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

The voltammetric behavior of weak and strong polyprotic acids was studied at platinum microelectrodes under steady-state conditions in solutions of very low ionic strength, including those without added supporting electrolyte. Three diprotic acids (sulfuric, oxalic, and malonic), one triprotic acid (phosphoric acid), and one tetraprotic acid (pyrophosphoric acid) were chosen for the investigation. The reduction of hydrogen ion in solutions of strong diprotic acid (sulfuric acid) with no supporting electrolyte results in a voltammetric wave 1.5 times higher than the diffusional wave obtained with excess electrolyte. This agrees well with the theoretical prediction for one-electron reduction of monovalent cation accompanied by divalent anion. The voltammetric reduction waves of oxalic, malonic, and phosphoric acids in solutions without supporting electrolyte were twice as high as the diffusion-controlled waves with excess electrolyte. This is what theory predicts for a monoprotic strong acid. The strong (H+) and weak (HA(-) for oxalic and 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 (phosphoric and pyrophosphoric) voltammetric waves. The influence of the concentration of supporting electrolyte on the height of reduction waves of acids was examined for two kinds of supporting electrolyte, with mono- and divalent cations, over a wide range of their concentrations. The experimental results are compared with theoretical predictions. The diffusion coefficients of H(COO)2(-) and HCH2(COO)(2)(-) ions were calculated from the steady-state diffusion-controlled currents.