Anodic Chronopotentiometry with Graphite Electrode. Analytical Applications

Abstract

Electrolysis at constant current (chronopotentiometry) using a graphite indicating electrode was investigated. Poor results were obtained with inorganic reductions; much better results with organic oxidations. Electrolysis of the three dihydroxybenzene isomers at pH 5.5 results in oxidation of hydroquinone and catechol to pand o-benzoquinone, respectively, and in destructive oxidation or postelectrolytic polymerization in the case of resorcinol. Binary and ternary mixtures of the dihydroxybenzene isomers can be analyzed quantitatively; if both hydroquinone and catechol are present, the concentration of one has to be determined separately. Anodic chronopotentiometry of the three isomeric phenylenediamines indicates the differing nature of the mechanisms involved. At pH 2, 5, and 11, o- and p-phenylenediamine undergo a 2e oxidation, presumably to the diimine; at pH 5.5 the ortho isomer undergoes a subsequent 1e process. The latter may be due to reaction of the diimine with o-phenylenediamine to produce diaminophenazine, which is oxidized by a 1e step to a free radical which then polymerizes. The meta isomer seems to undergo at pH 5.5 a 3e oxidation to a final polymeric product and at pH 11.2 a 2e oxidation which leads either to an insoluble polymer or to destruction of the compound.