Oximes half-wave potentials

Fig. 13. Dependence of half-wave potentials for the reduction of substituted benzenes on the total polar substituent constant, ax- Examples shown benzophenones [pH 0], benzophenone-oximes [pH 0], thiobenzophenones [pH 0], nitrobenzenes [pH 2-0], azo derivatives [pH 2-6]. (Taken from Zuman, 1969.)...

Polarography of oximes and phenylhydrazones in aqueous buffers at pH 1.0. All give four-electron waves with half-wave potential dependent on pH. 

Reduction of aliphatic oximes is specifically catalysed by protons, The process exhibits a single four-electron wave on polarography and Ey, is dependent on pH. Some half-wave potentials are given in Table 10.7, Around pH 6-8 this wave decreases in height and finally disappears because protons can no longer be supplied... 

Fig. 26. Relation of half-wave potentials for the reduction of substituted benzene derivatives in acid media to Hammett substituent constant trx. Benzophenones, benzophenone oximes and thiobenzophenones at pH 0 nitrobenzenes at pH 2,0 azo dyes at pH 2.6...

Cis- and /ra s-isomers sometimes differ in their half-wave potentials, so that the individual isomers can be distinguished and in some instances their mixtures analysed. Differences in the values for cis- and trans-isomers have been reported for azocompounds (115, 116) (but cf. (117 bis 119)), oximes (120), unsaturated carbonyl compounds (121—123), unsaturated hydrocarbons (124), etc. In some cases, the heights of the waves of isomers at a given pH, or the pK values differ, so that mixtures of isomers can be analysed. This is possible for mixtures of fumaric and maleic acids (125—128), citraconic and mesaconic acids (125), etc. 

A further example is the determination of pyridoxal and pyridoxal-5-phosphate mixtures. In alkaline solution the phosphoric acid grouping of the pyridoxal-5-phosphate undergoes dissociation. The effect of the ionized group causes a shift more than 0-3 V to more negative values when compared with pyridoxal (Fig. 32). At pH values below 10 the half-wave potentials of pyridoxal and pyridoxal-5-phosphate differ so little that their waves coalesce. However, if transformed into the corresponding oximes, it is possible to distinguish the waves by employing a derivative circuit (Fig. 33). Finally in acid solution, where the half-wave potentials coincide, the analysis of the mixture can be... 

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