VOLTAMMETRIC REDUCTION OF POLYPROTIC ACIDS AT THE PLATINUM MICROELECTRODE - DEPENDENCE ON SUPPORTING ELECTROLYTE

The voltammetric behavior of weak and strong polyprotic acids was stud ied at platinum microelectrodes under steady-state conditions in solut ions of very low ionic strength, including those without added support ing electrolyte. Three diprotic acids (sulfuric, oxalic, and malonic), one triprotic acid (phosphoric acid), and one tetraprotic acid (pyrop hosphoric acid) were chosen for the investigation. The reduction of hy drogen ion in solutions of strong diprotic acid (sulfuric acid) with n o supporting electrolyte results in a voltammetric wave 1.5 times high er than the diffusional wave obtained with excess electrolyte. This ag rees well with the theoretical prediction for one-electron reduction o f monovalent cation accompanied by divalent anion. The voltammetric re duction waves of oxalic, malonic, and phosphoric acids in solutions wi thout supporting electrolyte were twice as high as the diffusion-contr olled waves with excess electrolyte. This is what theory predicts for a monoprotic strong acid. The strong (H+) and weak (HA(-) for oxalic a nd malonic, H(2)A(-) for phosphoric, and H(2)A(2-) for pyrophosphoric acid) forms of acids are reduced in one (oxalic and malonic) or two (p hosphoric and pyrophosphoric) voltammetric waves. The influence of the concentration of supporting electrolyte on the height of reduction wa ves of acids was examined for two kinds of supporting electrolyte, wit h mono- and divalent cations, over a wide range of their concentration s. The experimental results are compared with theoretical predictions. The diffusion coefficients of H(COO)2(-) and HCH2(COO)(2)(-) ions wer e calculated from the steady-state diffusion-controlled currents.