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.