Chronoamperometry of uncharged species in the presence of a very low concentration of supporting electrolyte. A simulation

A simulation technique was applied to model the time-dependent chronoamperometric current of uncharged species in the presence of varying concentrations of supporting electrolyte. For a low value of the support ratio (the ratio of the concentrations of supporting electrolyte and analyte) and the potentials usually employed, the chronoamperometric current grows slowly in time, and the surface concentration of the analyte drops slowly. The ion formed during the electrolysis, and the corresponding counterion that migrates to the electrode surface, increase the conductivity, decrease the ohmic drop, increase the electrode potential, and finally increase the current. After an appropriate time (this depends on the support ratio and the potential applied), the accumulation of ions in the depletion layer is so large, that the current measured is diffusion controlled and fits very well the equations derived for an excess of supporting electrolyte. This situation might take place for a wide range of the support ratio, and well before steady state is reached.