Redox titrants

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. 

Redox titrants (mainly in acetic acid) are bromine, iodine monochloride, chlorine dioxide, iodine (for Karl Fischer reagent based on a methanolic solution of iodine and S02 with pyridine, and the alternatives, methyl-Cellosolve instead of methanol, or sodium acetate instead of pyridine (see pp. 204-205), and other oxidants, mostly compounds of metals of high valency such as potassium permanganate, chromic acid, lead(IV) or mercury(II) acetate or cerium(IV) salts reductants include sodium dithionate, pyrocatechol and oxalic acid, and compounds of metals at low valency such as iron(II) perchlorate, tin(II) chloride, vanadyl acetate, arsenic(IV) or titanium(III) chloride and chromium(II) chloride. 

Rizk et al. [3] used 2,3-dichloro-5,6-dcyano-/)-benzoquinone as a redox titrant in the aqueous titration of penicillamine. Finely ground tablets were mixed with H20 and the mixture was filtered. The filtrate (or an injectable solution) was diluted with H20 and acidified with H3PO4 before titration with the redox titrant. The titration was conducted in anhydrous acetic acid using thiethylperazine dihydrochloride as the indicator. The endpoint was detected by a color change to green, and recoveries of penicillamine were 98.4-100.5%. 

Berka, A., J. Vulterin and J. Zyka, Newer Redox Titrants, New York, Pergamon Press Inc., 1965. 

A. BERKA, j. vuLTERiN, and j. ZYKA Newer Redox Titrants, translated by H. Weisz, Pergamon, New York, 1965. 

Reduction of horse cytochrome C with [Colsepll ", [Co(diAMsar)]2+, and [Co(NOcapten)]2+ cations was reported in Refs. 316-320. The intrinsic reactivity of these complexes with proteins make it possible the use of clathrochelates as potential protein redox titrants, electrochemical mediators, and electrode modifiers. 

Relatively simple syntheses for the majority of macrobicyclic complexes, compared with conventional techniques for the preparation of macrocyclic compounds, have made such complexes attractive not only for research, but also for practical application as electron carriers, catalysts for electro- and photochemical processes, and some other purposes (e.g., protein redox titrants, biological electrochemical mediators, and ionophore and electrode modifiers). 

This method was developed to overcome several disadvantages of the earlier dithionite titration method, the most severe being the complexation of the flavoprotein oxidases by bisulfite, an oxidation product of dithionite [15]. The present method presents additional advantages it is able to generate many redox titrants in situ without standardization by titration, and has a more flexible experimental design, as indicated in the following section. 

Berka, A., Vulterin, J. and Zyka, J., Newer Redox Titrants, Pergamon, Oxford, 1965. 

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. 

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. 

You will recall from Chapter 6 that the Nernst equation relates the electrochemical potential to the concentrations of reactants and products participating in a redox reaction. Consider, for example, a titration in which the analyte in a reduced state, Ared) is titrated with a titrant in an oxidized state, Tox- The titration reaction is... 

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. 

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. 

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. 

The redox half-reaction when 1 is used as a titrant is... 

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. 

In this titration the analyte is oxidized from Fe + to Fe +, and the titrant is reduced from CryOy to Cr +. Oxidation of Fe + requires only a single electron. Reducing CryOy, in which chromium is in the +6 oxidation state, requires a total of six electrons. Conservation of electrons for the redox reaction, therefore, requires that... 

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. 

Thus far we have examined titrimetric methods based on acid-base, complexation, and redox reactions. A reaction in which the analyte and titrant form an insoluble precipitate also can form the basis for a titration. We call this type of titration a precipitation titration. 

Titrimetric methods have been developed using acid-base, complexation, redox, and precipitation reactions. Acid-base titrations use a strong acid or strong base as a titrant. The most common titrant for a complexation titration is EDTA. Because of their... 

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. 

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. 

The purity of a sample of Na2S203 was determined by a coulometric redox titration using as a mediator, and as the titrant. A sample weighing 0.1342 g is transferred to a 100-mL volumetric flask and diluted to volume with distilled water. A 10.00-mL portion is transferred to an electrochemical cell along with 25 mL of 1 M KI, 75 mL of a pH 7.0 phosphate buffer, and several drops of a starch indicator solution. Electrolysis at a constant current of 36.45 mA required 221.8 s to reach the starch indicator end point. Determine the purity of the sample. 

Description of the Method. The concentration of Cr207 in a sample is determined by a coulometric redox titration using Fe + as a mediator and electrogenerated Fe + as the "titrant." The end point of the coulometric redox titration is determined potentiometrically. 

In a titration the analytical utility of the measured potential Hes not in its value, which may drift or be otherwise unstable, but in the magnitude of the change of its value near an end point. In a redox titration, the potential changes from something close to the of the analyte to something close to the E of the titrant. This works fine provided the electrochemistries of both analyte and titrant are reversible. The technique may fail, however, if the electrode responds slowly to concentration changes because of irreversibiUty. 

The Kad Fischer jack on the back of most pH meters, used to monitor Kad Fischer titrations, suppHes a constant regulated current to the cell, which can consist of two identical (platinum) working electrodes. The voltammograms shown in Figure 9 illustrate the essential features of this technique. The initial potential difference, AH, is small because both redox forms of the sample coexist to depolarize the electrodes. The sample corresponds to the wave on the right-hand (cathodic) side of each figure and is therefore easily oxidized. The titrant is represented by the wave on the left-hand (anodic) side and is therefore easily reduced. Halfway to the end point the potential difference,, remains small, but at the end point the potential difference,... 

The holistic thermodynamic approach based on material (charge, concentration and electron) balances is a firm and valuable tool for a choice of the best a priori conditions of chemical analyses performed in electrolytic systems. Such an approach has been already presented in a series of papers issued in recent years, see [1-4] and references cited therein. In this communication, the approach will be exemplified with electrolytic systems, with special emphasis put on the complex systems where all particular types (acid-base, redox, complexation and precipitation) of chemical equilibria occur in parallel and/or sequentially. All attainable physicochemical knowledge can be involved in calculations and none simplifying assumptions are needed. All analytical prescriptions can be followed. The approach enables all possible (from thermodynamic viewpoint) reactions to be included and all effects resulting from activation barrier(s) and incomplete set of equilibrium data presumed can be tested. The problems involved are presented on some examples of analytical systems considered lately, concerning potentiometric titrations in complex titrand + titrant systems. All calculations were done with use of iterative computer programs MATLAB and DELPHI. 

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