Oxidation REDOX titrations

Oxidation-Reduction Titrations. Potentiometry can be used to follow reduction-oxidation (redox) titrations. For example, the oxidation of stannous ions by ceric ions follows the chemical reaction... 

Fig 3. An oxidative redox titration of the rate of PgjQ oxidation before ( — > preillumination, after 15 min aerobic treatment (O—O) and after 4 hr anaerobic treatment (A—A),... 

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. 

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 oxidize 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. 

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. 

A common laboratory technique for determining the concentration of a solute is titration (Fig. L.2). Titrations are usually either acid-base titrations, in which an acid reacts with a base, or redox titrations, in which the reaction is between a reducing agent and an oxidizing agent. Titrations are widely used to monitor water purity and blood composition and for quality control in the food industry. 

The concentration of Fe2+ ions in an acid solution can be determined by a redox titration with either KM11O4 or K2Cr207. The reduction products of these reactions are Mir4 and Cr5+, respectively, and in each case the iron is oxidized to Fe3+. In one titration of an acidified Fe2 solution, 25.20 mL of 0.0210 m K2Cr207(aq) was required for complete reaction. If the titration had been carried out with 0.0420 M KMn()4(aq), what volume of the permanganate solution would have been required for complete reaction ... 

The one-electron reduction of the Ni-C form results in the diamagnetic species Ni-R. From the redox titration studies of Lindahl s group, a plausible catalytic cycle can be postulated where the enzyme in the Ni-Sl state binds H2 (77) and becomes the two-electron more reduced Ni-R state. Sequential one-electron oxidations from Ni-R to Ni-C and then to Ni-Sl will close the cycle (Fig. 6). The various redox states differ not only in the extent of their reduction, but also in their protonation, as shown by the pH dependence of their redox potentials (87). It is remarkable that both EPR (which monitors the magnetic... 

If existence of a persulphide or other potentially electron accepting sulphur group is confirmed, this might explain why redox titration experiments have shown the number of electron equivalents which the xanthine oxidase molecule can accept to be greater than is required for reduction of the three non-protein components (58, 91). Certainly, this interpretation seems more probable than the original suggestion (58, 91) that the molybdenum can be reduced to lower oxidation states than Mo(IV) by some substrates. 

FIGURE 13.2 An EPR-monitored redox titration of an Fe-O-Fe cluster with three stable oxidation states. The dinuclear iron center (= +210 mV and = +50 mV) in Pyrococcus furio-sus ferritin was titrated in the presence of a mediator mix. The fit is based on Equation 13.14. (Data from Tatur and Hagen 2005.)... 

Figure 5.10 Redox titration of the Ni-C EPR signal in D. gigas hydrogenase, in the presence of mediators under partial pressure of H2. (A) Titration monitored by EPR spectroscopy (data from Cammack et al. 1982, 1987).The data points were obtained by removing samples from a vessel as shown in Fig. 5.8. Data NiA signal A NiC signal. (B) Titration monitored by FTIR spectroscopy (data from De Lacey et al. 1997).The spectra were recorded directly in a sealed optically transparent thin-layer electrode cell. Note that the oxidized and reduced species, which are undetectable by EPR, can be measured. Data o I946cm (NiB state) 1914+ 1934cm (NiSR state) A 1952cm (NiA state) 1940cm (NiR state).

D. gigas hydrogenase can be considered a case study for the application of Mossbauer and EPR spectroscopies in conjunction with redox titration methodologies. H2 (the substrate/product of the reaction) was used to control the redox state of the enzyme by varying the partial pressure of the gas. By doing so, several samples of the enzyme were obtained in different oxidation states and investigated in parallel both by Mossbauer and EPR spectroscopies. 

The end point of a redox titration can be detected by use of a redox indicator, that is, a redox couple (0,R) in which the oxidized and reduced forms of the couple have distinctly different UV-visible spectra and hence, present different colours to the naked eye ... 

---