EntranceThe essence of redox analysis methods. Redox titration methods
Titrimetric analysis. Basic concepts (aliquot, titrant, equivalence point, indicator, titration curve). Requirements for reactions in titrimetry. Reagents used in titrimetry. Standard substances, titrants.
A method of quantitative analysis based on measuring the volume of a solution with a precisely known concentration of the reagent required to react with a given amount of the analyte. Aliquot-an accurately measured multiple of the sample (volume of solution) taken for analysis, which retains the properties of the main sample. Titrant or working solution is the solution with which the titration is carried out. Equivalence point point in titration when the amount of titrant added is chemically equivalent to the amount of the substance being titrated. TE can also be called the stoichiometric point, the theoretical end point. Indicator- a substance that changes its color in TE is characterized by low concentration and transition interval. Titration curve-shows a graphical dependence of the logarithm of the concentration of a participant in the reaction occurring during titration, or some type of solution, on the volume of the added titrant (or on the degree of titration). For example, for an acid-base reaction, titer curves. The pH-volume of the titrant is plotted in coordinates.
Requirements for reactions in titrimetry: 1. The interaction of the titrant with the analyte must occur in strict accordance with the stoichiometric reaction equation, and the titrant must be spent only on the reaction with the analyte. At the same time, the analyte must react only with the titrant and not interact, for example, with atmospheric oxygen, as can, in principle, be the case when titrating reducing agents.
2. The titration reaction must proceed quantitatively, that is, the equilibrium constant of the titration reaction must be sufficiently large.
3. The interaction of the analyte with the titrant must occur at high speed.
4. There must be a way to determine the end of the titration.
5. The titrant solution must be standardized.
Reagents: Based on the properties of substances and the method of their preparation, titrants are of two types: standard, with a prepared titer, standardized or with a set titer. Standard solutions or prepared titres are called primary standard solutions. It is prepared by dissolving a precise amount of pure chemical substance in a certain volume of solvent. Primary standard substances include: Na2CO3, Na2B4O7*10H2O, Na2SO4, CaCO3, CaCI2, MgSO4, MgCI2, H2C2O4*2H2O, Na2C2O4, K2Cr2O7, sodium bicarbonate, potassium bromate, potassium iodate and others.
The first type of titrants (with a prepared titer) are used in titrimetry for quantitative determinations of certain substances and for establishing titers of the second type - secondary standard solutions.
A secondary standard solution is a solution of such substances, the concentration of which is established (standardized) by the concentration of the primary standard solutions or prepared by the known mass of the secondary standard substance.
The second type of titrants includes solutions of substances that do not meet the requirements for primary standard substances. These include: alkalis, acid solutions HCI, H2SO4, HNO3, CH3COOH, KMnO4, AgNO3, Na2S2O3 and others.
Typical calculations in titrimetry. Methods of expressing concentrations in titrimetry (molar concentration, molar concentration equivalent, titer, correction factor. Calculation of the mass of a standard sample for the preparation of titrant, calculation of titrant concentration
Molar concentration c(A) is the amount of dissolved substance A in moles contained in one liter of solution: mol/l. c(A) = n(A)/V(A) = m(A)/M/(A)V(A), where n(A)- amount of dissolved substance A, mol; V(A)- volume of solution, l; t(A)- mass of dissolved substance A, g; M/(A) - molar mass of solute A, g/mol. Molar concentration of equivalent c(1/zA),, - the amount of dissolved substance A in moles, corresponding to the equivalent of A contained in one liter of solution: mol/l c(1/z A) = n(1/z A)/V(A)= t(A)/M(1/z A)V(A), where 1/z is the equivalence factor; calculated for each substance based on the stoichiometry of the reaction; n(1/zA)- amount of substance equivalent to A in solution, mol; M(1/zA)- molar mass of equivalent solute A, g/mol. Titre T(A) dissolved substance A is the mass of dissolved substance A contained in one ml of solution: measured in ml T(A)= m(A)/V(A) = с(1/z А)М(1/z A)/1000. Titer of the solution for the analyte X, or titrimetric conversion factor t(T/X), is the mass of the titrated substance X interacting with one ml of titrant T: t(T/X) = T(T)M(1/zX) /M(1/zT) = c(1/zT) M(1/zX)/1000. Measured in g/ml. Correction factor F (or K)- a number expressing the ratio of the actual (practical) concentration c(1/zA) pr of substance A in solution to its given (theoretical) concentration c(1/z A) theor: F = c(1/zA) pr /c(1/zA) theor. Calculation of the mass of a sample of a standard substance. Hitch weight t(A) standard substance A, necessary to obtain a solution with a given molar concentration of the equivalent с(1/zА), calculated using the formula: m(A) = с(1/z А)М(1/z A)VA), where M(1/z A) is the molar mass of the equivalent of substance A. If the molar concentration c(A) is specified, then the mass of the sample is calculated similarly using the formula: t(A) = c(A)M(A)V(A), Where M/(A) is the molar mass of substance A. The mass of the sample is usually weighed on an analytical balance with a weighing error of ±0.0002 g. The concentration of titrant T when standardized against a standard solution of substance A is calculated as follows. Let the reaction T + A = B occur during standardization. According to the law of equivalents, equivalent quantities substances T, A And B are equal to n (1/z T) = n (1/z A) = n (1/z V), the equivalent amount of a substance is equal to the product of the molar concentration of the equivalent of this substance by the volume of its solution: c(1/z T)= c(1/z A)V(A)/V(T) = c( 1/z IN) V(B)/V(T).
Classification of titrimetric analysis methods - acid-base, oxidation-reduction, precipitation, complexometric. Types of titration (direct, reverse, indirect). Methods for establishing the titration point.
1) Acid-base titration (neutralization method)- tit
ation based on the proton transfer reaction from one reacting
particles to another in solution. There are acidimetry and alkalimetry.
Acidimetry (acidimetric titration)- determination of substances by titration with a standard acid solution.
Alkalimetry (alkalimetric titration)- determination of substances by titration with a standard solution of a strong base.
2) Redox titration (redoxmetry)-
titration accompanied by the transition of one or more
electrons from a donor ion or molecule (reducing agent) to an acceptor (oxidizing agent).
3) Precipitation titration- a titration in which the titrated substance, upon interaction with the titrant, is released from the solution in the form of a precipitate
4) Compleximetric titration- titration of a substance with a solution [of a compound that forms a weakly dissociating soluble complex with the titrated substance.
A type of compleximetric titration is complexometric titration (complexometry)- such a titration when the titrated substance, when interacting with a titrant - a solution of complexones - forms metal complexonates.
Direct titration- this is a titration when the analyte is directly titrated with a standard titrant solution or vice versa. Back titration (residue titration)- titration of unreacted substance, which is added in excess to the analyzed solution in the form of a standard solution. Indirect titration (substitution titration)- titration, in which the substance being determined does not react with the titrant directly, but is determined indirectly as a result of the use of a stoichiometric reaction with the formation of another substance that reacts with the titrant. Methods for establishing titration endpoints There are two groups of methods for fixing CTT: visual and instrumental.
Visual methods. The progress of the reaction is monitored visually, observing the change in color (or other property) of the specially introduced indicator | by neutralization, oxidation-reduction, precipitation or complexation. CTT is determined by a sharp change in the visible property of the system in the presence of an indicator or without it: the appearance, change, or disappearance of color, the formation or dissolution of a precipitate. B indicator In visual methods, an indicator is added to the titrated solution. IN non-indicator visual methods use the color of the titrant or titrated substance. CTT is determined by the appearance of the color of the titrant or the disappearance of the color of the titrated substance.
Instrumental methods. CTT is determined by changes in the physicochemical properties of the solution - fluorescence, optical density, potential, electrical conductivity, current, radioactivity, etc. Change physical and chemical properties recorded on various devices.
Acid–base titration. Basic reactions and titrants of the method. Types of acid-base titration (alkalimetry and acidimetry). Indicators, requirements for them. Ionic, chromophore, ion-chromophoric theories of acid-base titration indicators.
ACID-BASE TITRATION - this is a method for determining acids, bases, salts, based on the interaction reaction between proto-lites - acid NA and base B: NA + B = A "+ HB + In aqueous solutions - this is the neutralization reaction of H 3 0 + +0H = 2H 2 0 therefore the acid-base titration method is also called the neutralization method. The titrants of the method are solutions of strong acids and bases: HC1, H 2 S0 4, NaOH, KOH. These substances do not meet the requirements for standard substances, therefore the concentration of titrants is established by standardization their solutions.Borax Na 2 B 4 0 7 10H 2 O, anhydrous sodium carbonate Na 2 C0 3, oxalic acid dihydrate H 2 C 2 0 4 2H 2 0 and some others are most often used as primary standards. Acidimetric titration (acidimetry)- a method for determining strong and weak bases, salts of weak acids, basic salts and other compounds with basic properties by titration with a standard solution of a strong acid. Alkalimetric titration (alkalimetry)- a method for determining strong and weak acids, acid salts, salts of weak bases by titration with a standard solution of a strong base. Indicator- is a substance that exhibits a visible change at or near its equivalence point.
The acid-base indicator is itself an acid or base and during acid-base titration changes its color in TE or
near her. (Methyl orange рТ=4 pH transition interval and indicator color 3.1–4.4 Red – orange-yellow; Phenolphthalein рТ=9.0 8.2–10 Colorless – violet).
Requirements for indicators:1) coloring d.b. intense, different in acidic and alkaline environments 2) color change d.b. clear in a narrow pH range of the solution 3) indicator d.b. sensitive 4) ind-r d.b. stable, does not decompose in air, in solution. Indicator theories:
1) ionic (Ostwald theory) - indicators are weak acids or bases that ionize in aqueous solutions
HInd↔H+ +Ind-. Disadvantages: 1) it only states the differences in color in acidic and alkaline. Wed, but does not explain the nature of the color 2) ion reaction occurs instantly, and the indicator changes color only over time
2) Chromophore - the presence of color is explained by the appearance of chromophore groups. Indices in the solution are present in the form of tautomeric forms. Disadvantages: does not explain why tautomeric transformations occur when the pH changes.
3) ion-chromophoric-acid-base indicators are weak acids and bases, with the neutral indicator molecule and its ionized form containing different chromophoric groups. Indicator molecules in aqueous solution are capable of either donating hydrogen ions (indicator - weak acid) or accepting them (indicator - weak base), while undergoing tautomeric transformations.
REACTION (see notebook topic acid-base titration)
Acid-base titration curves. Calculation, construction and analysis of typical titration curves of a strong acid with an alkali and a strong and weak base with an acid. Selection of indicators based on the titration curve. Titration of polyprotic acids. Errors in acid-base titration, their calculation and elimination.
Acid-base titration curves graphically display the dependence of the change in pH of the titrated solution on the volume of added titrant or on the degree of titration f= V(T)/V, where V(T) and V are, respectively, the volume of added titrant in this moment and in TE. Most often (though not always), when constructing acid-base titration curves, the volume of added titrant or the degree of titration is plotted along the abscissa axis, and the pH value of the titrated solution is plotted along the ordinate axis.
Calculation, construction and analysis of titration curves. To construct an acid-base titration curve, the pH values of the titrated solution are calculated at various points in the titration, i.e. V different points titration: for the initial solution, for solutions before TE, in TE and after TE.
After the start of titration and before TE, the pH value of the solution is determined as pH = -1 8 s(X)
Calculation of pH at the equivalence point. When titrating a strong acid with a strong base, the medium in the fuel cell is neutral, pH = 7.
Calculation of pH after TE. determined by concentration c(T) alkali added in excess of the stoichiometric amount. Considering that pH + pOH = 14, we can write: pH = 14-pOH
The formulas are used to calculate the pH values of the solution at different moments of titration, and based on the calculated data, a titration curve is constructed in pH-V coordinates (T).
Calculated titration curve for 20 ml of 0.1000 mol/l HC1 solution with 0.1000 mol/l NaOH solution
To determine the CTT in this case, you can use acid-base titration indicators such as methyl orange (pT = 4), methyl red (pT = 5.5), bromothymol blue (pT = 7.0), phenolphthalein (pT = 9) and others, for which the pT value lies in the range from 3 to 11. Methyl orange and phenolphthalein are most often used as the most accessible indicators of acid-base titration. Usually, one strives to choose an indicator so that, other things being equal, the pH value of the indicator would be as close as possible to the pH value of the solution in TE, since this reduces the titration error.
Titration of a strong base with a strong acid. When titrating a strong base with a strong acid, for example, a solution of Sodium hydroxide with a solution of hydrochloric acid, processes similar to those discussed in the previous section occur, but only in the opposite Direction: as the titrant is added, the pH value of the solution does not increase, but decreases. For the initial solution of a strong base and the titrated of a solution, the pH value before TE is determined by the concentration of alkali in the solution. In TE, the solution is neutral, pH = 7. After TE, the pH value of the solution is determined by the presence of excess titrant - a strong acid
Titration of polyacid bases. Solutions of polyacid bases are titrated with a solution of a strong acid sequentially, stepwise. At an acceptable level of titration, jumps in the titration curve are separated if differences in the values рК b, successive stages of base dissociation are at least 4 units, as in the case of titration of solutions of polybasic acids with a solution of a strong base.
Errors in main title: 1) measurement error (error of the burette, pipettes) If the solution is taken using a burette, then two measurements of the volume of the solution in the burette are carried out: before and after taking the solution. The random error of each such measurement when using conventional laboratory burettes is approximately ±(0.01-0.02) ml. If the volume of the sampled solution is equal to V, then the maximum random relative error e of measuring the volume taken for titration will be (in percent): έ = ±ν*100%/V, where ν = 0.02 + 0.02 = 0.04 ml. With the volume of the selected solution V= 20 ml, the maximum relative error in measuring the volume of a solution using a burette will be έ= ±0.04 100%/20 =0.2%.
The value of έ can be reduced by increasing the volume V sample solution.
2) methodological errors 3) systematic errors (incorrect indicator selection, discrepancy between the equivalence point and the end point of titration) a) indicator - the difference in the amount of titrant found at the end point of titration and the amount of titrant in t.eq.
a.1.) hydrogen error (X H3O+, XH+) - associated with overtitration of the solution with a strong acid (then the error is +) or undertitration (-) XH3O+ = a/a*100%
a-number of excess equivalents of H+ ions to total number of equivalents
а′=СН3о+ *V
a=СН3о+ * V(а+в)=СН3о+ * (Va+Vb)
C Н3o+=10(in step – рН)
Substitute into our enemy
X n3o+= +-(10 - pT)*(Va+Vb)/Cb*Vb)*100%
b-acid a-alkali.pT-display titre ind
a.2.) hydroxide error (main) - associated with an excess number of OH groups during titration with a strong base, or with under titration of a base solution
a.3.) acid error - caused by the presence of a certain amount of subtitric acid at the end point of the titration (weak acid)
Oxidation-reduction titration. The essence of the method. Classification of redox methods. Conditions for redox titration. Requirements for reactions. Types of redox titration (direct, reverse, substitution). Examples of redox indicators. Formulas, color transition at the equivalence point.
Oxidation-reduction titration(redoximetry, oxidimetry.)
Redox methods include a wide group of titrimetric analysis methods based on the occurrence of redox reactions. Redox titrations use various oxidizing and reducing agents. In this case, it is possible to determine reducing agents by titration with standard solutions of oxidizing agents, and vice versa, determining oxidizing agents with standard solutions of reducing agents. Due to the wide variety of redox reactions, this method makes it possible to determine a large number of different substances, including those that do not directly exhibit redox properties. In the latter case, back titration is used. For example, when determining calcium, its ions precipitate oxalate - an ion
Ca 2+ + C 2 O 4 2- ® CaC 2 O 4 ¯
The excess oxalate is then titrated with potassium permanganate.
Redox titration has a number of other advantages. Redox reactions occur quite quickly, allowing titration to be carried out in just a few minutes. Many of them occur in acidic, neutral and alkaline environments, which significantly expands the possibilities of using this method. In many cases, fixing the equivalence point is possible without the use of indicators, since the titrant solutions used are colored (KMnO 4, K 2 Cr 2 O 7) and at the equivalence point the color of the titrated solution changes from one drop of titrant. The main types of redox titrations are distinguished by the oxidizing agent used in the reaction.
Redox titration (redoximetry), depending on the nature of the reagent, is divided into permanganate, dichromate, cerium, iodo, bromato and iodotometry. They are based on the occurrence of a redox reaction, the essence of which is the transfer of an electron from a reducing agent to an oxidizing agent.
Types of OM titration:
Direct titration is that the solution of the analyte A titrate with standard titrant solution IN. The direct titration method is used to titrate solutions of acids, bases, carbonates, etc.
Back titration used in cases where direct titration is not applicable: for example, due to a very low content of the analyte, the inability to determine the equivalence point, when the reaction proceeds slowly, etc. During back titration to an aliquot of the analyte A pour in a precisely measured volume of a standard solution of the substance IN taken in excess. Unreacted excess substance IN determined by titration with a standard solution of the excipient WITH. Based on the difference in the initial amount of the substance IN and its amount remaining after the reaction, determine the amount of substance IN, which reacted with the substance A, on the basis of which the substance content is calculated A.
Indirect titration or titration by substituent. Based on the fact that it is not the substance being determined that is titrated, but the product of its reaction with the auxiliary substance WITH.
Substance D must be formed strictly quantitatively in relation to the substance A. Having determined the content of the reaction product D titration with a standard solution of the substance IN, Using the reaction equation, the content of the analyte is calculated A.
Redox titration curves, errors, their origin, calculation, elimination. Permanganatometry. The essence of the method, titration conditions, titrant, its preparation, standardization, establishment of the equivalence point. Application of permanganatometry.
Redox titration curves
Redox titration curves show the change in redox potential during the titration process: E = ƒ(V PB), (Fig. 2.7) Redox titration involves two redox systems - the titrated substance and the titrant. The potential of each of them can be calculated using the Nernst equation using the corresponding half-reaction. After adding each portion of titrant, equilibrium is established in the solution and the potential can be calculated using any of these pairs. It is more convenient to calculate the potential for the substance that is in excess in the titrated solution at the moment of titration, i.e. Before the equivalence point, the potential is calculated from the half-reaction involving the titrated substance, and after the equivalence point, from the half-reaction involving the titrant. Before titration begins, it is considered that for the titrated substance the concentrations of the oxidized and reduced forms differ by 1000 or 10,000 times. At the equivalence point, both conjugate forms of the oxidizing agent and the reducing agent are present in equal amounts, so the redox potential can be calculated as the sum of the potentials: 
Transforming the equation, we get:
Where n 1, n 2 – the number of electrons participating in half-reactions of oxidation and reduction, respectively; E 0 1 , E 0 2 – standard redox potential for an oxidizing agent and a reducing agent, respectively.
Rice. Titration curves in the redoximetry method:
1 – the reducing agent is titrated with the oxidizing agent; 2 – the oxidizing agent is titrated with a reducing agent
Near the equivalence point on the titration curve there is a potential jump, the magnitude of which is greater, the greater the difference between E 0 ok and E 0 v-la. Indicator titration is possible if EMF = E 0 ok – E 0 v-la ≥ 0.4 V. If EMF = 0.4 - 0.2 V, you can use instrumental titration, where the equivalence point is fixed using instruments. If EMF< 0,2 IN direct redoximetric titration is not possible. The magnitude of the jump is significantly affected by a decrease in the concentration of one of the components of the redox pair. This is sometimes used to increase the jump in the titration curve, which is sometimes necessary when choosing an indicator.
For example, if Fe 2+ is titrated with any oxidizing agent, the redox couple Fe 3+ /Fe 2+ is used to calculate the redox potential to the equivalence point. The initial potential can be reduced by binding Fe 3+ ions into some low-dissociation complex, by adding, for example, fluorides or phosphoric acid. This is what is done when determining Fe 2+ by dichromatometry. The jump is observed in the range of 0.95 - 1.30 V. To carry out titration in the presence of the redox indicator diphenylamine ( E 0 = 0.76 V), it is necessary to shift the jump towards lower potential values. When adding the specified complexing agents, the jump is in the range of 0.68 - 1.30 V . The color transition potential of diphenylamine is within the jump range and can be used for Fe 2+ titration. The magnitude of the jump also depends on the pH of the medium in which the reaction is carried out. For example, for the half-reaction: MnO 4 - + 8H + + 5e – → Mn 2+ + 4H 2 O system potential
will increase with decreasing pH of the medium, which will affect the magnitude of the jump in the titration curve. Redox titration curves are asymmetrical around the equivalence point if the number of electrons involved in the oxidation and reduction half-reactions are not equal ( n 1 ≠ n 2). The equivalence point in such cases is shifted towards E 0 of the substance for which n more. When titrating mixtures of oxidizing or reducing agents, there may be several jumps in the titration curve if the difference between the redox potentials of the corresponding redox pairs is large enough, in which case separate determination of the components of the mixture is possible.
PERMANGANOMETRY
Permanganatometry- a method based on the use of potassium permanganate as a titrant for the determination of compounds that have reducing properties.
The reduction products of permanganate ions can be different depending on the pH of the environment:
Ø in a strongly acidic environment
+ 5e+ MnO 4 - + 8H + ↔ Mn 2+ + 4H 2 O E 0= 1.51 V
Ø slightly acidic or neutral environment
+ 3e+ MnO 4 - + 4H + ↔ MnO 2 ↓ + 2H 2 O E 0= 1.69 V
Ø slightly alkaline environment
+ 3e+ MnO 4 - + 2H 2 O ↔ MnO 2 ↓ + 4OH - E 0= 0.60 V
For analysis, the oxidative properties of MnO 4 - - ions in a strongly acidic environment are most often used, since the product of their reduction in this case is colorless ions Mn 2+ ( in contrast to the brown precipitate MnO 2), which do not interfere with observing a change in the color of the titrated solution from an excess drop of titrant. Required value The pH of the medium is created using a solution of sulfuric acid. Others are strong mineral acids do not use. Thus, nitric acid itself has oxidizing properties, and in its presence, side reactions become possible. In a solution of hydrochloric acid (in the presence of traces of Fe 2+), an oxidation reaction of chloride ions occurs. Titrant method- solution 0.1 * (0.05) mol/dm 3 potassium permanganate - prepared as a secondary standard solution and standardized according to standard substances: oxalic acid, sodium oxalate, arsenic (ΙΙΙ) oxide, Mohr's salt (NH 4) 2 Fe(SO 4) 2 ∙ 6H 2 O and etc.
It is impossible to prepare a titrated solution of potassium permanganate from an accurate weighing of the crystalline preparation, since it always contains a certain amount of MnO 2 and other decomposition products. Before establishing the exact concentration, the KMnO 4 solution is kept in a dark bottle for 7-10 days. During this time, oxidation of reducing agents occurs, the presence of which in distilled water cannot be completely eliminated (dust, traces of organic compounds and so on.). To speed up these processes, a solution of potassium permanganate is sometimes boiled. It must be taken into account that water has redox properties and can reduce permanganate. This reaction is slow, but MnO 2 and direct sunlight catalyze the decomposition process of KMnO 4, so after 7-10 days the MnO 2 precipitate must be removed. The KMnO 4 solution is usually carefully drained from the sediment or filtered through a glass filter. The KMnO 4 solution prepared in this way is not of too low a concentration (0.05 mol/dm 3 or higher) and does not change the titer for a long time. The titer of a potassium permanganate solution is most often determined by anhydrous sodium oxalate Na 2 C 2 O 4 or oxalic acid H 2 C 2 O 4 ∙ 2H 2 O:
МnО 4 - + 5НС 2 О 4 - + 11H + ↔ 2Мn 2+ + 10СО 2 + 8Н 2 О
The first drops of permanganate, even in a hot solution, discolor very slowly. During titration, the concentration of Mn 2+ ions increases and the reaction rate increases. The titer of potassium permanganate can also be determined by arsenic (II) oxide or metallic iron. The use of metallic iron to establish the titer is especially advisable if permanganatometric determination of this element is planned in the future.
In permaganatometry, solutions of reducing agents are also used - Fe (II) salts, oxalic acid and some others - to determine oxidizing agents by back titration. Fe(II) compounds slowly oxidize in air, especially in a neutral solution. Acidification slows down the oxidation process, but it is usually recommended to check its titer before using a Fe (II) solution in an analysis. Oxalates and oxalic acid in solution slowly decompose:
H 2 C 2 O 4 ↔ CO 2 + CO + H 2 O
This process accelerates in light, so it is recommended to store oxalate solutions in dark bottles. Acidified oxalate solutions are more stable than neutral or alkaline solutions.
In permanganatometry, the use of a special indicator is often dispensed with, since the permanganate itself has an intense color, and an excess drop of it causes the appearance of a pink color of the solution that does not disappear within 30 s. When titrating with dilute solutions, redox indicators are used, such as diphenylamine sulfonic acid or ferroin (a coordination compound of Fe (II) with 1,10-phenanthroline). Determination of the titration end point is also performed using potentiometric or amperometric methods.
The permanganometric method can be used to determine:
Ø reducing agents H 2 O 2, NO 2, C 2 O 4 2-, Fe 2+ etc.,
Ø Ca 2+, Ba 2+ and other cations in various preparations;
Ø MnO 2, PbO 2, K 2 Cr 2 O 7, persulfates and other oxidizing agents by back titration. The second standard solution in this case is a solution of a reducing agent (usually oxalic acid or Mohr's salt). In this case, the oxidizing agents are reduced with a titrated solution of oxalic acid or Mohr's salt, the excess of which is titrated with a solution of potassium permanganate.
For example, when analyzing lead dioxide, the sample is dissolved in a sulfate solution of oxalic acid:
MnO 2 + HC 2 O 4 - + 3H + ↔ Mn 2+ + 2 CO 2 + 2H 2 O
and excess oxalic acid is titrated with potassium permanganate.
Permanganatometry can be used to determine ions that do not have redox properties (substituent titration). This method can be used to determine, for example, cations of calcium, strontium, barium, lead, zinc and others, which form poorly soluble oxalates.
Analysis of organic compounds. Oxidation of organic compounds of potassium permanganate occurs at a low rate, which inhibits practical use this method for analysis organic matter. Nevertheless, some organic substances can be successfully determined by this method using the reduction of MnO 4 - in an alkaline medium. Organic compounds are usually oxidized to carbonate. At the end of the reduction reaction of the permanganate in an alkaline medium, the solution is acidified and titrated with MnO 4 - a solution of iron (II) or another suitable reducing agent. This is how, for example, methanol is determined, which in an alkaline environment is oxidized with potassium permagane according to the following scheme:
CH 3 OH + 6MnO 4 - + 8OH- ↔ CO 3 2- + 6MnO 4 2- + 6H 2 O
This method can also determine formic, tartaric, citric, salicylic and other acids, glycerin, phenol, formaldehyde and other organic compounds.
Permanganatometry is pharmacopoeial method of analysis.
Dichromatometry. The essence of the method, titration conditions, titrant, its preparation, establishment of the equivalence point. Iodine - Iodometric titration. The essence of the method, titration conditions, titrant, its preparation, establishment of the equivalence point.
Dichromatometry- determination method based on the oxidation of substances with dichromate ions. It is based on the half-reaction:
+ 6e+ Cr 2 O 7 2- + 14H + ↔ 2Cr 3+ + 7H 2 O E 0= 1.33 V;
f (K 2 Cr 2 O 7) = 1/6.
in an acidic environment, K 2 Cr 2 O 7 is a strong oxidizing agent, therefore, this method can determine a number of inorganic and organic reducing agents, for example Fe 2+, 4-, SO 3 2-,
Redox titration methods are based on the use of reactions associated with electron transfer, that is, redox processes.
Oxidation-reduction reactions are reactions in which reactants gain or lose electrons. An oxidizing agent is a particle (ion, molecule, element) that adds electrons and moves from a higher oxidation state to a lower one, i.e. is being restored. A reducing agent is a particle that donates electrons and moves from a lower oxidation state to a higher one, i.e. oxidizes.
2КМnО 4 +10FeSO 4 +8Н 2 SO 4 ↔2МnSO 4 + 5Fe 2 (SO 4) 3 +К 2 SO 4 + 8Н 2 О
Fe 2+ - e ↔ Fe 3+
MnO 4 - + 5e + 8H + ↔ Mn 2+ + 4H 2 O
Redox titration methods are suitable for the determination of many organic compounds, including pharmaceuticals, the vast majority of which are potential reduction agents.
Depending on the titrant used, permanganatometry, iodometry, dichromatometry, and bromatometry are distinguished. In these methods, KMnO 4, I 2, K 2 Cr 2 O 7, KBrO 3, etc. are used as standard solutions, respectively.
Of all types chemical reactions, used in quantitative analysis, redox reactions (ORR) are the most complex in mechanism.
A distinctive feature of ORR is the transfer of electrons between reacting particles, as a result of which the oxidation state of the reacting particles changes.
In this case, two processes occur simultaneously - the oxidation of some and the reduction of others. Thus, any OVR recorded in general view
aOx 1 + bRed 2 = aRed 1 + bOx 2
Can be represented as two half-reactions:
Red 2 – a= Ox 2
The initial particle and the product of each half-reaction constitute an OB pair. For example, in the oxidation reaction of iron(II) with potassium permanganate, two OM pairs are involved: Fe 3 /Fe 2+ and MnO 4 - /Mn 2+.
During the titration process using the oxidation-reduction method, a change occurs in the RH potentials of the interacting systems. If the conditions differ from standard ones, i.e. The activities of potential-determining ions are not equal to 1 (a≠1), the equilibrium potential of the OM half-reaction aOx 1 + n= bRed 1 can be calculated using the Nernst equation:
E Ox 1/ Red 1 = E º + ,
R – universal gas constant(8.314 J/mol∙deg., F – Faraday’s constant (9.6585 cells/mol), E – OB potential of the system, E º – standard OB potential.
If we substitute the values of constant quantities, T = 298 K (i.e. 25 º C) and replace the natural logarithm with a decimal one, and activity with concentration, then the Nernst equation will take the following form:
E Ox 1/ Red 1 = E º + 
.
Oxidation-reduction reactions (ORR) are more complex than ion exchange reactions and have the following features:
1. The potential of the system depends on the value of the standard RH potential of the system, the concentrations of the oxidizing agent and reducing agent, the concentration of hydrogen ions and temperature.
2. Reactions often occur in several stages, each of them proceeding at a different rate.
3. The rate of ORR is lower than the rate of ion exchange reactions. Often special conditions are required to ensure reactions proceed to completion.
4. The presence of precipitants or complexing agents, causing a change in the concentrations of oxidized or reduced forms, leads to a change in the RH potential of the system.
The oxidation-reduction reactions on the basis of which titration is carried out must satisfy all the requirements for reactions during titration. To increase the speed of OVR use various techniques: increase the temperature, concentration of reactants, change the pH of the solution or introduce a catalyst.
The equivalence point is most often fixed using Red/Ox – indicators, i.e. organic compounds that change their color depending on the potential of the system. With an excess of the oxidizing agent, an oxidized form of the indicator is formed, and an excess of the reducing agent leads to the formation of its reduced form. The process of transition from the oxidized form to the reduced form and back, accompanied by a change in color, can be repeated many times without destroying the indicator. Such indicators include diphenylamine (blue-violet in the oxidized state and colorless in the reduced state) and N-phenylanthranilic acid (oxidized form is red, reduced form is colorless).
For some reactions they use specific indicators are substances that change color when reacting with one of the titration components. For example, such an indicator is starch, which forms a blue adsorption compound with iodine.
In some cases, titration without an indicator is used if the color of the titrant is quite bright and changes sharply as a result of the reaction. An example is titration with potassium permanganate (KMnO 4), the raspberry solution of which becomes discolored when MnO 4 - is reduced to Mn 2+. When all the titrated substance has reacted, an extra drop of KMnO 4 solution will turn the solution pale pink.
Methods redox titration, or redox methods, are based on the use of electron transfer reactions - redox (OR) reactions. In other words, redox titration, or redoxmetry, - This is a titration accompanied by the transfer of one or more electrons from a donor ion or molecule (reducing agent) Red 1 to an acceptor (oxidizing agent) Ox 2:
Red 1 + Ox 2 = Ox 1 + Red 2
The reduced form of one substance Red 1, donating electrons, goes into the oxidized form Ox 1 of the same substance. Both of these forms form one redox pair Ox l Red l.
The oxidized form Ox 2 of the second substance participating in the OB reaction, accepting electrons, goes into the reduced form Red 2 of the same substance. Both of these forms also form a redox couple Ox 2 Red 2.
Any redox reaction involves at least two redox pairs.
The higher the RH potential of the redox couple Ox 2 Red 2, the oxidized form of which plays the role of an oxidizing agent in this reaction, the greater the number of reducing agents Red 1 can be titrated and determined using this oxidizing agent Ox 2. Therefore, in redoxmetry, oxidizing agents are most often used as titrants, the standard OB potentials of redox pairs of which have the highest possible values, for example (at room temperature):
Se 4+, E°(Ce 4+ Ce 3+) = 1.44 V; МnО 4 - , E°(МnО 4 ‑, Н + Мn 2+) = 1.51 V,
Cr 2 O 7 2‑, E°(Cr 2 O 7 2‑, H + Сr 3+) = 1.33 V, etc.
On the contrary, if the substances to be determined are Ox 2 oxidizers, then for their titration it is advisable to use reducing agents whose standard redox vapor RH is as minimal as possible, for example
Jֿ E°(J 2 J⁻) = 0.54 V; S 2 O 3 2‑, E°(S 4 O 6 2‑ S 2 O 3 2‑) = 0.09 V, etc.
Redox methods are the most important pharmacopoeial methods of quantitative analysis.
4.2. Classification of redox methods
Several dozen different methods of OM titration are known. They are usually classified as follows.
Classification according to the nature of the titrant. In this case, RH titration methods are divided into two groups:
oxidimetry - methods for determining reducing agents using an oxidizing titrant;
reductometry - methods for determining oxidizing agents using a reducing titrant.
Classification according to the nature of the reagent, interacting with the analyte. Below, after the name of the corresponding method, the main active ingredient of this method is indicated in parentheses: bromatometry(potassium bromate KBrO 3, bromometry(bromoBr 2), dichromatometry(potassium dichromate K 2 Cr 2 O 7), iodotometry(potassium iodate KJO 3), iodymetry(iodJ 2), iodometry(potassium iodide KJ, sodium thiosulfate Na 2 S 2 O 3, nitritometry(sodium nitriteNaNO 2), permanganatometry(potassium permanganate KMnO 4). chloriodimetry(iodine chloride JC1), cerimetry(cerium(IV) sulfate).
Some other RH titration methods are less commonly used, such as: ascorbinometry(ascorbic acid), titanometry(titanium(III) salts), vanadatometry(ammonium vanadate NH 4 VO 3), etc.
4.3. Conditions for redox titration
Reactions used in RH titration methods must meet a number of requirements, the most important of which are the following:
The reactions should proceed almost to completion. The higher the equilibrium constant, the more complete the OB reaction is. TO, which is determined by the relation
lg K =n( E 1°‑ E 2°)/0.059
at room temperature, where E 1° and E 2 ° - respectively, standard OB potentials of redox pairs participating in a given OB reaction, P - the number of electrons given up by a reducing agent to an oxidizing agent. Therefore, the greater the difference E° =E 1 ° - E 2 °, the higher the equilibrium constant, the more complete the reaction proceeds. For reactions like
A + B = Reaction products
at n =1 and TO 10 8 (with this value TO the reaction proceeds no less than 99.99%) we obtain for E°:
E°0.059lg10 8 0.47 V.
The reaction must proceed quickly enough so that equilibrium, in which the real OB potentials of both redox pairs are equal, is established almost instantly. Typically, RH titrations are carried out at room temperature. However, in the case of slow OM reactions, solutions are sometimes heated to speed up the reaction. Thus, the oxidation reaction of antimony(III) with bromate ions in an acidic medium at room temperature proceeds slowly. However, at 70-80 °C it proceeds quite quickly and becomes suitable for the bromatometric determination of antimony.
To speed up the achievement of equilibrium, homogeneous catalysts are also used. Consider, for example, the reaction
HAsO 2 + 2Ce 4+ + 2H 2 O=H 3 AsO 4 + 2Ce 3+ + 2H +
Standard OB potentials of redox pairs participating in the reaction are equal at room temperature E°(Ce 4+ Ce 3+) = 1.44 V, Eº (H 3 AsO 4 HAsO 2 = 0.56 V. Hence, for the equilibrium constant of this reaction we obtain (n = 2)
lg K = (1,44 ‑ 0,56)/0,059≈30;TO≈ 10 30
The equilibrium constant is large, so the reaction proceeds with a very high degree of completeness. However, under normal conditions it proceeds slowly. To speed it up, catalysts are introduced into the solution.
Sometimes the reaction products themselves are the catalyst. Thus, during permanganatometric titration of oxalates in an acidic medium according to the scheme
5C 2 O 4 2‑ + 2МnО 4 ‾ + 16Н + = 2Mn 2+ + 10CO 2 + 8H 2 O
Manganese(II) cations Mn 2+ act as a catalyst. Therefore, at first, when a titrant solution - potassium permanganate - is added to a titrated solution containing oxalate ions, the reaction proceeds slowly. Therefore, the titrated solution is heated. As manganese(II) cations are formed, the achievement of equilibrium accelerates and titration is carried out without difficulty.
The reaction must proceed stoichiometrically , side processes must be excluded.
The end point of the titration must be determined accurately and unambiguously either with indicators or without indicators.
Redoxometry methods are based on oxidation-reduction reactions. A lot of methods have been developed. They are classified according to the standard (working, titrant) solution used. The most commonly used methods are:
Permanganatometry is a method that is based on the oxidizing ability of a working solution of potassium permanganate KMnO4. Titration is carried out without an indicator. Used to determine only reducing agents during direct titration.
Iodometry is a method in which the working titrated solution is a solution of free iodine in CI. The method allows the determination of both oxidizing agents and reducing agents. Starch serves as an indicator.
Dichromatometry is based on the use of potassium dichromate K2Cr2O7 as a working solution. The method can be used for both direct and indirect determination of reducing agents.
Bromatometry is based on the use of potassium bromate KBrO3 as a titrant in the determination of reducing agents.
Iodatometry uses a solution of potassium iodate KIO3 as a working solution when determining reducing agents.
Vanadatometry makes it possible to use the oxidizing ability of ammonium vanadate NH4VO3. In addition to the listed methods, such methods as cerimetry (Ce4+), titanometry and others are also used in laboratory practice.
To calculate molar mass equivalent of oxidizing agents or reducing agents, the number of electrons taking part in the redox reaction is taken into account (Me = M/ne, where n is the number of electrons e). To determine the number of electrons, it is necessary to know the initial and final oxidation states of the oxidizing agent and the reducing agent.
From large number redox reactions for chemical analysis Use only those reactions that:
- · proceed to the end;
- · pass quickly and stoichiometrically;
- form products of a certain chemical composition(formulas);
- · allow you to accurately fix the equivalence point;
- · do not react with by-products present in the test solution.
Most important factors that influence the reaction rate are:
- · concentration of reacting substances;
- · temperature;
- · pH value of the solution;
- presence of a catalyst.
In most cases, the reaction rate is directly dependent on the temperature and pH of the solution. Therefore, many determinations by redox titration must be carried out at a certain pH value and under heating.
Redox titration indicators
oxidative reduction titration
When analyzing by redox titration methods, direct, reverse and substitution titration are used. The equivalence point of redox titration is fixed both using indicators and without indicators. The indicator-free method is used in cases where the oxidized and reduced forms of the titrant differ. At the equivalence point, the introduction of 1 drop of excess titrant solution will change the color of the solution. Determinations can be made using the permanganatometric method without an indicator, since at the equivalence point, one drop of potassium permanganate solution turns the titrated solution pale pink.
In the indicator method of fixing the equivalence point, specific and redox indicators are used. Specific indicators include starch in iodometry, which in the presence of free iodine turns intense blue due to the formation of a blue adsorption compound. Redox indicators are substances whose color changes when a certain redox potential value is reached. Redox indicators include, for example, diphenylamine NH(C6H5) 2. When exposed to colorless solutions by its oxidizing agents, it turns blue-violet.
Redox indicators have the following requirements:
- · the color of the oxidized and reduced forms must be different;
- · the color change should be noticeable with a small amount of indicator;
- · the indicator must react at the equivalence point with a very small excess of reducing agent or oxidizing agent;
- · its action interval should be as short as possible;
- The indicator must be resistant to components environment(O2, air, CO2, light, etc.).
The action interval of the redox indicator is calculated by the formula:
E = Ео ± 0.058/n,
where Eo is the normal redox potential of the indicator (in the reference book), n is the number of electrons accepted in the process of oxidation or reduction of the indicator.
oxidation states
For example:
For example:
Methods for establishing T.E.
To determine the equivalence point during redox titration, use:
a) non-indicator methods. In the case where the solution of the titrated substance or titrant is colored, TE can be determined by the disappearance or appearance of this color, respectively;
b) specific indicators - changing color when the titrant appears or the substance being determined disappears. For example, for the J 2 /2J - system, the specific indicator is starch, which colors solutions containing J 2 blue, and for Fe 3+ ions the specific indicator is SCN - ions (thiocyanate ions), the resulting complex is colored blood-red ;
c) RH (redox) indicators – changing color when the RH potential of the system changes. Single-color indicators are diphenylamine, two-color indicators are ferroin.
Redox indicators exist in two forms - oxidized (Ind ok) and reduced (Ind rec), and the color of one form is different from the other. The transition of an indicator from one form to another and a change in its color occurs at a certain transition potential, which is observed when the concentrations of the oxidized and reduced forms of the indicator are equal and according to the Nernst-Peters equation:
The transition interval of redox indicators is very short, unlike acid-base indicators.
RH titration curves
RH titration curves depict the change in the RH potential of the system as the titrant solution is added.
Reductometry, when a solution of an oxidizing agent is titrated with a standard solution of a reducing agent
In reductometry, titration curves are calculated:
2) 
3) 
Oxidimetry, when a reducing agent solution is titrated with a standard oxidizing agent solution
 |
In oxidimetry, titration curves are calculated:
2)
3) 
Example. Let's calculate the titration curve of a 100 cm 3 solution of FeSO 4 with a molar concentration equivalent to 0.1 mol/dm 3 with a KMnO 4 solution of the same concentration.
Reaction equation:
The equilibrium constant of this reaction is
Big numeric value The equilibrium constant shows that the equilibrium of the reaction is almost entirely shifted to the right. After adding the first drops of titrant, two OM pairs are formed in the solution: , the potential of each of which can be calculated using the Nernst equation:
In this case, the reducing agent solution is titrated with an oxidizing agent solution, i.e. Titration refers to the oxidimetry method; the titration curve is calculated according to the appropriate scheme.
3) After T.E.
Calculation data for constructing a titration curve
| No. | τ | Calculation formula | E, B |
| 0,10 |  | 0,71 | |
| 0,50 | 0,77 | ||
| 0,90 | 0,83 | ||
| 0,99 | 0,89 | ||
| 0,999 | 0,95 | ||
 | 1,39 | ||
| 1,001 |  | 1,47 | |
| 1,01 | 1,49 | ||
| 1,10 | 1,50 | ||
| 1,50 | 1,505 |
Using the table data, we construct a titration curve:
For titration error ±0.1% titration jump
∆E = E τ =1.001 - E τ =0.999 = 1.47 – 0.95 = 0.52.
For titration error ± 1.0% titration jump
∆E = E τ =1.01 - E τ =0.99 = 1.49 – 0.89 = 0.60.
In the region of TE, when moving from a solution undertitrated by 0.1% to a solution overtitrated by 0.1%, the potential changes by more than 0.5 V. The potential jump makes it possible to use directly potentiometric measurements or RH indicators, the color of which changes with change in potential. In addition, in this case, a colored solution is used as a titrant, therefore T.E. can be determined by the appearance of a faint pink color from one excess drop of potassium permanganate.
PERMANGANOMETRY
The method is based on the oxidation of solutions of reducing agents with potassium permanganate KMnO 4. The oxidation of reducing agents can be carried out in various environments, and manganese (VII) is reduced in an acidic environment to Mn 2+ ions, in a neutral environment to manganese (IV) and in an alkaline environment to manganese (VI). Typically, in the permanganatometry method, the reaction is carried out in an acidic environment. In this case, a half-reaction occurs
A titrated solution cannot be prepared using an exact weighing, because it contains . Therefore, first prepare a solution of approximately the required concentration, leave it in a dark bottle for 7-10 days, filter off the precipitate, and then set the exact concentration of the resulting solution. Standardization of the solution is carried out using a titrated solution of oxalic acid ( 
) or sodium oxalate ().
The indicator is the permanganate itself, colored red-violet. The end of the reaction is easily determined by the change in color from one excess drop of permanganate. In an acidic environment, the titrated solution turns pink due to excess MnO 4 - ions. A big disadvantage of redox reactions is their low speed, which complicates the titration process. Heat is used to speed up slow reactions. As a rule, with every 10° increase in temperature, the reaction rate increases by 2-3 times. The oxidation reaction with oxalic acid permanganate is carried out at a temperature of 70-80 °C. Under these conditions, titration proceeds normally, since the reaction rate increases significantly.
If heating cannot be used (volatilization of one of the substances, decomposition, etc.), the concentrations of the reacting substances are increased to speed up the reaction. The reaction rate can be affected by the introduction of a catalyst into the solution.
The oxidation reaction of oxalic acid permanganate can be catalytically accelerated by the addition of MnSO 4, the role of which is as follows:
The resulting manganese dioxide oxidizes oxalic acid, reducing to manganese (III):
Thus, manganese (II) added to the solution is completely regenerated and is not consumed in the reaction, but greatly accelerates the reaction. In permanganatometry, one of the products of the oxalic acid oxidation reaction is Mn 2+ ions, which, as they form in solution, accelerate the reaction process. Such reactions are called autocatalytic. The first drops of permanganate during the titration of a hot acidified solution of oxalic acid become discolored slowly. As a small amount of Mn 2+ ions is formed, further discoloration of the permanganate occurs almost instantly, since the formed Mn 2+ ions play the role of a catalyst.
Redox titration
Redox processes include chemical processes that are accompanied by changes oxidation states atoms of substances participating in the reaction.
Substances whose atoms reduce their oxidation state during a reaction due to the addition of electrons are called oxidizing agents, i.e. they are electron acceptors. In this case, the oxidizing agents themselves are reduced. Reducing agents, being electron donors, are oxidized.
The product of reduction of an oxidizing agent is called the reduced form, and the product of oxidation of a reducing agent is its oxidized form. The oxidizing agent with its reduced form constitutes a half-pair of the redox system, and the other half-pair is the reducing agent with its oxidized form. Thus, a reducing agent with an oxidized form and an oxidizing agent with its reduced form constitute two semi-pairs (redox pairs) of the redox system.
All OM processes (redox reactions) can be divided into three types
a) intermolecular, when during the OB reaction the transfer of electrons occurs between particles of different substances. For example
In this reaction, the role of the oxidizing agent in the presence of H 3 O + is played by ions, and the ions act as a reducing agent
b) dismutation (disproportionation), during which the transfer of electrons occurs between particles of the same substance. As a result of disproportionation, the oxidation state of one part of the atoms decreases at the expense of another part of the same atoms, the oxidation state of which becomes greater.
For example:
c) intramolecular, in which the transfer of electrons occurs between two atoms that are part of the same particle of a substance, leading to the decomposition of the substance into simpler ones.