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Physico-chemical methods of analysis: practical application. Quantitative Analysis

Lecture 9. Basics of quantitative analysis.

1. Classification of methods of chemical analysis.

2. Types of gravimetric determinations.

3. General characteristics of the gravimetric method of analysis.

4. Volumetric titrimetric method of analysis.

5. Calculations in titrimetric analysis.

6. Methods of titrimetric analysis.

D.Z. according to school Pustovalova pp. 181-218.

Classification of methods of chemical analysis.

Number And honest A liz – Col.a. - a set of chemical, physicochemical and physical methods for determining the quantitative ratio of the components that make up the substance being analyzed.

Quantitative analysis methods:

1) chemical (gravimetry, titrimetry, gas analysis);

2) physicochemical method (photometry, electrochemical, chromatographic analysis);

3) physical-spectral: luminescent, etc.

Along with the qualitative analysis of Col. A. is one of the main branches of analytical chemistry. Based on the amount of substance taken for analysis, macro-, semi-micro, micro- and ultra-micro methods of analysis are distinguished. In macromethods, the sample weight is usually >100 mg, solution volume > 10 ml; in ultramicromethods - 1-10 -1, respectively mg and 10 -3 -10 -6 ml(see also Microchemical analysis, Ultramicrochemical analysis) . Depending on the object of study, a distinction is made between inorganic and organic CA, which in turn is divided into elemental, functional, and molecular analysis. Elemental analysis allows you to determine the content of elements (ions), functional analysis - the content of functional (reactive) atoms and groups in the analyzed object. Molecular K. a. involves the analysis of individual chemical compounds characterized by a certain molecular weight. The so-called phase analysis is a set of methods for separating and analyzing individual structural (phase) components of heterogeneous systems. In addition to specificity and sensitivity (see Qualitative analysis), an important characteristic of K. a. methods. - accuracy, that is, the value of the relative error of determination; accuracy and sensitivity in CA. expressed as a percentage.

To the classical chemical methods of CA. include: gravimetric analysis, based on precise measurement of the mass of the substance being determined, and volumetric analysis. The latter includes titrimetric volumetric analysis - methods for measuring the volume of a reagent solution consumed in the reaction with the analyte, and gas volumetric analysis - methods for measuring the volume of analyzed gaseous products (see Titrimetric analysis, Gas analysis) .

Along with classical chemical methods, physical and physicochemical (instrumental) methods of CA are widely used, based on the measurement of optical, electrical, adsorption, catalytic and other characteristics of the analyzed substances, depending on their quantity (concentration). Typically, these methods are divided into the following groups: electrochemical (conductometry, polarography, potentiometry, etc.); spectral or optical (emission and absorption spectral analysis, photometry, colorimetry, nephelometry, luminescent analysis, etc.); X-ray (absorption and emission X-ray spectral analysis, X-ray phase analysis, etc.); chromatographic (liquid, gas, gas-liquid chromatography, etc.); radiometric (activation analysis, etc.); mass spectrometric. The listed methods, while inferior to chemical ones in accuracy, are significantly superior to them in sensitivity, selectivity, and speed of execution. Accuracy of chemical methods of CA. is usually in the range of 0.005-0.1%; errors in determination by instrumental methods are 5-10%, and sometimes significantly more. Sensitivity of some methods K. a. is given below (%):

Volume................................................. ......10 -1

Gravimetric........................................ 10 -2

Emission spectral........................10 -4

Absorption X-ray spectral...... 10 -4

Mass spectrometric........................10 -4

Coulometric......................................... 10 -5

Luminescent........................................ 10 -6 -10 -5

Photometric colorimetric......... 10 -7 -10 -4

Polarographic........................................10 -8 -10 -6

Activation................................................10 -9 -10 -8

When using physical and physicochemical methods, K. a. As a rule, microquantities of substances are required. The analysis can in some cases be performed without destroying the sample; Sometimes continuous and automatic recording of results is also possible. These methods are used to analyze high-purity substances, evaluate product yields, study the properties and structure of substances, etc. See also Electrochemical methods of analysis, Spectral analysis, Chromatography, Kinetic methods of analysis, Nephelometry, Colorimetry, Activation analysis.

1) chemical methods of analysis:

Gravimetric– is based on determining the mass of a substance isolated in pure form or in the form of a compound of known composition.

positive side “+” - gives the result of high strength,

the negative side “-” is very labor-intensive work.

Titrimetric -(volumetric) - based on an accurate measurement of the reagent spent on the reaction with a certain component. The reagent is taken in the form of a solution of a certain concentration (titrated solution).

High speed of analysis;

Less accurate result compared to gravimetry.

Depending on the type of reactions occurring during the titration process, the following are distinguished: methods:

Acid-base titration methods,

Reductive titration method,

Deposition method

Complexation.

2) Physico-chemical method- based on the measurement of absorption, transmission, and scattering of light by the solution being determined.

Most photometric methods use assessment of the color intensity of the solution visually or using appropriate instruments.

It is used for a specific component that is part of the analyte in very small quantities;

The accuracy of the method is lower than in gravimetry and titrimetry.

Electrochemical methods- electrogravimetric analysis, conductometry, potentiometry and polarography.

Chromatographic method- based on the use of the phenomenon of selective adsorption of a solution of a substance and ions by various substances or adsorbents: Al 2 O 3, silica gel, starch, talc,

permutide, synthetic resins and other substances.

Application: in both quantitative analysis and qualitative analysis, especially widely used for the determination of substances and ions.

The vast majority of information about substances, their properties and chemical transformations was obtained through chemical or physicochemical experiments. Therefore, the main method used by chemists should be considered a chemical experiment.

The traditions of experimental chemistry have evolved over centuries. Even when chemistry was not an exact science, in ancient times and in the Middle Ages, scientists and artisans, sometimes by accident, and sometimes purposefully, discovered methods for obtaining and purifying many substances that were used in economic activities: metals, acids, alkalis, dyes and etc. Alchemists contributed greatly to the accumulation of such information (see Alchemy).

Thanks to this, by the beginning of the 19th century. chemists were well versed in the basics of experimental art, especially methods for purifying all kinds of liquids and solids, which allowed them to make many important discoveries. And yet, chemistry began to become a science in the modern sense of the word, an exact science, only in the 19th century, when the law of multiple ratios was discovered and atomic-molecular science was developed. Since that time, chemical experiment began to include not only the study of the transformations of substances and methods of their isolation, but also the measurement of various quantitative characteristics.

A modern chemical experiment involves many different measurements. Both the equipment for conducting experiments and chemical glassware have changed. In a modern laboratory you will not find homemade retorts - they have been replaced by standard glass equipment produced by industry and adapted specifically for performing a particular chemical procedure. Working methods have also become standard, which in our time no longer has to be reinvented by every chemist. A description of the best of them, proven by many years of experience, can be found in textbooks and manuals.

Methods for studying matter have become not only more universal, but also much more diverse. An increasingly important role in the work of a chemist is played by physical and physicochemical research methods designed to isolate and purify compounds, as well as to establish their composition and structure.

The classical technique of purifying substances was extremely labor intensive. There are cases where chemists spent years of work isolating an individual compound from a mixture. Thus, salts of rare earth elements could be isolated in pure form only after thousands of fractional crystallizations. But even after this, the purity of the substance could not always be guaranteed.

The perfection of technology has reached such a high level that it has become possible to accurately determine the rate of even “instantaneous”, as previously believed, reactions, for example, the formation of water molecules from hydrogen cations H + and anions OH –. With an initial concentration of both ions equal to 1 mol/l, the time of this reaction is several hundred billionths of a second.

Physicochemical research methods are specially adapted for the detection of short-lived intermediate particles formed during chemical reactions. To do this, the devices are equipped with either high-speed recording devices or attachments that ensure operation at very low temperatures. These methods successfully record the spectra of particles whose lifespan under normal conditions is measured in thousandths of a second, for example, free radicals.

In addition to experimental methods, calculations are widely used in modern chemistry. Thus, thermodynamic calculation of a reacting mixture of substances makes it possible to accurately predict its equilibrium composition (see.

The study of substances is a rather complex and interesting matter. After all, they are almost never found in nature in their pure form. Most often, these are mixtures of complex composition, in which the separation of components requires certain efforts, skills and equipment.

After separation, it is equally important to correctly determine whether a substance belongs to a particular class, that is, to identify it. Determine boiling and melting points, calculate molecular weight, test for radioactivity, and so on, in general, research. For this purpose, various methods are used, including physicochemical methods of analysis. They are quite diverse and usually require the use of special equipment. They will be discussed further.

Physico-chemical methods of analysis: general concept

What are these methods for identifying compounds? These are methods that are based on the direct dependence of all physical properties of a substance on its structural chemical composition. Since these indicators are strictly individual for each compound, physicochemical research methods are extremely effective and give 100% results in determining the composition and other indicators.

Thus, the following properties of a substance can be taken as a basis:

light absorption ability;
thermal conductivity;
electrical conductivity;
boiling temperature;
melting and other parameters.

Physicochemical research methods have a significant difference from purely chemical methods of identifying substances. As a result of their work, a reaction does not occur, that is, the transformation of a substance, either reversible or irreversible. As a rule, the compounds remain intact both in mass and composition.

Features of these research methods

There are several main features characteristic of such methods for determining substances.

The research sample does not need to be cleaned of impurities before the procedure, since the equipment does not require this.
Physicochemical methods of analysis have a high degree of sensitivity, as well as increased selectivity. Therefore, a very small amount of the test sample is required for analysis, which makes these methods very convenient and effective. Even if it is necessary to determine an element that is contained in the total wet mass in negligible quantities, this is not an obstacle for the indicated methods.
The analysis takes only a few minutes, so another feature is its short duration, or expressiveness.
The research methods under consideration do not require the use of expensive indicators.

Obviously, the advantages and features are enough to make physicochemical research methods universal and in demand in almost all studies, regardless of the field of activity.

Classification

Several characteristics can be identified on the basis of which the methods under consideration are classified. However, we will present the most general system that unites and covers all the main methods of research related directly to physicochemical ones.

1. Electrochemical research methods. Based on the measured parameter, they are divided into:

potentiometry;
voltammetry;
polarography;
oscillometry;
conductometry;
electrogravimetry;
coulometry;
amperometry;
dielcometry;
high-frequency conductometry.

2. Spectral. Include:

optical;
X-ray photoelectron spectroscopy;
electromagnetic and nuclear magnetic resonance.

3. Thermal. Divided into:

thermal;
thermogravimetry;
calorimetry;
enthalpimetry;
delatometry.

4. Chromatographic methods, which are:

gas;
sedimentary;
gel penetrating;
exchange;
liquid.

It is also possible to divide physicochemical methods of analysis into two large groups. The first are those that result in destruction, that is, complete or partial destruction of a substance or element. The second is non-destructive, preserving the integrity of the test sample.

Practical application of such methods

The areas of use of the methods of work under consideration are quite diverse, but all of them, of course, relate to science or technology in one way or another. In general, we can give several basic examples, from which it will become clear why exactly such methods are needed.

Control over the flow of complex technological processes in production. In these cases, equipment is necessary for contactless control and tracking of all structural links in the work chain. These same instruments will record problems and malfunctions and provide an accurate quantitative and qualitative report on corrective and preventive measures.
Carrying out chemical practical work for the purpose of qualitative and quantitative determination of the yield of the reaction product.
Examination of a sample of a substance to determine its exact elemental composition.
Determination of the quantity and quality of impurities in the total mass of the sample.
Accurate analysis of intermediate, main and secondary participants in the reaction.
A detailed report on the structure of a substance and the properties it exhibits.
Discovery of new elements and obtaining data characterizing their properties.
Practical confirmation of theoretical data obtained empirically.
Analytical work with high-purity substances used in various fields of technology.
Titration of solutions without the use of indicators, which gives a more accurate result and has completely simple control, thanks to the operation of the device. That is, the influence of the human factor is reduced to zero.
Basic physicochemical methods of analysis make it possible to study the composition of:

minerals;
mineral;
silicates;
meteorites and foreign bodies;
metals and non-metals;
alloys;
organic and inorganic substances;
single crystals;
rare and trace elements.

Areas of use of methods

nuclear power;
physics;
chemistry;
radio electronics;
laser technology;
space research and others.

The classification of physicochemical methods of analysis only confirms how comprehensive, accurate and universal they are for use in research.

Electrochemical methods

The basis of these methods is reactions in aqueous solutions and on electrodes under the influence of electric current, that is, in simple terms, electrolysis. Accordingly, the type of energy that is used in these analysis methods is the flow of electrons.

These methods have their own classification of physicochemical methods of analysis. This group includes the following species.

Electrical gravimetric analysis. Based on the results of electrolysis, a mass of substances is removed from the electrodes, which is then weighed and analyzed. This is how data on the mass of compounds is obtained. One of the varieties of such work is the method of internal electrolysis.
Polarography. It is based on measuring current strength. It is this indicator that will be directly proportional to the concentration of the desired ions in the solution. Amperometric titration of solutions is a variation of the considered polarographic method.
Coulometry is based on Faraday's law. The amount of electricity spent on the process is measured, from which they then proceed to calculate the ions in the solution.
Potentiometry - based on measuring the electrode potentials of the participants in the process.

All the processes considered are physical and chemical methods for the quantitative analysis of substances. Using electrochemical research methods, mixtures are separated into their component components and the amount of copper, lead, nickel and other metals is determined.

Spectral

It is based on the processes of electromagnetic radiation. There is also a classification of the methods used.

Flame photometry. To do this, the test substance is sprayed into an open flame. Many metal cations give a certain color, so their identification is possible in this way. These are mainly substances such as: alkali and alkaline earth metals, copper, gallium, thallium, indium, manganese, lead and even phosphorus.
Absorption spectroscopy. Includes two types: spectrophotometry and colorimetry. The basis is the determination of the spectrum absorbed by the substance. It acts in both the visible and hot (infrared) parts of radiation.
Turbidimetry.
Nephelometry.
Luminescent analysis.
Refractometry and polarometry.

Obviously, all the methods considered in this group are methods for qualitative analysis of a substance.

Emission analysis

This causes the emission or absorption of electromagnetic waves. Based on this indicator, one can judge the qualitative composition of the substance, that is, which specific elements are included in the composition of the research sample.

Chromatographic

Physicochemical studies are often carried out in different environments. In this case, chromatographic methods become very convenient and effective. They are divided into the following types.

Adsorption liquid. It is based on the different adsorption abilities of the components.
Gas chromatography. Also based on adsorption capacity, only for gases and substances in vapor state. It is used in mass production of compounds in similar aggregate states, when the product comes out in a mixture that must be separated.
Partition chromatography.
Redox.
Ion exchange.
Paper.
Thin layer.
Sedimentary.
Adsorption-complexation.

Thermal

Physicochemical research also involves the use of methods based on the heat of formation or decomposition of substances. Such methods also have their own classification.

Thermal analysis.
Thermogravimetry.
Calorimetry.
Enthalpometry.
Dilatometry.

All these methods make it possible to determine the amount of heat, mechanical properties, and enthalpy of substances. Based on these indicators, the composition of the compounds is quantitatively determined.

Methods of analytical chemistry

This section of chemistry has its own characteristics, because the main task facing analysts is the qualitative determination of the composition of a substance, their identification and quantitative accounting. In this regard, analytical methods of analysis are divided into:

chemical;
biological;
physico-chemical.

Since we are interested in the latter, we will consider which of them are used to determine substances.

The main types of physicochemical methods in analytical chemistry

Spectroscopic - all the same as those discussed above.
Mass spectral - based on the action of electric and magnetic fields on free radicals, particles or ions. Physicochemical analysis laboratory assistants provide the combined effect of the designated force fields, and the particles are separated into separate ion flows based on the ratio of charge and mass.
Radioactive methods.
Electrochemical.
Biochemical.
Thermal.

What can we learn about substances and molecules from such processing methods? Firstly, the isotopic composition. And also: reaction products, the content of certain particles in especially pure substances, the masses of the sought compounds and other things useful for scientists.

Thus, methods of analytical chemistry are important ways of obtaining information about ions, particles, compounds, substances and their analysis.

Analysis method name the principles underlying the analysis of matter, that is, the type and nature of the energy that causes disturbance of the chemical particles of the substance.

The analysis is based on the relationship between the detected analytical signal and the presence or concentration of the analyte.

Analytical signal is a fixed and measurable property of an object.

In analytical chemistry, analytical methods are classified according to the nature of the property being determined and the method of recording the analytical signal:

1.chemical

2.physical

3.physical and chemical

Physicochemical methods are called instrumental or measuring methods, since they require the use of instruments and measuring instruments.

Let's consider the complete classification of chemical methods of analysis.

Chemical methods of analysis- are based on measuring the energy of a chemical reaction.

During the reaction, parameters associated with the consumption of starting materials or the formation of reaction products change. These changes can either be observed directly (precipitate, gas, color) or measured by quantities such as reagent consumption, mass of product formed, reaction time, etc.

By goals chemical analysis methods are divided into two groups:

I.Qualitative analysis– consists of detecting the individual elements (or ions) that make up the analyte.

Qualitative analysis methods are classified:

1. cation analysis

2. Anion analysis

3. analysis of complex mixtures.

II.Quantitative analysis– consists in determining the quantitative content of individual components of a complex substance.

Quantitative chemical methods classify:

1. Gravimetric(weight) method of analysis is based on isolating the analyte in its pure form and weighing it.

Gravimetric methods are divided according to the method of obtaining the reaction product:

a) chemogravimetric methods are based on measuring the mass of the product of a chemical reaction;

b) electrogravimetric methods are based on measuring the mass of the product of an electrochemical reaction;

c) thermogravimetric methods are based on measuring the mass of a substance formed during thermal exposure.

2. Volumetric analysis methods are based on measuring the volume of the reagent spent on interaction with the substance.

Volumetric methods, depending on the state of aggregation of the reagent, are divided into:

a) gas-volumetric methods, which are based on selective absorption of the determined component of the gas mixture and measurement of the volume of the mixture before and after absorption;

b) liquid-volumetric (titrimetric or volumetric) methods are based on measuring the volume of liquid reagent consumed for interaction with the substance being determined.

Depending on the type of chemical reaction, volumetric analysis methods are distinguished:

· protolitometry – a method based on the occurrence of a neutralization reaction;

· redoxometry – a method based on the occurrence of redox reactions;

· complexometry – a method based on the occurrence of a complexation reaction;

· precipitation methods – methods based on the occurrence of precipitation formation reactions.

3. Kinetic analytical methods are based on determining the dependence of the rate of a chemical reaction on the concentration of reactants.

Lecture No. 2. Stages of the analytical process

The solution to the analytical problem is carried out by performing an analysis of the substance. According to IUPAC terminology analysis [‡] called the procedure for obtaining experimentally data on the chemical composition of a substance.

Regardless of the chosen method, each analysis consists of the following stages:

1) sampling (sampling);

2) sample preparation (sample preparation);

3) measurement (definition);

4) processing and evaluation of measurement results.

Fig1. Schematic representation of the analytical process.

Sample selection

Chemical analysis begins with the selection and preparation of a sample for analysis. It should be noted that all stages of analysis are interconnected. Thus, a carefully measured analytical signal does not provide correct information about the content of the component being determined if the sample is selected or prepared for analysis incorrectly. Sampling error often determines the overall accuracy of component determination and makes the use of highly accurate methods pointless. In turn, sample selection and preparation depend not only on the nature of the analyzed object, but also on the method of measuring the analytical signal. The methods and procedure for sampling and its preparation are so important when conducting chemical analysis that they are usually prescribed by the State Standard (GOST).

Let's consider the basic rules for sampling:

· The result can only be correct if the sample is sufficiently representative, that is, it accurately reflects the composition of the material from which it was selected. The more material selected for the sample, the more representative it is. However, very large samples are difficult to handle and increase analysis time and costs. Thus, the sample must be taken so that it is representative and not very large.

· The optimal sample mass is determined by the heterogeneity of the analyzed object, the size of the particles from which the heterogeneity begins, and the requirements for the accuracy of the analysis.

· To ensure the representativeness of the sample, batch homogeneity must be ensured. If it is not possible to form a homogeneous batch, then the batch should be separated into homogeneous parts.

· When taking samples, the aggregate state of the object is taken into account.

· The condition for the uniformity of sampling methods must be met: random sampling, periodic, chess, multi-stage sampling, “blind” sampling, systematic sampling.

· One of the factors that must be taken into account when choosing a sampling method is the possibility of changes in the composition of the object and the content of the component being determined over time. For example, the variable composition of water in the river, changes in the concentration of components in food products, etc.

Chemical methods of analysis

(a. chemical methods of analysis; n. chemische Analyseverfahren; f. procedes chimiques de l'analyse; And. metodos quimicos de analisis) - a set of methods of qualities. and quantities. analysis of substances, basic on the use of chemicals reactions.
Quality X. m. a. ( cm. Qualitative analysis) include the use of detection reactions characteristic of inorganic. ions in solutions and for organic functional groups. connections. These reactions are usually accompanied by a change in the color of the solution, the formation of precipitation, or the release of gaseous products. Depending on the amount of the analyzed substance, macroanalysis (1-0.1 g), semi-microanalysis (0.1-0.01 g), microanalysis (0.01-0.001 g) and ultramicrochemical are distinguished. (0.0001 g) analysis ( cm. Microchemical analysis).
K quantitative X. m. a. ( cm. Quantitative analysis) usually includes “classical” methods: gravimetry ( cm. Gravimetric analysis), titrimetry ( cm. Titrimetric analysis) with visual indication of the titration end point, and gas volumetric analysis. Gas volumetric analysis (gas volumetric analysis) is based on the selective absorption of the components of a gas mixture in vessels filled with one or another absorber, followed by measurement of the decrease in gas volume using a burette. Thus, carbon dioxide is absorbed with a solution of potassium hydroxide - a solution of pyrogallol, carbon monoxide - with an ammonia solution of copper chloride. Gas volumemetry refers to rapid methods of analysis. It is widely used for the determination of carbonates in minerals and minerals.
X. m. a. widely used for the analysis of ores, minerals, minerals and other materials when determining components in them with contents from tenths to several. tens of percent. X. m.a. are characterized by high accuracy (the analysis error is usually tenths of a percent). However, these methods are gradually being replaced by more express physical-chemical methods. and physical ( cm. Physical methods of analysis) methods of analysis. Literature: Kreshkov A.P., Fundamentals of Analytical Chemistry, 3rd ed., vol. 2, M., 1970; Zolotov Yu. A., Essays on Analytical Chemistry, M., 1977. H. B. Trofimov.

Mountain encyclopedia. - M.: Soviet Encyclopedia. Edited by E. A. Kozlovsky. 1984-1991 .

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