Potentiometric titration, acid-base oxidation-reduction

Potentiometric titration can determine the end point more accurately than the color indicators. Thus, the quantitative consumption of a titrant in an acid-base neutralization, oxidation-reduction reaction, or complex formation reaction can be determined precisely and very accurately by potentiometric titration. The titration involves the addition of large increments of the titrant to a measured volume of the sample at the initial phase and, thereafter, adding smaller and smaller increments as the end point approaches. The cell potential is recorded... 

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. 

Since potentiometric titrations are an old and well-known technique, particularly in regard to acid-base and oxidation-reduction titrations, only a few selected examples will be presented here. For more detailed treatments, the student is urged to consult the bibliography at the end of the chapter. It will be assumed that the student is already familiar with titration curves and their calculation from ionic equilibria and other pertinent data. 

The enhancement of the current-function for the anodic wave (I), which involves proton dissociation, upon addition of pyridine is evident [Fig. 1(a) and (b)], whereas the decrease of the current-function for wave (II) conceivably results from the reaction of pyridine with the complex product of the first oxidation (however, in other complexes, namely of the dinuclear dicarbene type [3], this current-function is promoted by base). The second anodic process, which possibly involves a sequence of electron transfer/deprotonadon steps (> 4 electrons, by CPE), is more affected by the temperature and scan rate than the first one (which involves a smaller number of electrons) either a decrease of the temperature [Fig. 1(c), (a)] or an increase of the scan rate leads to a reduction of the current function of wave (II) relative to that of wave (I), by hampering the sequence of the chemical steps involved. Potentiometric titration of the exhaustively electrolyz solution at the second wave indicates the presence of an acidic product (besides the liberated protons), possibly with an acidic methylene group. 

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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