Differential Electrolytic Potentiometry Determination of Nanogram

electrode crevices suggests that these thin layer electrodes might be useful for rapid analysis of very small volumes ml.) of solutions. (ca. 1 0 - ~ The precision of the present micrometer electrode is limited primarily by the precision of the caliper movement which has as its smallest scale division 0.001 em. A more precise unit is currently under construction. The cavity thickness of the present electrode could be varied from 1 to 10 X 10-3 cm. without noticeable effect on the performance of the electrode except for an accurately predictable change in the cavity volume and in the transition time for the trial. I n Table I1 are presented data indicating the dependence of the observed transition times on current and cavity thickness. The reason that the shape of the chronopotentiograms deteriorates a t higher current densities and the transition times become shorter than predicted by Faraday's law is probably that the ohmic resistance within the crevice itself causes the

concentration of Fe(II1) to decrease to zero a t different times at different parts of the electrode surface so that the area of the working electrode nearest to the salt bridge may be forced to hydrogen evolution potentials before all the Fe(II1) has been consumed a t areas deeper within the crevice. It is this practical feature of the thin layer electrode behavior that sets the lower limit on usable transition times rather than the theoretical limit set by the time necessary for the reactant to diffuse through the thin solution layer to the electrode surface-Le., P / 3 D