Titrations, redox

Redox titrations are based on the transfer of electrons between the titrant and the analyte. These types of titrations are usually followed by potentiometry, although dyes which change colour when oxidised by excess titrant may be used. 

Reduction potential is a measure of how thermodynamically favourable it is for a compound to gain electrons. A high positive value for a reduction potential indicates that a compound is readily reduced and consequently is a strong oxidising agent, i.e. it removes electrons from substances with lower reduction potentials. The oxidised and reduced form of a substance are known as a redox pair. Table 3.2 lists the standard reduction potentials for some typical redox pairs. 

A substance with a higher reduction potential will oxidise one with a lower reduction potential. The difference in potential between two substances is the reaction potential and is approximately the potential difference which would be 

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Where a reference electrode has a reduction potential 0, then the predicted reading of the potential for a redox pair is obtained by subtracting the reduction potential for the reference electrode, e.g. for an Ag/AgCI reference electrode 0.223 V is subtracted. 

REDOX titrations are titrations that involve the processes of oxidation and reduction. These two processes always occur together and are of huge importance in chemistry. Everything from simple ionic reactions to the generation of energy within human mitochondria depends on these two processes. 

Other REDOX reagents include iodine (I2), either by itself in a forward titration or in a back titration with sodium thiosulfate (Na2S2Os), and complex salts of the metal cerium (such as ammonium cerium sulfate, 

Ce(S04)2-2(NH4)2S04,2H20). Salts of this type are complex by name as well as by formula, but in reality behave as 

Bromine is a volatile liquid at room temperature and pressure and so cannot be measured accurately by pipette. It is also an extremely corrosive compound, irritant to eyes, lungs and mucous membranes. To overcome these difficulties, the bromine required for reaction with the resorcinol is generated in situ by reaction of potassium bromate and potassium bromide in the presence of strong mineral acid. 

To ensure that the bromination reaction proceeds quantitatively to the right-hand side, an excess of bromine is generated and the volume of bromine that does not react with resorcinol is determined by back titration. Bromine cannot be titrated easily, so the excess bromine is determined by addition of an excess of potassium iodide and titration of the liberated iodine with sodium thiosulfate, to give sodium iodide and sodium tetrathionate. 

Like acid-base titrations, redox titrations normally require an indicator that clearly changes color. In the presence of large amounts of reducing agent, the color of the 

these oxidizing agents can thanselves be used as internal indicator in a redox titration because they have distinctly different colors in the oxidized and reduced forms. 

Redox titrations require the same type of calculations (based on the mole method) as acid-base neutralizations. The difference is that the equations and the stoichiometry tend to be more complex for redox reactions. The following is an example of a redox titration. 

42-mL volume of 0.1327 M KMn04 solution is needed to oxidize 25.00 ruL of a FeS04 solution in an acidic medium. What is the concentration of the FeS04 solution in molarity The net ionic equation is 

Strategy We want to calculate the molarity of the FeS04 solution. From the definition of molarity 

Permanent magnet levitates above superconducting disk cooled in a pool of liquid nitrogen. Redox titrations are crucial in measuring the chemical composition of a superconductor. [Photo courtesy D. Cornelius, Michelson Laboratory, with materials from 1 Vanderah.J 

A prototypical high-temperature superconductor is yttrium barium copper oxide, YBa2Cu307, in which two-thirds of the copper is in the +2 oxidation state and one-third is in the unusual +3 state. Another example is BijSrjlCaQgYoyjCujOgjys, in which the average oxidation state of copper is +2.105 and the average oxidation state of bismuth is +3.090 (which is formally a mixture of Bi3+ and Bi5+). The most reliable means to unravel these complex formulas is through wet oxidation-reduction titrations, described in this chapter. 

A redox titration is based on an oxidation-reduction reaction between analyte and titrant. In addition to the many common analytes in chemistry, biology, and environmental and materials science thai can be measured by redox titrations, exotic oxidation states of elements in uncommon materials such as superconductors and laser materials are measured by redox titrations. For example, chromium added to laser crystals to increase their efficiency is found in the common oxidation states +3 and +6, and the unusual +4 state. A redox titration is a good way to unravel the nature of this complex mixture of chromium ions. 

This chapter introduces the theory of redox titrations and discusses some common reagents. A few of the oxidants and reductants in Table 16-1 can be used as titrants.2 Most reductants react with 02 and require protection from air to be used as titrants. 

Iron and its compounds are environmentally acceptable redox agents that are finding increased use in remediating toxic waste in groundwaters 1 

One of the most common oxidants is potassium permanganate which in acidic solution can undergo the following reaction  

Unfortunately, potassium permanganate is not obtainable in high enough purity and can undergo decomposition by exposure to sunlight. Therefore it cannot be used as a primary standard (p. 143). However, it can be used in redox titrations provided it is standardized with sodium oxalate (which is available in high purity). The redox reaction involving oxalate is as follows  

The overall reaction between permanganate and oxalate can be obtained by balancing the electrons on each side of the equation. This can be achieved by multiplying eqn [24.1] by 2 and eqn [24.2] by 5, and then combining them as 

Another common method for the standardization of potassium permanganate is to use iron (II)  

Potassium permanganate has a major advantage when used for titrations in that it can act as its own indicator. 

When a bright platinum plate is immersed in a solution containing the oxidised and the reduced forms of a system, of equal unit activity (concentration) eg Fe and Fe in aqueous solution, the potential of the Pt electrode can be measured by coupling it with a 

Although a reagent e.g. permanganate may act as a self indicator because of its intense colour and the very faint colour of its reduced form Mn in acid solution, generally a redox indicator is necessary to find the end-point visually. A redox indicator is usually an organic molecule which has different colours in the oxidised and reduced forms i.e. In ,( and ln d. The ratio of their concentration is given by  

In some cases, an external indicator is used i.e. drops of the solution are withdrawn near the end-point and are used as spot tests for a product. Tbe redox titration curve is similar to that of acid/base titration curves since the logarithmic function of (2.33) is similar to that of (2.28). It is worth noting here that E values of redox couples are greatly altered in presence of complexing agents. Only in a few acid/base titrations does pK. change in presence of other compounds e.g. the addition of polyhydroxyl organics increases K, of boric acid markedly. 

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Equations for the principal methods for the redox determinations of the elements are given in Table 11.29. Volumetric factors in redox titrations for the common titrants are given in Table 11.28. 

Types of Chelometric Titrations. Chelometric titrations may be classified according to their manner of performance direct titrations, back titrations, substitution titrations, redox titrations, or indirect methods. 

Examples of titration curves for (a) a complexation titration, (b) a redox titration, and (c) a precipitation titration. 

To evaluate a redox titration we must know the shape of its titration curve. In an acid-base titration or a complexation titration, a titration curve shows the change in concentration of H3O+ (as pH) or M"+ (as pM) as a function of the volume of titrant. For a redox titration, it is convenient to monitor electrochemical potential. 

Sketching a Redox Titration Curve As we have done for acid-base and complexo-metric titrations, we now show how to quickly sketch a redox titration curve using a minimum number of calculations. 

How to sketch a redox titration curve see text for explanation. 

The equivalence point of a redox titration occurs when stoichiometrically equivalent amounts of analyte and titrant react. As with other titrations, any difference between the equivalence point and the end point is a determinate source of error. 

Where Is the Equivalence Point In discussing acid-base titrations and com-plexometric titrations, we noted that the equivalence point is almost identical with the inflection point located in the sharply rising part of the titration curve. If you look back at Figures 9.8 and 9.28, you will see that for acid-base and com-plexometric titrations the inflection point is also in the middle of the titration curve s sharp rise (we call this a symmetrical equivalence point). This makes it relatively easy to find the equivalence point when you sketch these titration curves. When the stoichiometry of a redox titration is symmetrical (one mole analyte per mole of titrant), then the equivalence point also is symmetrical. If the stoichiometry is not symmetrical, then the equivalence point will lie closer to the top or bottom of the titration curve s sharp rise. In this case the equivalence point is said to be asymmetrical. Example 9.12 shows how to calculate the equivalence point potential in this situation. 

The most important class of redox indicators, however, are substances that do not participate in the redox titration, but whose oxidized and reduced forms differ in color. When added to a solution containing the analyte, the indicator imparts a color that depends on the solution s electrochemical potential. Since the indicator changes color in response to the electrochemical potential, and not to the presence or absence of a specific species, these compounds are called general redox indicators. 

A -visual indicator used to signal the end point in a redox titration. 

Finding the End Point Potentiometrically Another method for locating the end point of a redox titration is to use an appropriate electrode to monitor the change in electrochemical potential as titrant is added to a solution of analyte. The end point can then be found from a visual inspection of the titration curve. The simplest experimental design (Figure 9.38) consists of a Pt indicator electrode whose potential is governed by the analyte s or titrant s redox half-reaction, and a reference electrode that has a fixed potential. A further discussion of potentiometry is found in Chapter 11. 

Experimental arrangement for recording a potentiometric redox titration curve. 

This is an indirect method of analysis because the chlorine-containing species do not react with the titrant. Instead the total chlorine residual oxidizes l to l3 , and the amount of 13 is determined by the redox titration with Na282 03. 

As with acid-base and complexation titrations, redox titrations are not frequently used in modern analytical laboratories. Nevertheless, several important applications continue to find favor in environmental, pharmaceutical, and industrial laboratories. In this section we review the general application of redox titrimetry. We begin, however, with a brief discussion of selecting and characterizing redox titrants, and methods for controlling the analyte s oxidation state. 

A reagent used to reduce the analyte before its analysis by a redox titration. 

Another important example of a redox titration for inorganic analytes, which is important in industrial labs, is the determination of water in nonaqueous solvents. The titrant for this analysis is known as the Karl Fischer reagent and consists of a mixture of iodine, sulfur dioxide, pyridine, and methanol. The concentration of pyridine is sufficiently large so that b and SO2 are complexed with the pyridine (py) as py b and py SO2. When added to a sample containing water, b is reduced to U, and SO2 is oxidized to SO3. 

The amount of Fe in a 0.4891-g sample of an ore was determined by a redox titration with K2Cr20y. The sample was dissolved in HCl and the iron brought into the +2 oxidation state using a Jones reductor. Titration to the diphenylamine sulfonic acid end point required 36.92 mL of 0.02153 M K2Cr20y. Report the iron content of the ore as %w/w FeyOy. 

The scale of operations, accuracy, precision, sensitivity, time, and cost of methods involving redox titrations are similar to those described earlier in the chapter for acid-base and complexometric titrimetric methods. As with acid-base titrations, redox titrations can be extended to the analysis of mixtures if there is a significant difference in the ease with which the analytes can be oxidized or reduced. Figure 9.40 shows an example of the titration curve for a mixture of Fe + and Sn +, using Ce + as the titrant. The titration of a mixture of analytes whose standard-state potentials or formal potentials differ by at least 200 mV will result in a separate equivalence point for each analyte. 

The purity of ferrous ammonium sulfate is determined by a redox titration with K2Gt207, using the weight of the reagents as the signal in place of volume. 

The content of ascorbic acid, in milligrams per 100 mL, in orange juice is determined by a redox titration using either 2,6-dichlorophenolindephenol or N-bromosuccinimide as the titrant. 

Calculate or sketch (or both) titration curves for the following (unbalanced) redox titration reactions at 25 °C. Assume that the analyte is initially present at a concentration of 0.0100 M and that a 25.0-mL sample is taken for analysis. The titrant, which is the underlined species in each reaction, is 0.0100 M. 

The amount of Cr + in inorganic salts can be determined by a redox titration. A portion of sample containing... 

Potcntiomctric Titrations In Chapter 9 we noted that one method for determining the equivalence point of an acid-base titration is to follow the change in pH with a pH electrode. The potentiometric determination of equivalence points is feasible for acid-base, complexation, redox, and precipitation titrations, as well as for titrations in aqueous and nonaqueous solvents. Acid-base, complexation, and precipitation potentiometric titrations are usually monitored with an ion-selective electrode that is selective for the analyte, although an electrode that is selective for the titrant or a reaction product also can be used. A redox electrode, such as a Pt wire, and a reference electrode are used for potentiometric redox titrations. More details about potentiometric titrations are found in Chapter 9. 

End Point Determination Adding a mediator solves the problem of maintaining 100% current efficiency, but does not solve the problem of determining when the analyte s electrolysis is complete. Using the same example, once all the Fe + has been oxidized current continues to flow as a result of the oxidation of Ce + and, eventually, the oxidation of 1T20. What is needed is a means of indicating when the oxidation of Fe + is complete. In this respect it is convenient to treat a controlled-current coulometric analysis as if electrolysis of the analyte occurs only as a result of its reaction with the mediator. A reaction between an analyte and a mediator, such as that shown in reaction 11.31, is identical to that encountered in a redox titration. Thus, the same end points that are used in redox titrimetry (see Chapter 9), such as visual indicators, and potentiometric and conductometric measurements, may be used to signal the end point of a controlled-current coulometric analysis. For example, ferroin may be used to provide a visual end point for the Ce -mediated coulometric analysis for Fe +. 

Coulometric Titrations Controlled-current coulometric methods commonly are called coulometric titrations because of their similarity to conventional titrations. We already have noted, in discussing the controlled-current coulometric determination of Fe +, that the oxidation of Fe + by Ce + is identical to the reaction used in a redox titration. Other similarities between the two techniques also exist. Combining equations 11.23 and 11.24 and solving for the moles of analyte gives... 

Controllcd-Currcnt Coulomctry The use of a mediator makes controlled-current coulometry a more versatile analytical method than controlled-potential coulome-try. For example, the direct oxidation or reduction of a protein at the working electrode in controlled-potential coulometry is difficult if the protein s active redox site lies deep within its structure. The controlled-current coulometric analysis of the protein is made possible, however, by coupling its oxidation or reduction to a mediator that is reduced or oxidized at the working electrode. Controlled-current coulometric methods have been developed for many of the same analytes that may be determined by conventional redox titrimetry. These methods, several of which are summarized in Table 11.9, also are called coulometric redox titrations. 

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