Analytical Chemistry, May 1, 1994 v66 n9 p1507(6)

                  Self-enhancement of voltammetric waves of weak acids in the absence of
                  supporting electrolyte.
                  Zbigniew Stojek; Malgorzata Ciszkowska; Janet G. Osteryoung

              Weak acids such as acetic, ascorbic, and salicylic acids are easily reduced at platinum microelectrodes in
             the absence of the supporting electrolyte. The current is mass transport controlled, and the reduction of
             proton is preceded by dissociation of the acid in the reaction layer. Since these acids are only slightly
             dissociated, transport should not be enhanced by migration and the heights of their voltammetric waves
             should be nearly independent of supporting electrolyte concentration. However, transport-limited
             currents diminish by 50% when a small amount of a supporting electrolyte is added to the solution. Thus
             the wave height in the absence of electrolyte exceeds that with electrolyte present by a factor of 2, as
             expected for one-electron reduction of a singly charged reactant. This change in the wave height is
             connected with the small amount of hydrogen ion in the solution arising from the dissociation of the acid.
             Since the equilibrium concentration of hydrogen ion, which is the only cation available in the solution, is
             much lower than that of undissociated acid, only a small ratio of the concentration of the supporting
             electrolyte to that of the acid is needed to eliminate this effect. Dihydrogen phosphoric acid, which is
             negatively charged, behaves differently. Taking into account this outcome and the limited dissociation of
             weak acids, the diffusion coefficients of acetic, ascorbic, salicylic, and dihydrogen phosphoric acids were
             determined at 20 degrees C. They are 0.97 x 10^-5, 5.6 x 10^-6, 7.7 X 10^-6, and
             6.4 x 10^-6 cm^2 s^-1, respectively.