The Polarographic Reduction of Acids in Pyridine, The Cause of the Differences in Half-Wave Potentials with Various Supporting Electrolytes

The polarographic reductions of acetic, benzoic, p-toluenesulfonic, and perchloric acids have been examined in pyridine solutions with tetrabutylammonium, tetraethylammonium, sodium, and lithium perchlorate as the supporting electrolytes. The difference in the half-wave potentials of the acids was explained in terms of the difference in the degree of interaction between the pyridinium ion and the conjugate bases (e.g., CH3COO−) made of the acids. The alkali-metal ions interact with the acetate or benzoate ion to form RCOO−(M+)2 (M=Li and Na). The formation constants of RCOO−(M+)2 were evaluated by means of the positive shift in the half-wave potential of the pyridinium ion in the presence of a large excess of the alkali metal ions. Substituted acetic and benzoic acids and phenols were also examined so as to ascertain the effect of the substituent groups.