Polarography of α-Keto Acids in Aqueous and Nonaqueous Solutions

Abstract

Abstract Phenylglyoxylic acid (I), phenylpyruvic acid (II), and pyruvic acid (III) were polarographically studied in buffer solutions and in DMF. Their reduction waves were reinvestigated in a wider pH range. Cyclic voltammetry showed that most electrode reactions of these acids in acidic pH are irreversible. pH-Dependence of E1⁄2 and limiting current was studied. The observed decrease of the limiting currents of II and III in the strongly alkaline region was attributed to the dissociation of their corresponding enol forms. The i-E curves of these acids in DMF showed successive poorly-defined reduction waves, while those of the corresponding tetraethylammonium salts (TEA salts) were found to be much simpler in shape. A small oxidation wave was also observed with TEA salts of II and III, suggesting the presence of the enol form in DMF. The reduction products of I and its salt in DMF were studied by means of controlled potential electrolysis and NMR-spectroscopy. The electrode reaction mechanism of α-keto acids in nonaqueous solution is discussed, emphasizing the role of the free acid form as a proton-donor and the formation of a radical anion followed by a disproportionation reaction.