Influence of ph on the redox titration process. Essence and classification of 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 a reagent required for a reaction with a given amount of an analyte. Aliquot- an accurately measured multiple of the sample (solution volume) 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 the moment of 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 FC, is characterized by a low concentration and a transition interval. Titration curve-shows a graphical dependence of the logarithm of the concentration of the participant in the reaction occurring during titration, or some St-va solution on the volume of the added titrant (or on the degree of titration). Build in coordinates pH-volume of titrant.
Requirements for reactions in titrimetry: 1. The interaction of the titrant with the analyte must take place in strict accordance with the stoichiometric reaction equation, and the titrant must be consumed only for 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 it can in principle be in the case of titration of reducing agents.
2. The titration reaction must proceed quantitatively, i.e., the equilibrium constant of the titration reaction must be sufficiently large.
3. The interaction of the analyte with the titrant must take place at a high speed.
4. There must be a way to determine the end of the titration.
5. The titrant solution must be standardized.
Reagents: According to the properties of substances and the method of their preparation, titrants are of two types: standard, with a prepared titer, standardized or with a fixed titer. Standard solutions or with prepared titers are called primary standard solutions. It is prepared by dissolving a precise amount of a pure chemical in a specific 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 titre) is used in titrimetry for the quantitative determination of certain substances and for setting titers of the second type - secondary standard solutions.
Secondary standard solution - these are solutions of such substances, the concentration of which is established (standardized) by the concentration of primary standard solutions or prepared by a known mass of the secondary standard substance.
The second type of titrants includes solutions of such 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 for expressing concentrations in titrimetry (molar concentration, molar equivalent concentration, titer, correction factor. Calculation of the mass of a standard sample for the preparation of titrant, calculation of titrant concentration
Molar concentration c(A) - the amount of solute A in moles contained in one liter of solution: mol / l. c(A) = n(A)/V(A) = m(A)/M/(A)V(A), where p(A)- amount of dissolved substance A, mol; V(A)- volume of solution, l; t(A)- mass of the dissolved substance A, g; M / (A) - molar mass of the solute A, g / mol. Molar concentration equivalent c(1/zA),, - the amount of solute 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)= m(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)- the amount of substance equivalent to A in solution, mol; M(1/zA) is the molar mass of the equivalent of solute A, g/mol. Titer T(A) solute A is the mass of dissolved substance A contained in one ml of solution: measured in ml T(A)\u003d m (A) / V (A) \u003d c(1/z A)M(1/z A)/1000. Solution titer for analyte X, or titrimetric conversion factor t(T/X), is the mass of the titratable 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 \u003d c (1 / zA) pr / c (1 / zA) theor. Calculation of the mass of a sample of a standard substance. Sample weight t(A) standard substance A, necessary to obtain a solution with a given molar concentration of the equivalent c(1/zA), calculated by the formula: m (A) \u003d c(1/z A)M(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 given, then the mass of the sample is calculated similarly by the formula: m(A) = c(A)M(A)V(A), Where M / (A) is the molar mass of substance A. The sample mass is usually weighed on an analytical balance with a weighing error of ± 0.0002 g. The calculation of the concentration of titrant T when it is standardized according to a standard solution of substance A is carried out as follows. Let the reaction T + A = B proceed during standardization. According to the law of equivalents, the equivalent amounts of 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) \u003d c ( 1/z AT) V(B)/V(T).
Classification of titrimetric analysis methods - acid-base, redox, precipitation, complexometric. Types of titration (direct, reverse, indirect). Methods for establishing the titration point.
1) Acid-base titration (neutralization method)- tit
based on the reaction of proton transfer from one reacting
particles to another in solution. Distinguish between 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 followed by the transition of one or more
electrons from a donor ion or molecule (reductant) to an oxidizing acceptor).
3) Precipitation titration- such a titration, when the titratable in-in, when interacting 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 such a titration when the analyte is directly titrated with a standard titrant solution or vice versa. Back titration (titration by residue)- 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 analyte does not react directly with the titrant, but is determined indirectly as a result of using a stoichiometrically proceeding reaction with the formation of another substance that reacts with the titrant. Methods for establishing endpoint titration There are two groups of methods for fixing CTT: visual and instrumental.
visual methods. The course of the reaction is monitored visually by observing a change in color (or other property) of a specially introduced indicator | during 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, disappearance of color, the formation or dissolution of a precipitate. indicator By visual methods, an indicator is introduced into the titrated solution. AT 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 by the disappearance of the color of the titrated substance.
Instrumental methods. CTT is determined by the change in the physicochemical properties of the solution - fluorescence, optical density, potential, electrical conductivity, current strength, radioactivity, etc. Changes in the physicochemical properties are 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-chromophore theories of indicators of acid-base titration.
ACID-BASE TITRATING - this is a method for determining acids, bases, salts, based on the reaction of interaction between protoliths - acid HA and base B: HA + B \u003d A "+ HB + In aqueous solutions - this is the neutralization reaction H 3 0 + + 0H \u003d 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, so the concentration of titrants is set by standardization 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 the determination of 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 the equivalence point.
An acid-base indicator is itself an acid or a base, and during acid-base titration it changes its color in TE or
near her. (Methyl orange pT=4 pH transition interval and indicator color 3.1–4.4 Red – orange-yellow; Phenolphthalein pT=9.0 8.2–10 Colorless – violet).
Requirements for indicators:1) coloring d.b. intense, excellent in acidic and alkaline environments 2) discoloration d.b. clear in a narrow range of pH r-ra 3) indicator d.b. sensitive 4) ind-r d.b. stable, do not decompose in air, in solution. Theories of indicators:
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) the ionic solution proceeds instantly, and the indicator changes color only with time
2) Chromophoric - the presence of color is explained by the appearance of chromophore groups. Ind-ry in the solution are present in the form of tautomeric forms. Disadvantages: does not explain why tautomeric transformations occur when the pH is changed.
3) ionic-chromophoric-acid-base indicators are weak acids and bases, and the neutral indicator molecule and its ionized form contain different chromophore groups. Indicator molecules in an aqueous solution are capable of either donating hydrogen ions (the indicator is a weak acid) or accepting them (the indicator is a 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 according to the titration curve. Titration of polyprotic acids. Acid-base titration errors, their calculation and elimination.
Acid-base titration curves graphically display the dependence of the change in the pH of the titrated solution on the volume of the added titrant or on the degree of titration f = V(T)/V, where V(T) and V are, respectively, the volume of the added titrant at a given moment and in the fuel cell. Most often (though not always), when constructing acid-base titration curves, the volume of the added titrant or the degree of titration is plotted along the abscissa axis, and along the axis ordinate - pH values of the titrated solution.
Calculation, construction and analysis of titration curves. To construct an acid-base titration curve, the pH values of the titrated solution are calculated at different points in the titration, i.e. at different titration points: for initial solution, for solutions before FC, in FC and after FC.
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 a strong acid is titrated 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. Given that pH + pOH = 14, we can write: pH = 14-pOH
According to the formulas, the pH values of the solution are calculated at different moments of the titration, and according to the calculated data, a titration curve is built in the pH-V coordinates (T).
Calculated titration curve 20 ml 0.1000 mol/l HC1 solution 0.1000 mol/l NaOH solution
To determine the CTT in this case, you can use such acid-base titration indicators as methyl orange (pT = 4), methyl red (pT = 5.5), bromthymol 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, an attempt is made to choose an indicator so that, other things being equal, the pT value of the indicator would be as close as possible to the pH value of the solution in fuel cells, since this reduces the titration error.
Titration of a strong base with a strong acid. When a strong base is titrated 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 titrated 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 due to the presence of an excess of "exact 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 the differences in values pK 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 to-basic title: 1) measurement error (error of the burette, pipettes) If the solution is taken with a burette, then two measurements of the volume of the solution in the burette are carried out: before and after the solution is taken. 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 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 value of the maximum relative error in measuring the volume of the 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 the selected solution.
2) methodological errors 3) systematic errors (incorrect selection of the indicator, mismatch between the equivalence point and the end point of titration)
a.1.) hydrogen error (X n3o +, Xn +) - is associated with overtitration of the solution with a strong acid (then the error is +) or under-titration (-) Xn3o + \u003d a / a * 100%
a-number of excess equivalents of H+ ions to the total number of equivalents
a′=CH3o+ *V
a \u003d CH3o + * V (a + c) \u003d CH3o + * (Va + Vb)
C n3o + = 10 (in the step - pH)
Substitute in our hostility
X h3o + = + - (10 - pT) * (Va + Vb) / Cb * Vb) * 100%
b-acid a-alkali.pT-display titer-i ind
a.2.) hydroxide error (basic) - associated with an excess of the number of OH groups with a titer with a strong base, or with an undertiter with a solution of the base
a.3.) acid error - caused by the presence of a certain amount of undertiter acid at the end point of thyr-i (weak acid)
Redox titration. The essence of the method. Classification of redox methods. Conditions for redox titrations. Reaction requirements. Types of redox titration (direct, reverse, substitution). Examples of redox indicators. Formulas, color transition at the equivalence point.
Redox Titration(redoximetry, oxidimetry.)
Redox methods include an extensive group of titrimetric analysis methods based on the occurrence of redox reactions. Redox titrations use a variety of 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, the determination of 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 a wide variety of 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 are fast enough to allow titrations to be carried out in just a few minutes. Many of them proceed in acidic, neutral and alkaline environments, which greatly 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 permanganato-, 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 RH titration:
direct titration is that the solution of the analyte BUT titrated with standard titrant solution AT. 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, with a slow reaction, etc. During back titration to an aliquot of the analyte BUT add an accurately measured volume of a standard solution of a substance AT taken in excess. Unreacted excess of a substance AT determined by titration with a standard solution of the excipient With. By the difference in the initial amount of the substance AT and its amount remaining after the reaction, determine the amount of substance AT that has reacted with a substance BUT, on the basis of which the content of the substance is calculated BUT.
Indirect titration or substituent titration. It is based on the fact that it is not the substance itself that is being titrated, but the product of its reaction with an auxiliary substance With.
Substance D must be formed strictly quantitatively with respect to the substance BUT. Determining the content of the reaction product D titration with a standard solution of a substance AT, according to the reaction equation, the content of the analyte is calculated BUT.
Curves of redox titration, errors, their origin, calculation, elimination. Permanganatometry. Essence of the method, titration conditions, titrant, its preparation, standardization, establishment of the equivalence point. The use of permanganatometry.
Redox titration curves
The redox titration curves show the change in the redox potential during the titration process: E = ƒ(V PB), (Fig. 2.7) Two redox systems are involved in the redox titration - 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 the 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. up to the equivalence point, the potential is calculated from the half-reaction with the participation of the titrated substance, and after the equivalence point, from the half-reaction with the participation of the titrant. Before the start of titration, it is considered that for the titrated substance, the concentrations of the oxidized and reduced forms differ by a factor of 1000 or 10,000. At the equivalence point, both conjugated forms of the oxidizing agent and 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 is the number of electrons participating in the oxidation and reduction half-reactions, respectively; E 0 1 , E 0 2 – standard redox potential for the oxidizing agent and reducing agent, respectively.
Rice. Titration curves in redox method:
1 - the reducing agent is titrated with an oxidizing agent; 2 - oxidizing agent is titrated with a reducing agent
Near the equivalence point on the titration curve, a potential jump is observed, the magnitude of which is the greater, the greater the difference between E 0 ok-la and E 0 in-la. Indicator titration is possible if EMF = E 0 ok-la - E 0 v-la ≥ 0.4 V. If the EMF = 0.4 - 0.2 V, instrumental titration can be used, where the equivalence point is fixed using instruments. If EMF< 0,2 AT direct redox 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 on the titration curve, which is necessary when choosing an indicator.
For example, if Fe 2+ is titrated with an oxidizing agent, the Fe 3+ /Fe 2+ redox pair is used to calculate the redox potential up to the equivalence point. It is possible to reduce the initial potential by binding Fe 3+ ions into some low-dissociating complex, by adding, for example, fluorides or phosphoric acid. This is done in the determination of 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. With the addition of these complexing agents, the jump is in the range of 0.68 - 1.30 V . In this case, the diphenylamine color transition potential 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 on the titration curve. The redox titration curves are not symmetrical with respect to the equivalence point if the number of electrons involved in the oxidation and reduction half-reactions are not equal to each other ( n 1 ≠ n 2). The equivalence point in such cases is shifted towards E 0 of the substance in which n more. When titrating mixtures of oxidizing or reducing agents, there may be several jumps on the titration curve, if the difference between the redox potentials of the corresponding redox pairs is large enough, in this case, separate determination of the components of the mixture is possible.
PERMANGANATOMETRY
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 medium:
Ø in strongly acid 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 oxidizing 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 Мn 2+ ( in contrast to the brown precipitate of MnO 2), which do not interfere with observing the change in color of the titrated solution from an excess drop of titrant. The required pH value of the medium is created using a solution of sulfuric acid. Other strong mineral acids are not used. So, 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+), the oxidation reaction of chloride ions occurs. Method titrant- a solution of 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 according to an accurate sample of a 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, the oxidation of reducing agents occurs, the presence of which in distilled water cannot be completely excluded (dust, traces of organic compounds, etc.). 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 process of decomposition of KMnO 4, so after 7-10 days the precipitate of MnO 2 must be removed. The KMnO 4 solution is usually carefully decanted from the precipitate or filtered through a glass filter. The KMnO 4 solution prepared in this way is not too low in concentration (0.05 mol / dm 3 and above) and does not change the titer for a long time. The titer of a solution of potassium permanganate 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:
MnO 4 - + 5HC 2 O 4 - + 11H + ↔ 2Mn 2+ + 10CO 2 + 8H 2 O
The first drops of permanganate, even in a hot solution, discolor very slowly. During the 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 a permanganometric determination of this element is expected 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 are slowly oxidized in air, especially in a neutral solution. Acidification slows down the oxidation process, but it is usually recommended to check its titer before using the Fe (II) solution in the analysis. Oxalates and oxalic acid in solution slowly decompose:
H 2 C 2 O 4 ↔ CO 2 + CO + H 2 O
This process is accelerated in the 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 its excess drop causes the appearance of a pink color of the solution that does not disappear for 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 end point of the titration is also performed by potentiometric or amperometric methods.
The permanganometric method can be used to determine:
Ø reducers 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 reducing agent solution (often oxalic acid or Mohr's salt). In this case, 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 sulfuric acid 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.
Ions that do not have redox properties can be determined permanganatometrically (substituent titration). This method can be used to determine, for example, cations of calcium, strontium, barium, lead, zinc and others, which form sparingly soluble oxalates.
Analysis of organic compounds. The oxidation of organic compounds with potassium permanganate occurs at a low rate, which hinders the practical application of this method for the analysis of organic substances. 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 permanganate reduction reaction 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 medium is oxidized by potassium permaganate according to the 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. Essence of the method, titration conditions, titrant, its preparation, establishment of the equivalence point. Iodi - Iodometric titration. Essence of the method, titration conditions, titrant, its preparation, establishment of the equivalence point.
dichromatometry- method of determination based on the oxidation of substances by dichromate ions. It is based on a 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-,
This method of titrimetric analysis is based on redox reactions between the titrant and the analyte. Oxidation-reduction reactions are associated with the transfer of electrons. The substance that donates electrons in these reactions is reducing agent(Red), and acquiring electrons - oxidizing agent(Oh):
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. A conjugated redox pair Ox 1 /Red 1 (redox pair) is formed. The oxidized form of another substance (Ox 2), accepting electrons, goes into the reduced form (Red 2) of the same substance. Another redox pair Ox 2 /Red 2 is formed. Thus, at least two redox pairs are involved in the redox reaction. A measure of the redox properties of substances is the redox potential E 0 . Comparing the standard potentials of the OB pairs participating in the OVR, one can predetermine the direction of spontaneous reaction. The redox reaction spontaneously proceeds in the direction of the transformation of a strong oxidizing agent into a weak reducing agent, a strong reducing agent into a weak oxidizing agent.
The higher the standard potential of a redox pair, the stronger the oxidizing agent is its oxidized form and the weaker the reducing agent is its reduced form. The lower the standard potential of the OB-pair, the stronger the reducing agent is the reduced form, the weaker the oxidizing agent is the oxidized form. Therefore, in redox titration (redoximetry), as titrants in the determination of reducing agents, such oxidizing agents (Ox 2) are used, the standard OB potentials of the redox pairs of which are as high as possible, thereby using them to titrate a greater number of reducing agents ( Red 1). For example, E 0 (MnO 4 -, H +, Mn 2+) = + 1.51V, E 0 (Cr 2 O 7 2-, H +, Cr 3+) = + 1.33V, etc.
When determining oxidizing agents (Ox 2), reducing agents (Red 1) are used as titrants, the standard OB potential of redox pairs of which has the minimum possible value. For example, E 0 (I 2 / 2I -) \u003d + 0.536V, E 0 (S 4 O 6 2- / 2S 2 O 3 2-) \u003d + 0.09 V, etc.
To establish equivalence points used in redox redox indicators(redox indicators), which are substances that can be reversibly oxidized and reduced, and their oxidized and reduced forms have a different color. An example of such an indicator is diphenylamine. Often in redoximetry, the so-called indicatorless titration, for example, in permanganatometry, the role of an indicator is performed by a titrant - potassium permanganate. Quantitative calculations in RH titration, as in other methods of titrimetric analysis, are based on the law of equivalents.
Molar mass of oxidizing agent equivalent:
(39)
Molar mass of reducing agent equivalent:
(40)
One of the methods of redox titration is permanganometric titration. This is an analysis method in which a solution of potassium permanganate KMnO 4 is used as an oxidizing titrant. The MnO 4 anion - exhibits oxidizing properties in acidic, neutral and alkaline environments, recovering, respectively, to the Mn 2+ cation (colorless ions), manganese (IV) oxide MnO 2 (brown precipitate) and the MnO 4 2- anion (green solution turning brown on air).
Half reaction equations:
acid environment
MnO 4 - + 8H + + 5e - → Mn 2+ + 4H 2 O
E 0 (MnO 4 -, H +, Mn 2+) = + 1.51V
Neutral environment
MnO 4 - + 2H 2 O + 3e - → MnO 2 ↓ + 4OH -
E 0 (MnO 4 - / MnO 2) = + 0.60V
Alkaline environment
MnO 4 - + e - → MnO 4 2-
E 0 (MnO 4 - / MnO 4 2-) = + 0.56V
In permanganatometry, titration is carried out in an acidic environment, because:
1) the MnO 4 permanganate ion has the strongest oxidizing properties in an acidic environment compared to a neutral and alkaline one, as evidenced by the values of standard OB potentials (+1.51V versus +0.60V and +0.56V);
2) the determination of the end point of the titration in a neutral medium will interfere with the brown precipitate of MnO 2 ; in an alkaline environment, the formed manganate ions MnO 4 2- , which have a green color, also make it difficult to fix the end point of the titration. The Mn 2+ cations formed in an acidic medium are colorless;
3) when titrating in an acidic medium, it becomes possible to clearly fix the end point of the titration without the use of an extraneous indicator, since one extra drop of potassium permanganate stains a colorless solution in a pale pink color.
titrant: potassium permanganate solution (acidic).
Indicator: potassium permanganate.
Substances to be determined: Fe 2+ ions, Cr 3+ , NO 2 - , hydrogen peroxide H 2 O 2 , ethyl alcohol, uric acid in biological studies, glucose, the content of some vitamins, catalase enzyme activity, oxidizability of domestic and waste water, organic pollution in the atmosphere .
One of the disadvantages of permanganatometry is the need to standardize the potassium permanganate solution, since it A titrated solution cannot be prepared by accurately weighing it. In addition, the concentration of potassium permanganate, transferred to the solution, decreases markedly. Therefore, the exact concentration of the KMnO 4 solution is set no earlier than 5-7 days after its preparation. For standardization, oxalic acid or its salts (sodium or ammonium oxalates) are used.
Standard Substances: H 2 C 2 O 4 2H 2 O, Na 2 C 2 O 4, (NH 4) 2 C 2 O 4 ∙H 2 O.
The equation of the reaction that occurs when the KMnO 4 solution is standardized with oxalic acid:
H 2 C 2 O 4 + KMnO 4 + H 2 SO 4 → CO 2 + Mn 2+ + ...
C 2 O 4 2- - 2e - → 2CO 2 5
MnO 4 - + 8H + + 5e - → Mn 2+ + 4H 2 O 2
Redox methods are based on redox reactions. A lot of methods have been developed. They are classified according to the standard (working, titrant) solution used. The following methods are most commonly used:
Permanganatometry - method, which is based on the oxidizing ability of the working solution of potassium permanganate KMnO4. Titration is carried out without an indicator. It is used to determine only reducing agents in direct titration. Permanganatometry is based on the oxidation reaction of various reducing agents with a working solution of potassium permanganate, i.e. MnO4- ion. Oxidation with potassium permanganate can be carried out in an acidic, neutral, and alkaline environment. In a strongly acidic environment, permanganate ions (MnO4-) have a high redox potential, reducing to Mn2+, and they are used to determine many reducing agents: MnO4- + 8H+ + 5e = Mn2+ + 4H2O
In an alkaline environment, MnO4- is reduced to the manganate ion: MnO4- + e \u003d MnO42-
In a neutral or slightly alkaline medium, the permanganate ion is reduced to manganese acid MnO (OH) 2 or to MnO2: MnO4- + 2H2O + 3e \u003d MnO2v + 4OH-
KMnO4 solution refers to titrants with a fixed titer. In this regard, before using it in the analysis as a titrant, a solution of KMnO4.
Iodometry- a method in which a solution of free iodine in KI serves as a working titrated solution. The method allows to determine both oxidizing agents and reducing agents. Starch serves as an indicator. The iodometric method of titrimetric analysis is based on the reaction: I2 + 2e = 2I-
As a titrant in the determination of oxidizing agents, a solution of sodium thiosulfate is used, which interacts with the released iodine (substituent) in an equivalent amount. Na 2 S 2 O 3 -thiosulfate
32. Potentiometry- a research method based on the thermodynamic relationships between the EMF of electrochemical circuits, on the one hand, and the physicochemical and parameters of solutions and chemical reactions, on the other.
Inert electrodes- a plate or wire made of difficult-to-oxidize metals - platinum, gold, palladium. They are used to measure E in solutions containing a redox pair (for example, /).
Membrane electrodes of various types have a membrane on which the membrane potential E arises. The value of E depends on the difference in concentrations of the same ion on different sides of the membrane. The simplest and most widely used membrane electrode is the glass electrode.
Mixing insoluble salts such as AgBr, AgCl, AgI and others with some plastics (rubber, polyethylene, polystyrene) led to the creation of ion-selective electrodes that selectively adsorb these ions from the solution due to the Panet-Fajans-Han rule. Since the concentration of the ions to be determined outside the electrode differs from that inside the electrode, the equilibria on the membrane surfaces differ, which leads to the appearance of a membrane potential.
Most often, potentiometers are used for direct measurements of pH, concentrations of other ions pNa, pK, pNH₄, pCl and mV. Measurements are made using appropriate ion-selective electrodes.
A glass electrode and a silver chloride reference electrode are used to measure pH. Before carrying out analyzes, it is necessary to check the calibration of pH meters using standard buffer solutions, the fix channels of which are applied to the device.
pH meters, in addition to direct determinations of pH, pNa, pK, pNH₄, pCl and others, allow potentiometric titration of the ion to be determined.
Potentiometric titration.
Potentiometric titration is carried out in cases where chemical indicators cannot be used or in the absence of a suitable indicator.
In potentiometric titration, potentiometer electrodes dipped into the titrated solution are used as indicators. In this case, electrodes sensitive to the titratable ions are used. In the process of titration, the concentration of ions changes, which is recorded on the scale of the measuring probe of the potentiometer. Having recorded the readings of the potentiometer in units of pH or mV, they build a graph of their dependence on the volume of titrant (titration curve), determine the equivalence point and the volume of titrant used for titration. Based on the data obtained, a potentiometric titration curve is built.
The potentiometric titration curve has a form similar to the titration curve in titrimetric analysis. The equivalence point is determined from the titration curve, which is in the middle of the titration jump. To do this, draw tangents to sections of the titration curve and determine the equivalence point in the middle of the tangent of the titration jump. The change in ∆рН/∆V acquires the greatest value at the equivalence point.
Even more precisely, the equivalence point can be determined by the Grant method, according to which the dependence of ∆V / ∆E on the volume of the titrant is built. The Gran method can be used to carry out potentiometric titration without bringing it to the equivalence point.
Potentiometric titration is used in all cases of titrimetric analysis.
Acid-base titration uses a glass electrode and a reference electrode. Since the glass electrode is sensitive to changes in the pH of the medium, when they are titrated, changes in the pH of the medium are recorded on the potentiometer. Acid-base potentiometric titration is successfully used in the titration of weak acids and bases (рК≤8). When titrating mixtures of acids, it is necessary that their pK differ by more than 4 units, otherwise a part of the weaker acid is titrated together with a strong one, and the titration jump is not clearly expressed.
This allows you to use potentiometry to build experimental titration curves, select indicators for titration and determine acidity and basicity constants.
In precipitation potentiometric titration, a metal electrode is used as an indicator, constituting an electrode pair with the ions to be determined.
In complexometric titration, the following are used: a) a metal electrode reversible to the ion of the metal to be determined; b) a platinum electrode in the presence of a redox pair in the solution. When one of the components of the redox pair is bound by a titrant, its concentration changes, which causes changes in the potential of the indicator platinum electrode. Back titration of an excess of an EDTA solution added to a metal salt with a solution of an iron (III) salt is also used.
In redox titration, a reference electrode and a platinum indicator electrode sensitive to redox pairs are used.
Potentiometric titration is one of the most used methods of instrumental analysis due to its simplicity, accessibility, selectivity and wide possibilities.
33. Electrode potentials and mechanisms of their occurrence. To determine the direction and completeness of the course of redox reactions between redox systems in aqueous solutions, the values are used electrode potentials these systems. The mechanism of the occurrence of electrode potentials, their quantitative determination, processes that are accompanied by the occurrence of an electric current or caused by an electric current, are studied by a special section of chemistry - electrochemistry. By combining an electrode representing the redox system under study with a standard hydrogen electrode, the electrode potential E of this system is determined. In order to be able to compare the redox properties of different systems by their electrode potentials, it is necessary that the latter also be measured under standard conditions. These are usually an ion concentration of 1 mol / l, a pressure of gaseous substances of 101.325 kPa and a temperature of 298.15 K. Potentials measured under such conditions are called standard electrode potentials and are designated Eo. They are often also called redox or redox potentials, representing the difference between the redox potential of the system under standard conditions and the potential of a standard hydrogen electrode. The standard electrode potential is the potential of a given electrode process at concentrations of all substances participating in it equal to one. Standard electrode potentials of redox systems are given in the reference literature. These systems are written in the form of reduction half-reaction equations, on the left side of which there are atoms, ions or molecules that accept electrons (oxidized form). its ions; metals having negative electrode potentials, i.e. standing in a series of voltages to the left of hydrogen, are able to displace it from dilute acid solutions; each metal is able to displace (restore) those metals that have a higher electrode potential from salt solutions. Under conditions different from the standard, the numerical value of the equilibrium electrode potential for the redox system, written in the form, is determined by Nernst equation: where and are the electrode and standard potentials of the system, respectively; R is the universal gas constant; T is the absolute temperature; F is Faraday's constant; n is the number of electrons involved in the redox process. C(Red) and C(Ox) are the molar concentrations of the reduced and oxidized forms of the compound, respectively. For example, for a redox system, the Nernst equation has the form
(REDOXOMETRY, OXIDIMETRY)
Essence and classification of redox titration methods
Redox methods are based on redox reactions. A lot of methods have been developed. They are classified according to the standard (working, titrant) solution used. The following methods are most commonly used:
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. It is used to determine only reducing agents in direct titration.
Iodometry is a method in which a solution of free iodine in KI serves as a working titrated solution. The method allows to determine 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 determinations 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 in the determination of reducing agents.
Vanadatometry makes it possible to use the oxidizing power of ammonium vanadate NH4VO3. In addition to the above methods, laboratory practice also uses such methods as cerimetry (Ce4+), titanometry, and others.
To calculate the molar mass of the equivalent of oxidizing agents or reducing agents, the number of electrons involved 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 reducing agent.
Of the large number of redox reactions, only those reactions are used for chemical analysis that:
flow 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.
The most important factors affecting the reaction rate are:
the concentration of reactants;
· temperature;
the pH value of the solution;
the 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 should be carried out at a certain pH value and under heating.
Redox Titration Indicators
redox titration
In the analysis by redox titration methods, direct, reverse and substitution titrations are used. The equivalence point of the redox titration is fixed both with the help of indicators and in the non-indicator way. The non-indicator method is used in cases where the oxidized and reduced forms of the titrant differ. At the equivalence point, adding 1 drop of excess titrant solution will change the color of the solution. In a non-indicator way, it is possible to carry out determinations by the permanganometric method, tk. at the equivalence point from one drop of potassium permanganate solution, the titrated solution turns pale pink.
With 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 value of redox (redox potential) is reached. Redox indicators include, for example, diphenylamine NH(C6H5)2. When colorless solutions are exposed to its oxidizing agents, it turns blue-violet.
Redox indicators have the following requirements:
The color of the oxidized and reduced forms should be different;
The change in color should be noticeable with a small amount of the indicator;
· the indicator should react at the equivalence point with a very small excess of the reducing agent or oxidizing agent;
· its action interval should be as short as possible;
· the indicator must be resistant to environmental components (O2, air, CO2, light, etc.).
The interval of action of the redox indicator is calculated by the formula:
E \u003d Eo ± 0.058 / n,
where Eo is the normal redox potential of the indicator (in the directory), n is the number of electrons that the indicator receives in the process of oxidation or reduction.
permanganatometry
Permanganatometry is based on the oxidation reaction of various reducing agents with a working solution of potassium permanganate, i.e. MnO4- ion. Oxidation with potassium permanganate can be carried out in acidic, neutral and alkaline environments
In a strongly acidic medium, permanganate ions (MnO4-) have a high redox potential, reducing to Mn2+, and they are used to determine many reducing agents:
МnО4- + 8Н+ + 5е = Мn2+ + 4Н2О
E0 MnO4- / Mn2+ = 1.51 V
In an alkaline environment, MnO4- is reduced to the manganate ion:
MnO4- + e = MnO42-
In a neutral or slightly alkaline medium, the permanganate ion is reduced to manganese acid MnO (OH) 2 or to MnO 2:
MnO4- + 2H2O + 3e \u003d MnO2 ↓ + 4OH-
E0 MnO4- / MnO2 \u003d 0.59 V
When titrating with permanganate, indicators are not used, since the reagent itself is colored and is a sensitive indicator: 0.1 ml of a 0.01 M KMnO4 solution colors 100 ml of water in a pale pink color. As a result of the reaction of potassium permanganate with a reducing agent in an acidic medium, colorless Mn2+ ions are formed, which makes it possible to clearly fix the equivalence point.
KMnO4 solution refers to titrants with a fixed titer. In this regard, before using it in the analysis as a titrant, the KMnO4 solution is standardized according to the concentration of solutions of the initial substances of chavelic acid or sodium oxalate. A solution of potassium permanganate is very difficult to obtain in its pure form. It is usually contaminated with traces of manganese(IV) oxide. In addition, pure distilled water usually contains traces of substances that reduce potassium permanganate to form manganese (IV) oxide:
4 KMnO4 + 2H2O \u003d 4 MnO2 ↓ + 4OH- + 3O2
When stored in solid form, potassium permanganate decomposes under the influence of light, becoming contaminated also with MnO2:
KMnO4 \u003d K2MnO4 + MnO2 ↓ + O2
A solution of potassium permanganate can be prepared from a standard titer and according to a sample taken on a technical scale. In the first case, the contents of the ampoule are quantitatively transferred into a 2-liter volumetric flask, rinsing the ampoule and funnel with warm distilled water. Add a small volume of hot water to the volumetric flask to dissolve the crystals, then cool the resulting solution to room temperature, bring the volume of the solution to the mark and mix. The molar concentration of the resulting solution is 0.05 mol/l.
In the second case, weigh a portion of potassium permanganate weighing 1.6 g on a technical balance in a weighing bottle or on a watch glass, place it in a beaker and dissolve in hot distilled water with thorough stirring of the resulting solution, trying to dissolve all KMnO4 crystals. Then carefully pour the solution through a funnel into a volumetric flask with a capacity of 1 l and mix thoroughly, after closing the flask with a ground stopper (do not use a rubber stopper). Leave the prepared solution of KMnO4 for 7-10 days, then filter the solution through a funnel with glass wool or carefully pour it into another bottle using a siphon. It is obligatory to store the KMnO4 solution in dark bottles, protected from light, to prevent decomposition.
Setting the titer of a solution of potassium permanganate, prepared according to a sample taken, can be carried out using oxalic acid H2C2O4 * 2H2O or sodium oxalate Na2C2O4.
Determination of nitrite ions in solution
In a neutral or alkaline environment, nitrites do not react with potassium permanganate; in an acidic hot solution, they are oxidized to nitrates:
5KNO3 + 2KMnO4 + 3H2SO4 = 2MnSO4 + 5KNO2 + K2SO4 + 3H2O
When slowly titrating an acidified sodium nitrite solution with a potassium permanganate solution, lower results are obtained, because nitrites are easily oxidized by acids to form nitrogen oxides:
2NO2- + 2H+ → 2HNO2 → NO2- + NO + H2O
Therefore, in order to avoid losses, you can use the back titration method or the Lynge method - titration with a solution of sodium nitrite of an acidified solution of potassium permanganate.
Determination of calcium in calcium carbonate
Determination of calcium in solution by the method of permanganometric titration is possible by the method of back or substitution titration. In the first case, a precisely measured excess of a titrated solution of oxalic acid is introduced into a solution containing calcium. The resulting CaC2O4 + H2SO4 precipitate of CaC2O4 is filtered off, and the residue that did not enter into the reaction of oxalic acid is titrated with a standard solution of potassium permanganate. The difference between the introduced volume and the residue determines how much oxalic acid was required to precipitate Ca2+, which will be equivalent to the calcium content in the solution.
According to the method of substitution titration, Ca2+ is isolated as a precipitate of CaC2O4, which is filtered, washed and dissolved in H2SO4 or HC1.
CaC2O4 + H2SO4 → H2C2O4 + CaSO4
The resulting oxalic acid is titrated with a standard solution of potassium permanganate, the amount of which is equivalent to the calcium content in the solution.
Iodometry
The iodometric method of titrimetric analysis is based on the reaction:
I2 + 2e= 2I-; Eo I2 / 3I- = 0.545 V
This equation is written schematically, since in practice, to increase the solubility of I2, a solution of KI is used, which forms a complex with K with I2. Then the equation for iodometric determination looks like this:
The amount of the analyte is judged by the amount of absorbed or released iodine. Substances whose redox potential is below 0.545 V will be reducing agents (SO2, Na2S2O3, SnCl2, etc.) and, therefore, the reaction will proceed with the absorption of iodine. The balance will shift to the right. Substances whose redox potential is greater than 0.545 V will be oxidizing agents (KMnO4, MnO2, K2Cr2O7, Cl2, Br2, etc.) and direct the reaction to the left, towards the release of free iodine.
In this regard, the iodometric method is used both for the determination of reducing agents and oxidizing agents. Iodometric determinations are carried out in an acidic environment, since in an alkaline environment a hypoiodide ion can be formed, the oxidizing ability of which is higher than that of iodine, which can contribute to the occurrence of side processes, in particular, oxidize the thiosulfate ion to sulfate and the results will be distorted.
When determining strong reducing agents (Eo much more than 0.545 V), direct titration is used, and weak ones (Eo close to 0.545 V) are back titrated. The working solution (titrant) is I2 solution. Oxidizing agents are determined only by substitution titration, because when using potassium iodide as a working solution, it is impossible to fix the equivalence point (the moment the iodine release stops). As a titrant in the determination of oxidizing agents, a solution of sodium thiosulfate is used, which interacts with the released iodine (substituent) in an equivalent amount.
Freshly prepared 1% starch solution is used as an indicator in iodometry. When starch interacts with iodine, 2 processes occur - complexation and adsorption, as a result of which a blue compound is formed. The sensitivity of the reaction with starch is high, but decreases sharply with increasing temperature. Starch should be added to the titrated solution only when the main amount of iodine has already been titrated, otherwise the starch forms such a strong compound with an excess of iodine that sodium thiosulfate is overused.
Standardization of sodium thiosulfate solution by potassium dichromate
It is impossible to titrate thiosulfate directly with potassium dichromate, since it reacts non-stoichiometrically with all strong oxidizing agents (dichromate, permanganate, bromate, etc.). Therefore, the substitution method is used, first using the stoichiometric reaction between dichromate and iodide:
Cr2O72- + 6I- + 14H+ = 2Cr3+ + 3I2 + 7H2O (1)
Iodine, which is released in an amount equivalent to dichromate, is titrated with a standard solution of thiosulfate:
I2 + 2S2O32- = 2I- + S4O62- (2)
Reaction (1) requires a high concentration of hydrogen ions, because in an acidic environment, the redox potential of the Cr2O72-/ 2Cr3+ pair increases, i.e. the oxidizing ability of potassium dichromate is enhanced. An excess of I- dissolves the released iodine and lowers the potential of the I3-/3I- redox pair, thus increasing the EMF of reaction (1). Before titrating the released iodine, it is necessary to lower the acidity of the solution by diluting it with water in order to prevent the occurrence of a side reaction:
2H+ + S2O32- = H2S2O3 = H2O + SO2 + S
dichromatometry
The essence of dichromatometric titration
Dichromatometric titration is one of the methods of redox titration based on the use of potassium dichromate K2Cr207 as an oxidizing agent. When exposed to reducing agents, the dichromate ion Cr2O72- acquires six electrons and is reduced to Cr3+
Cr2O72- + 6e + 14H+ = 2Cr3+ + 7H20
Therefore, the molar mass of the equivalent of potassium dichromate is 1/6 of the molar mass. It can be seen from the reaction equation that the reduction of Cr2O72- anions to Cr3+ cations occurs in the presence of H+ ions.
Therefore, titrate with dichromate in an acidic medium. The redox potential of the Cr2O72-/2Cr3+ system is 1.36 V. At [H+] = 1 mol/l. Therefore, in an acidic environment, potassium dichromate is a strong oxidizing agent. Therefore, dichromatometry is successfully used to determine almost all reducing agents determined permanganatometrically. Dichromatometry even has some advantages over permanganatometry.
Potassium dichromate is easily obtained in chemically pure form by recrystallization. Therefore, its standard solution is prepared by dissolving an accurate sample. Potassium dichromate solutions are extremely stable when stored in closed vessels; it does not decompose even when the acidified solution is boiled and practically does not change when the solution is standing.
In addition, potassium dichromate is more difficult than permanganate to be reduced by organic substances. Therefore, it does not oxidize random impurities of organic substances. This also determines the constancy of its titer in solution. Potassium dichromate does not oxidize (without heating) chloride ions. This allows them to titrate reducing agents in the presence of HCl.
The indicator in dichromatometric titration is most often diphenylamine, which turns the solution blue with the slightest excess of dichromate. Diphenylamine belongs to the group of so-called redox indicators (redox indicators). They are redox systems that change color when the reduced form changes to the oxidized form, or vice versa.
If we designate the oxidized form of the indicator Indoxidized. restored form Indrest., and the number of electrons transferred is n, then the transformation of one form of such an indicator into another can be represented by a diagram;
Indooxide ↔ Ind restore -ne-
Each redox indicator is characterized by a certain redox potential. For dephenylamine, it is +0.76 V. The oxidized form of diphenylamine is colored blue, and the reduced form is colorless.
In addition to diphenylamine, redox indicators include ferroin, sodium diphenylaminosulfonate, phenylanthranilic acid, etc.
Fe2+ ions are determined dichromatometrically in HCl solutions or in sulfuric acid solutions. Chloride ions do not interfere with the determination if their concentration does not exceed 1 mol/L.
However, when Fe2+ salts are titrated with dichromate, Fe3+ cations accumulate in the solution, the redox potential of the Fe3+↔Fe2+ system increases, and diphenylamine is oxidized. Therefore, a blue color may appear when the equivalence point has not yet been reached.
To lower the redox potential of the Fe2+ ↔ Fe3+ system, orthophosphoric acid is added to the solution in addition to diphenylamine and hydrochloric acid. The latter masks the interfering Fe3+ ions by binding them into a strong colorless Fe(HP04)+ complex.
Preparation of the potassium dichromate standard solution
A standard solution is prepared by dissolving an accurate weigh of potassium dichromate (chemically pure) in a volumetric flask. Potassium dichromate must first be recrystallized from an aqueous solution and dried at 150°C.
Prepare 100 ml of approximately 0.1 N potassium dichromate standard solution. It was noted above that when interacting with reducing agents in an acidic medium, the dichromate ion Cr2O72- acquires six electrons. Therefore, the molar mass of the equivalent of K2Cr207 is 294.20:6 = 49.03 g / mol, and to prepare 0.1 l of a 0.1 N solution, 49.03 * 0.1 * 0.1 = 0.4903 g of potassium dichromate will be required .
Take about 0.5 g of freshly recrystallized potassium dichromate in a small test tube and weigh it on an analytical balance. Using a funnel, transfer the contents of the test tube to a 100 ml volumetric flask. Weigh the test tube again and find the mass of the sample using the difference
Dissolve a weighed portion of potassium dichromate in distilled water, remove the funnel and, using a pipette, bring the volume of the solution in the flask to the mark. Calculate the titer and normal concentration of the potassium dichromate solution.
Let us assume that the sample of potassium dichromate was 0.4916 g. Then the titer of the solution
T \u003d m / V \u003d 0.4916 / 100 \u003d 0.004916 g / ml,
and normal concentration (molar equivalent concentration)
c \u003d 0.004916 * 1000 / 49.03 \u003d 0.1003.
Determination of iron (II) content in solution
Dichromatometrically, iron is determined mainly in ores, alloys, slags, and other materials. However, upon dissolution, their iron partially transforms into Fe3+ ions. Therefore, before the determination, it is necessary to reduce Fe3+ to Fe2+. This is achieved by the action of metals (or their amalgams), for example, by the action of metallic zinc:
2Fe3++ Zn = 2Fe2+ + Zn2+
Excess zinc is removed from the solution by filtration (for example, through cotton wool). The essence of the reaction used for the dichromatometric determination of Fe2+ can be expressed by the equation
6Fe2++ Сr2О72- + 14Н+ → 6Fe3+ + 2Сr3+ + 7Н20
The determination consists in direct titration of the analyzed solution with a standard solution of potassium dichromate in the presence of diphenylamine:
6FeS04 + K2Cr207 + 7H2S04 = 3Fe 2 (S04)3 + Cr2 (S04)3 + K2S04 + 7H20
1 Cr2O72- + 14H+ + 6e = 2Cr3+ + 7H20
6 Fe 2+ - e \u003d Fe3 +
Sulfuric acid is added to the test solution to maintain a high acidity of the medium and phosphoric acid to bind the accumulated Fe3+ ions, which can prematurely convert diphenylamine into an oxidized (colored) form.
oxidation states
For example:
For example:
Methods for establishing T.E.
To determine the equivalence point in a redox titration, use:
a) non-indicator methods. In the case when the solution of the titrated substance or titrant has a color, TE can be determined by the disappearance or appearance of this color, respectively;
b) specific indicators - changing color when a titrant appears or the analyte disappears. For example, for the J 2 /2J - system, a specific indicator is starch, which stains solutions containing J 2 blue, and for Fe 3+ ions, SCN - ions (thiocyanate ions) are a specific indicator, the resulting complex is colored blood red ;
c) RH (redox-) indicators - changing color when the RH of the system potential changes. One-color indicators - diphenylamine, two-color - ferroin.
Redox indicators exist in two forms - oxidized (Ind ok) and reduced (Ind voss), and the color of one form differs from the other. The transition of the indicator from one form to another and the 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
The RH curves of the RH titration depict the change in the RH potential of the system as the titrant solution is added.
Reductometry, when the oxidizing agent solution is titrated with a standard reducing agent solution
In reductometry, titration curves are calculated:
2) 
3) 
Oxidimetry, when a reducing agent solution is titrated with a standard oxidizing solution
 |
In oxidimetry, titration curves are calculated:
2)
3) 
Example. We calculate the titration curve of 100 cm 3 solution of FeSO 4 with a molar concentration of the equivalent of 0.1 mol / dm 3 with a solution of KMnO 4 of the same concentration.
Reaction equation:
The equilibrium constant of this reaction is
A large numerical value of the equilibrium constant indicates that the equilibrium of the reaction is almost entirely shifted to the right. After adding the first drops of the titrant, two OB 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 the oxidizing agent solution, i.e. titration refers to the method of oxidimetry, the calculation of the titration curve is carried out according to the appropriate scheme.
3) After T.E.
Calculated data for plotting the titration curve
| No. p / p | 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 |
According to the table, we build a titration curve:
For titration error ± 0.1% titration step
∆E \u003d E τ \u003d 1.001 - E τ \u003d 0.999 \u003d 1.47 - 0.95 \u003d 0.52.
For titration error ± 1.0% titration step
∆E \u003d E τ \u003d 1.01 - E τ \u003d 0.99 \u003d 1.49 - 0.89 \u003d 0.60.
In the TE region, when going from a solution under-titrated by 0.1% to a solution overtitrated by 0.1%, the potential changes by more than 0.5 V. potential change. In addition, in this case, a colored solution is used as a titrant, therefore, T.E. can be identified by the appearance of a faint pink color from one excess drop of potassium permanganate.
PERMANGANATOMETRY
The method is based on the oxidation of solutions of reducing agents with KMnO 4 potassium permanganate. The oxidation of reducing agents can be carried out in various media, and manganese (VII) is reduced in an acidic medium to Mn 2+ ions, in a neutral one to manganese (IV) and in an alkaline medium to manganese (VI). Usually, in the method of permanganatometry, the reaction is carried out in an acidic medium. In this case, a half-reaction occurs
It is impossible to prepare a titrated solution according to an exact sample, because it contains in its composition. Therefore, first prepare a solution of approximately the desired concentration, leave it in a dark bottle for 7-10 days, filter out the precipitated, and then set the exact concentration of the resulting solution. Standardization of the solution is carried out according to a titrated solution of oxalic acid ( 
) or sodium oxalate ().
The indicator is permanganate itself, colored red-violet. The end of the reaction is easily determined by the color change 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, for every 10°C rise in temperature, the reaction rate increases by a factor of 2-3. The oxidation reaction with oxalic acid permanganate is carried out at a temperature of 70-80 °C. Under these conditions, the 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 reactants 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 with oxalic acid permanganate can be catalytically accelerated by the addition of MnSO 4 , whose role is as follows:
The resulting manganese dioxide oxidizes oxalic acid, being reduced 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 oxidation reaction of oxalic acid 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 titration of a hot acidified solution of oxalic acid become colorless slowly. As a small amount of Mn 2+ ions are formed, further discoloration of 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 a change oxidation states atoms of the substances involved in the reaction.
Substances whose atoms during the reaction lower their oxidation state due to the addition of electrons are called oxidizing agents, i.e. they are electron acceptors. In this case, the oxidants themselves are restored. Reducing agents, being electron donors, are oxidized.
The reduction product of an oxidizing agent is called the reduced form, and the oxidation product of the reducing agent is called 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 half-pairs (redox pairs) of the redox system.
All OM processes (redox reactions) can be divided into three types
a) intermolecular, when during the reaction of OB the transfer of electrons occurs between particles of different substances. for example
In this reaction, the role of an oxidizing agent in the presence of H 3 O + is played by ions, and ions act as a reducing agent
b) dismutation (disproportionation), during which the transition 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 higher.
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.
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