Amperometric titrations oxidation-reduction reaction

Phenylarsine oxide, C6H5As = O, is as effective as sodium thiosulfate in reducing iodine. It is more stable than thiosulfate. An advantage is that it is stable even in dilute solution. This substance is, however, highly toxic and is a suspected carcinogen. Because of its toxicity, its application is limited. One such application is in the amperometric titration of residual chlorine. The oxidation-reduction reaction of PAO is similar to thiosulfate. Its equivalent weight in iodine reaction is 168. 

Another important amperometric titrant is bromine solution, which undergoes stoichiometric oxidation-reduction reactions with many substances such as As(III), Sb(III), ammonium salts, and others. Often the titration involves adding an excess of KBr to an acidified solution of the substance to be oxidized and then titrating it with potassium bromate solution. Bromine is thereby generated in situ. 

Polarisation titrations are often referred to as amper-ometric or biamperometric titrations. It is necessary that one of the substances involved in the titration reaction be oxidisable or reducible at the working electrode surface. In general, the polarisation titration method is applicable to oxidation-reduction, precipitation and complex-ation titrations. Relatively few applications involving acid/base titration are found. Amperometric titrations can be applied in the determination of analyte solutions as low as ICE5 M to 10-6 M in concentration. 

The endpoint may be detected by addition of colored indicators, provided the indicator itself is not electroactive. Potentiometric and spectrophotometric indication is used in acid-base and oxidation-reduction titrations. Amperometric procedures are applicable to oxidation-reduction and ion-combination reactions especially for dilute solutions. 

The shape of the amperometric titration curve in this case, where both the titrant and the substance titrated undergo reversible redox reactions, is illustrated in Figure 3.21A. In the case where the substance titrated does not have a reversible voltammetric wave, the titration curve will have the shape illustrated in Figure 3.2IB. Prior to the equivalence point, the applied voltage is too small to cause both oxidation and reduction of the redox couple of the substance titrated. If the titrant has an irreversible wave, the titration curve will look like that in Figure 3.21C. This type of titration is commonly called a dead-stop titration, because the indicator current falls to zero at the equivalence point. 

Among them, volumetric methods are presumably the most widely used for water analysis. They are titrimetric techniques which involve a chemical reaction between a precise concentration of a reagent or titrant and an accurately known volume of sample. The most common types of reactions as used within this method are acid-base neutralization, oxidation-reduction, precipitation, and complexation. The use of an indicator which identifies the equivalence point is required to develop this kind of method. The modem laboratories usually employ automated endpoint titrators, which largely improve the efficiency and reliability of the determination. Moreover, spectrophotometric, potentiometric, or amperometric methods to determine the endpoint of the reaction can... 

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