbols and Definitions for Electroanalytical Chemistry // Pure & Appl. Chem. 1979. Vol. 51. P. 1159–1174.
10. On the use of the concept of “chemical equivalent” and related quantities: Zhurn. analyte chemistry. 1989. T. 44, no. 4. pp. 762–764; Journal analyte chemistry. 1982. T. 37, no. 5. P. 946; Journal analyte chemistry. 1982. T. 37, no. 5. P. 947.
11. Neiman E.Ya. Terminology of modern analytical chemistry and its formation // Journal. analyte chemistry. 1991. T. 46, no. 2. pp. 393–405.
12. Presentation of the results of chemical analysis (IUPAC Recommendations 1994) // Journal. analyte chemistry. 1998. T. 53. No. 9. pp. 999–1008.
13. Compendium of Analytical Nomenclature (Definitive Rules 1997). 3rd ed., IUPAC, Blackwell Science, 1998. 8.1–8.51 (Electrochemical Analysis).
Electrochemical methods of analysis are a set of methods of qualitative and quantitative analysis based on electrochemical phenomena occurring in the medium under study or at the interface and associated with changes in the structure, chemical composition or concentration of the analyte.
Electrochemical methods of analysis (ECMA) are based on processes occurring on electrodes or the interelectrode space. Their advantage is high accuracy and comparative simplicity of both equipment and analysis methods. High accuracy is determined by very precise patterns used in ECMA. A great convenience is that this method uses electrical influences, and the fact that the result of this influence (response) is also obtained in the form of an electrical signal. This ensures high speed and accuracy of reading, and opens up wide possibilities for automation. ECMA are distinguished by good sensitivity and selectivity; in some cases they can be classified as microanalysis, since sometimes less than 1 ml of solution is sufficient for analysis.
According to the types of analytical signal, they are divided into:
1) conductometry - measurement of the electrical conductivity of the test solution;
2) potentiometry - measurement of the current-free equilibrium potential of the indicator electrode, for which the test substance is potentiodetermining;
3) coulometry - measurement of the amount of electricity required for complete transformation (oxidation or reduction) of the substance under study;
4) voltammetry - measurement of stationary or non-stationary polarization characteristics of electrodes in reactions involving the test substance;
5) electrogravimetry - measurement of the mass of a substance released from a solution during electrolysis.
27. Potentiometric method.
potentiometry - measurement of the current-free equilibrium potential of the indicator electrode, for which the test substance is potentiation-determining.
A) standard (reference electrode) - has a constant potential, independent of external influences. Terms
B) individual electrode - its potential depends on the concentration of the substance.
Potential depends on concentration: E = f(c)
Nerist equation E= E° + lna kat
E° - standard. Electron. Potential (const)
R- Univer. Gas constantconst)
T – absolute temp (t)- +273 °
.п – number of electrons involved. In oxidation/reduction Reactions
. a – active concentration
Potentiometry method
Ionometry potentiometry (small solution is added to the research solution. Standard solution (titran) is added in portions, after each addition the potential is measured. - E)
Equivalence point
EСх Vх = l t *Vt
28. Conductometric method.
conductometry - measurement of the electrical conductivity of the test solution.
Conductometric titration
Conductometer (device)
Conductometric analysis (conductometry) is based on the use of the relationship between the electrical conductivity (electrical conductivity) of electrolyte solutions and their concentration.
The electrical conductivity of electrolyte solutions - conductors of the second type - is judged on the basis of measuring their electrical resistance in an electrochemical cell, which is a glass vessel (glass) with two electrodes soldered into it, between which the test electrolyte solution is located. An alternating electric current is passed through the cell. Electrodes are most often made of metal platinum, which, to increase the surface of the electrodes, is coated with a layer of spongy platinum by electrochemical deposition of platinum compounds from solutions (platinized platinum electrodes).
29.Polarography.
Polarography is a method of qualitative and quantitative chemical analysis based on obtaining curves of current versus voltage in a circuit consisting of the solution under study and electrodes immersed in it, one of which is highly polarizable and the other practically non-polarizable. Such curves - polarograms - are obtained using polarographs.
The polarographic method is characterized by high sensitivity. To perform the analysis, 3-5 ml of the test solution is usually sufficient. Analysis using an auto-recording polarograph lasts only about 10 minutes. Polarography is used to determine the content of toxic substances in objects of biological origin (for example, compounds of mercury, lead, thallium, etc.), to determine the degree of oxygen saturation of the blood, to study the composition of exhaled air, and harmful substances in the air of industrial enterprises. The polarographic method of analysis is highly sensitive and makes it possible to determine substances at very low (up to 0.0001%) concentrations in solution.
30. Classification of spectral analysis methods. The concept of spectrum.
Spectral analysis is a set of methods for determining quality and quantity. Composition, as well as structure of matter (based on the interaction of the research object with various types of radiation.)
All spectroscopic methods are based on the interaction of atoms, molecules or ions that make up the substance being analyzed with electromagnetic radiation. This interaction manifests itself in the absorption or emission of photons (quanta). Depending on the nature of the sample’s interaction with electromagnetic radiation, two groups of methods are distinguished:
Emission and absorption. Depending on which particles form the analytical signal, a distinction is made between atomic spectroscopy methods and molecular spectroscopy methods
Emission
In emission methods, the analyzed sample emits photons as a result of its excitation.
absorption
In absorption methods, radiation from an external source is passed through the sample, and some of the quanta are selectively absorbed by atoms or molecules
Range- distribution of values of a physical quantity (usually energy, frequency or mass). A graphical representation of such a distribution is called a spectral diagram. Typically, spectrum refers to the electromagnetic spectrum - the spectrum of frequencies (or the same thing as quantum energies) of electromagnetic radiation.
1.light reflection
2.rotation of the light beam (defraction)
3.light scattering: nephelometry, turbidimetry
4.light absorption
5re-emission
A) phosphorescence (lasts a long time)
B) fluorescence (very short)
According to the nature of the distribution of physical quantity values, spectra can be discrete (line), continuous (solid), and also represent a combination (superposition) of discrete and continuous spectra.
Examples of line spectra include mass spectra and spectra of bonded-bonded electronic transitions of an atom; examples of continuous spectra are the spectrum of electromagnetic radiation of a heated solid and the spectrum of free-free electronic transitions of an atom; examples of combined spectra are the emission spectra of stars, where chromospheric absorption lines or most sound spectra are superimposed on the continuous spectrum of the photosphere.
31. Photometry: principle of the method, application in forensic research.
Photometry - a spectral method based on the absorption of electromagnetic radiation in the visible and near ultraviolet range (the method is based on the absorption of light)
Molecular Atomic
Spectroscopy spectroscopy (In electron analysis)
Cuvette - light is passed through it
l
I (output light intensity)
I° is the intensity of the incident light.
Photometry is a section of physical optics and measurement technology devoted to methods for studying the energy characteristics of optical radiation in the process of its emission, propagation in various media and interaction with bodies. Photometry is carried out in the ranges of infrared (wavelengths - 10 -3 ... 7 10 -7 m), visible (7 10 -7 ... 4 10 -7 m) and ultraviolet (4 10 -7 ... 10 -8 m) optical radiation. When electromagnetic radiation of the optical range propagates in a biological environment, a number of main effects are observed: absorption and scattering of radiation by atoms and molecules of the medium, scattering of inhomogeneities of the medium by particles, depolarization of radiation. By recording data on the interaction of optical radiation with the medium, it is possible to determine quantitative parameters associated with the medical and biological characteristics of the object under study. To measure photometric quantities, instruments called photometers are used. In photometric terms, light is radiation capable of producing a sensation of brightness when exposed to the human eye. The basis of photometry as a science is the light field theory developed by A. Gershun.
There are two common methods photometry: 1) visual photometry, in which the ability of the human eye to perceive differences in brightness is used when aligning the brightness of two comparison fields by mechanical or optical means; 2) physical photometry, in which various light receivers of a different kind are used to compare two light sources - vacuum photocells, semiconductor photodiodes, etc.
32. Bouguer-Lambert-Beer law, its use in quantitative analysis.
A physical law that determines the attenuation of a parallel monochromatic beam of light as it propagates in an absorbing medium.
The law is expressed by the following formula:
,
where is the intensity of the incoming beam, is the thickness of the layer of substance through which the light passes, is the absorption index (not to be confused with the dimensionless absorption index, which is related to the formula, where is the wavelength).
The absorption index characterizes the properties of a substance and depends on the wavelength λ of the absorbed light. This dependence is called the absorption spectrum of the substance.
For solutions of absorbing substances in non-light-absorbing solvents, the absorption index can be written as
where is the coefficient characterizing the interaction of a molecule of an absorbing solute with light with wavelength λ, is the concentration of the solute, mol/l.
The statement that does not depend on is called Beer's law (not to be confused with Beer's law). This law assumes that the ability of a molecule to absorb light is not affected by other surrounding molecules of the same substance in solution. However, numerous deviations from this law are observed, especially at large .
If a light flux of intensity I passes through a certain layer of a solution or gas of thickness I, then according to the Lambert-Beer law, the amount of absorbed light will be proportional to the intensity /, the concentration c of the substance absorbing light, and the thickness of the LAYER) the BMB law, which relates the intensity of light incident on the substance and the substance that passed through it, with the concentration of the substance and the thickness of the absorbing layer. Well, this is the same as refraction, only attenuation in the substance. Which absorbs light at a certain percentage. That is, the remainder of the light output is
33.IR spectroscopy.
This analysis method is based on recording the infrared absorption spectra of a substance. Absorption by matter in the infrared region occurs due to vibrations of atoms in molecules. Vibrations are divided into stretching (when the distances between atoms change during the vibration) and vibrational (when the angles between the bonds change during the vibration). Transitions between different vibrational states in molecules are quantized, due to which absorption in the IR region has the form of a spectrum, where each vibration has its own wavelength. It is clear that the wavelength for each vibration depends on which atoms participate in it, and in addition, it depends little on their environment.
The IR spectroscopy method is not a separating method, that is, when studying any substance, it may turn out that what was actually studied was a mixture of several substances, which of course will greatly distort the results of deciphering the spectrum. Well, it’s not entirely correct to talk about the unambiguous identification of a substance using the IR spectroscopy method, since the method rather allows one to identify certain functional groups, rather than their quantity in a compound and their method of communication with each other.
The IR spectroscopy method is used to conduct research on polymer materials, fibers, paint coatings, and narcotic drugs (when identifying the filler, carbohydrates, including polysaccharides, are often used). The method is especially indispensable in the study of lubricants, as it makes it possible to simultaneously determine the nature of both the lubricant base and possible additives (additives) to this base.
34. X-ray fluorescence analysis.
(XRF) is one of the modern spectroscopic methods for studying a substance in order to obtain its elemental composition, that is, its elemental analysis. It can be used to analyze various elements from beryllium (Be) to uranium (U). The XRF method is based on the collection and subsequent analysis of a spectrum obtained by exposing the material under study to X-ray radiation. When irradiated, the atom goes into an excited state, which consists in the transition of electrons to higher energy levels. The atom remains in an excited state for an extremely short time, on the order of one microsecond, after which it returns to a quiet position (ground state). In this case, electrons from the outer shells either fill the resulting vacancies, and the excess energy is emitted in the form of a photon, or the energy is transferred to another electron from the outer shells (Auger electron)
Ecology and environmental protection: determination of heavy metals in soils, sediments, water, aerosols, etc.
Geology and mineralogy: qualitative and quantitative analysis of soils, minerals, rocks, etc.
Metallurgy and chemical industry: quality control of raw materials, production process and finished products
Paint and varnish industry: analysis of lead paints
35. Atomic emission spectroscopy.
Atomic emission spectral analysis is a set of elemental analysis methods based on the study of the emission spectra of free atoms and ions in the gas phase. Typically, emission spectra are recorded in the most convenient optical wavelength range from 200 to 1000 nm.
AES (atomic emission spectrometry) is a method of determining the elemental composition of a substance from the optical emission spectra of atoms and ions of the analyzed sample, excited in light sources. As light sources for atomic emission analysis, use a burner flame or different kinds plasma, including electric spark or arc plasma, laser spark plasma, inductively coupled plasma, glow discharge, etc. AES is the most common express, highly sensitive method for identifying and quantifying impurity elements in gaseous, liquid and solid substances, including high-purity ones .
Areas of use:
Metallurgy: analysis of the composition of metals and alloys,
Mining industry: study of geological samples and mineral raw materials,
Ecology: water and soil analysis,
Equipment: analysis of motor oils and other technical fluids for metal impurities,
Biological and medical research.
Operating principle.
The operating principle of an atomic emission spectrometer is quite simple. It is based on the fact that the atoms of each element can emit light of certain wavelengths - spectral lines, and these wavelengths are different for different elements. In order for atoms to start emitting light, they must be excited - by heat, electrical discharge, laser or some other means. The more atoms of a given element are present in the analyzed sample, the brighter the radiation of the corresponding wavelength will be.
The intensity of the spectral line of the analyzed element, in addition to the concentration of the analyzed element, depends on a large number of different factors. For this reason, it is impossible to theoretically calculate the relationship between line intensity and the concentration of the corresponding element. That is why standard samples that are similar in composition to the sample being analyzed are required for analysis. These standard samples are first exposed (burned) on the device. Based on the results of these burns, a calibration graph is constructed for each analyzed element, i.e. dependence of the intensity of the spectral line of an element on its concentration. Subsequently, when analyzing samples, these calibration graphs are used to recalculate the measured intensities into concentrations.
Preparation of samples for analysis.
It should be borne in mind that in reality several milligrams of a sample from its surface are analyzed. Therefore, to obtain correct results, the sample must be homogeneous in composition and structure, and the composition of the sample must be identical to the composition of the metal being analyzed. When analyzing metal in a foundry or smelter, it is recommended to use special molds for casting samples. In this case, the sample shape can be arbitrary. It is only necessary that the sample being analyzed has sufficient surface area and can be clamped in a stand. Special adapters can be used to analyze small samples such as rods or wires.
Advantages of the method:
Non-contact,
Possibility of simultaneous quantitative determination of a large number of elements,
High accuracy,
Low detection limits,
Ease of sample preparation,
Low cost.
36. Atomic absorption spectroscopy.
a method for quantitatively determining the elemental composition of a substance under study using atomic absorption spectra, based on the ability of atoms to selectively absorb electromagnetic radiation in decomp. parts of the spectrum. A.-a.a. carried out on special devices - absorption spectrophotometers. A sample of the analyzed material is dissolved (usually with the formation of salts); the solution in the form of an aerosol is fed into the burner flame. Under the influence of a flame (3000°C), salt molecules dissociate into atoms, which can absorb light. Then a beam of light is passed through the burner flame, in the spectrum of which there are spectral lines corresponding to one or another element. The spectral lines under study are isolated from the total radiation using a monochromator, and their intensity is recorded by a recording unit. Math. processing is carried out according to the formula: J = J0 * e-kvI,
where J and J0 are the intensities of transmitted and incident light; kv – coefficient absorption, depending on its frequency; I - thickness of the absorbing layer
more sensitive than nuclear power plants
37. Nephelometry and turbidimetry.
S = log (I°/I) intensity falling. In solution (I°) divide by the intensity leaving solution (I) =
k-const turbidity
b – light beam path length
N is the number of particles per unit. solution
Nephelometric and turbidimetric analysis uses the phenomenon of light scattering by solid particles suspended in solution.
Nephelometry - a method for determining dispersion and concentration colloidal systems by the intensity of the light scattered by them. Nephelometry, measurements are made in a special device, a nephelometer, the action of which is based on comparing the intensity of light scattered by the medium under study with the intensity of light scattered by another medium, which serves as a standard. The theory of light scattering by colloidal systems in which particle sizes do not exceed the half-wavelength of incident light was developed by the English physicist J. Rayleigh in 1871. According to Rayleigh's law, the intensity of light I scattered in a direction perpendicular to the incident beam is expressed by the formula I = QNvlk - where q is the intensity of the incident light, N is the total number of particles per unit volume, or partial concentration, v is the volume of one particle, \ is the wavelength of the incident light, k is a constant depending on the refractive indices of colloidal particles and the dispersion medium surrounding them, distance from the light source, as well as from the accepted units of measurement
Turbidimetry is a method for analyzing turbid media based on measuring the intensity of light absorbed by them. Turbidimetric measurements are carried out in transmitted light using visual turbidimeters or photoelectric colorimeters. The measurement technique is similar to the colorimetric one and is based on the applicability of the Bouguer-Lambert law to turbid media, which in the case of suspensions is valid only for very thin layers or at significant dilutions. Turbidimetry requires careful observance of the conditions for the formation of the dispersed phase, similar to the conditions observed for nephelometry. A significant improvement in turbidimetry is the use of turbidimetric peak turbidity titration using photoelectric colorimeters. Turbidimetry is successfully used for the analytical determination of sulfates, phosphates, chlorides, cyanides, lead, zinc, etc.
The main advantage of nephelometric and turbidimetric methods is their high sensitivity, which is especially valuable in relation to elements or ions for which there are no color reactions. In practice, for example, nephelometric determination of chloride and sulfate in natural waters and similar objects is widely used. In terms of accuracy, turbidimetry and nephelometry are inferior to photometric methods, which is mainly due to the difficulties of obtaining suspensions with the same particle sizes, stability over time, etc. In addition to the usual relatively small errors in photometric determination, errors associated with the insufficient reproducibility of chemical analytical methods are added properties of suspensions.
Nephelometry and turbidimetry are used, for example, to determine SO4 in the form of a suspension of BaSO4, Cl- in the form of a suspension of AgCl, S2- in the form of a suspension of CuS from the bottom. the limits of detectable contents are ~ 0.1 μg/ml. To standardize the conditions of analysis in experiments, it is necessary to strictly control the temperature, the volume of suspension, the concentration of reagents, the stirring speed, and the time of measurements. Precipitation must proceed quickly, and the precipitated particles must be small in size and have low pH. To prevent coagulation of large particles, a stabilizer is often added to the solution, for example. gelatin, glycerin.
38. Chromatography: history of origin, principle of the method, application in court. Research.
Chromatography is a dynamic sorption method for separating and analyzing mixtures of substances, as well as studying the physicochemical properties of substances. It is based on the distribution of substances between two phases - stationary (solid phase or liquid bound on an inert carrier) and mobile (gas or liquid phase, eluent). The name of the method is associated with the first experiments in chromatography, during which the developer of the method, Mikhail Tsvet, separated brightly colored plant pigments.
The chromatography method was first used by the Russian botanist Mikhail Semenovich Tsvet in 1900. He used a column filled with calcium carbonate to separate plant pigments. The first report on the development of the chromatography method was made by Tsvet on December 30, 1901 at XI Congress of Naturalists and Doctors in St. Petersburg. The first printed work on chromatography was published in 1903, in the journal Proceedings of the Warsaw Society of Naturalists. First time term chromatography appeared in two printed works by Color in 1906, published in a German magazine Berichte der Deutschen Botanischen Gesellschaft. In 1907, Tsvet demonstrates his method German Botanical Society.
In 1910-1930, the method was undeservedly forgotten and practically did not develop.
In 1931, R. Kuhn, A. Winterstein and E. Lederer, using chromatography, isolated α and β fractions in crystalline form from crude carotene, thereby demonstrating the preparative value of the method.
In 1941, A. J. P. Martin and R. L. M. Singh developed a new type of chromatography, which was based on the difference in the distribution coefficients of the separated substances between two immiscible liquids. The method was called " partition chromatography».
In 1947, T. B. Gapon, E. N. Gapon and F. M. Shemyakin developed the method of “ion exchange chromatography”.
In 1952, J. Martin and R. Singh were awarded the Nobel Prize in Chemistry for the creation of the method of partition chromatography.
From the mid-20th century to the present day, chromatography has developed intensively and has become one of the most widely used analytical methods.
Classification: Gas, Liquid
Fundamentals of chromatography process. To carry out chromatographic separation of substances or determination of their physical-chemical. characteristics are usually used special. devices - chromatographs. Basic chromatograph units - chromatographic. column, detector, and sample injection device. The column containing the sorbent performs the function of separating the analyzed mixture into its constituent components, and the detector performs the function of separating their quantities. definitions. The detector located at the outlet of the column automatically continuously determines the concentration of the separated compounds. in the flow of the mobile phase After introducing the analyzed mixture with the flow of the mobile phase into the column, the zones of all substances are located at the beginning of the chromatographic. columns (Fig. 1). Under the influence of the flow of the mobile phase, the components of the mixture begin to move along the column with decomposition. speeds, the values of which are inversely proportional to the distribution coefficients K of the chromatographed components. Well-sorbed substances, the distribution constant values for which are large, move along the sorbent layer along the column more slowly than poorly sorbed substances. Therefore, component A leaves the column the fastest, then component B, and the last one to leave the column is component C (K A<К Б <К В). Сигнал детектора, величина к-рого пропорциональна концентрации определяемого в-ва в потоке элюента, автоматически непрерывно записывается и регистрируется (напр., на диаграммной ленте). Полученная хроматограмма отражает расположение хроматографич. зон на слое сорбента или в потоке подвижной фазы во времени.
Rice. 1. Separation of a mixture of three components (A, B and C) on a chromatographic column K with detector D: a - position of the chromatographic zones of the separated components in the column at certain time intervals; b - chromatogram (C - signal, t - time) .
With flat layer chromatography When separating, a sheet of paper or a plate with a layer of sorbent with applied samples of the substance under study is placed in a chromatography. camera. After separation, the components are determined by any suitable method.
39. Classification of chromatographic methods.
Chromatography is a method of separation and analysis of substances based on the distribution of the analyte. The difference between 2 phases: mobile and stationary
A solution of a mixture of substances to be separated is passed through a glass tube (Adsorption column) filled with an adsorbent. As a result, the components of the mixture are retained at different heights of the adsorbent column in the form of separate zones (layers). The stuff is better than adsorbir. They are at the top of the column, and worse adsorbed at the bottom of the column. Substances that cannot be adsorbed pass through the column without stopping and are collected in the filter.
Classifications:
1. According to the state of aggregation of the phases.
1) Movable
A) gas (inert gases: helium, argon, azone)
B) liquid
2. according to the method of implementation
1) on a plane (planar); thin-layer paper
2) columnar
A) packed (packed column filled with sorbent)
B) capillary (thin glass/quartz capillary on the inner surface of which a stationary phase is applied)
You can def. Items in small quantities.
The volatile substances are separated.
40. Chromatogram. Basic parameters of the chromatograph peak.
A chromatogram is the result of recording the dependence of the concentration of components at the outlet of the column on time.
H S
Each peak in the chromatogram is characterized by two main parameters
1. Retention time ( t R) is the time from the moment the analyzed sample is introduced until the maximum of the chromatographic peak is recorded. It depends on the nature of the substance and is a qualitative characteristic.
2. Height ( h) or area ( S) peak
S = ½ ω × h. (4)
The height and area of the peak depend on the amount of substance and are quantitative characteristics.
The retention time consists of two components - the residence time of substances in the mobile phase ( t m) and residence time in the stationary phase ( t s):
Identification of peaks of unknown components of the analyzed mixture is carried out by comparison (comparison). values determined directly from the chromatogram, with corresponding tabulated data for known compounds. When identifying in chromatography, only negative is reliable. answer; for example, peak i is not item A if the retention times of peak i and item A do not coincide. The coincidence of the retention times of peak i and item A is a necessary but not sufficient condition for concluding that peak i is item A.
In practical work, the choice of one or another parameter for the quantitative decoding of chromatograms is determined by the combined influence of several factors: the speed and ease of calculation, the shape (wide, narrow) and degree of asymmetry of the chromatographic peak, the efficiency of the column used, the completeness of separation of the components of the mixture, the presence of the necessary automated devices (integrators, computer systems for processing chromatographic data).
The determined parameter of the chromatographic peak is measured manually by the operator on the chromatogram at the end of the cycle of separation of the components of the analyzed mixture
The determined parameter of the chromatographic peak is measured automatically using digital voltmeters, integrators or specialized computers simultaneously with the separation of the components of the analyzed mixture in the column and recording of the chromatogram
Since the technique of decoding chromatograms comes down to measuring the parameters of the chromatographic peaks of the compound of interest and the standard compound, the chromatography conditions should ensure their complete separation as possible; all other components of the original sample under the accepted analysis conditions may not be separated from each other or even not appear at all on the chromatogram (this is advantage of the internal standard method over the internal normalization method)
41.Qualitative chromatographic analysis.
With a sufficient column length, complete separation of the components of any mixture can be achieved. And after eluting the separated components into separate fractions (eluates), determine the number of components of the mixture (it corresponds to the number of eluates), establish their qualitative composition, determine the amount of each of them, using appropriate methods of quantitative analysis.
Qualitative chromatographic analysis, i.e. identification of a substance according to its chromatogram can be performed by comparing chromatographic characteristics, most often the retained volume (i.e., the volume of the mobile phase passed through the column from the beginning of the mixture input until the appearance of this component at the outlet of the column), found under certain conditions for the components of the analyzed substance mixtures and for the standard.
42.Quantitative chromatograph analysis.
Quantitative chromatographic analysis is usually carried out on a chromatograph. The method is based on measuring various parameters of the chromatographic peak, depending on the concentration of the chromatographed substances - height, width, area and retained volume or the product of retained volume and peak height.
In quantitative gas chromatography, the methods of absolute calibration and internal normalization, or normalization, are used. The internal standard method is also used. With absolute calibration, the dependence of the peak height or area on the concentration of the substance is experimentally determined and calibration graphs are constructed or the corresponding coefficients are calculated. Next, the same characteristics of the peaks in the analyzed mixture are determined, and the concentration of the analyte is found from the calibration graph. This simple and accurate method is the main one for determining trace impurities.
When using the internal normalization method, the sum of any peak parameters, for example, the sum of the heights of all peaks or the sum of their areas, is taken as 100%. Then the ratio of the height of an individual peak to the sum of the heights or the ratio of the area of one peak to the sum of the areas when multiplied by 100 will characterize the mass fraction (%) of the component in the mixture. With this approach, it is necessary that the dependence of the value of the measured parameter on concentration is the same for all components of the mixture.
43.Planar chromatography. Use of thin layer chromatography for ink analysis.
The first form of use of cellulose in thin layer chromatography was paper chromatography. Available TLC and high-throughput TLC plates allow the separation of mixtures of polar substances, using at least ternary mixtures of water, an immiscible organic solvent, and a water-soluble solvent that promotes the formation of one phase as eluents)