As a means to illustrate the calculation of an optimal cell-voltage control for a parallel-plate reactor, we determine the time-varying cell voltage that maximizes p-aminophenol produced from the electroreduction of nitrobenzene in a differential-conversion reactor operated in a batch mode; that is, the electrolyte is continuously recirculated from a batch holding tank through the reactor in which a low conversion per pass occurs. A rationale is given for restricting the search for the optimal control for this particular reaction network to a chattering-cell voltage that switches between a priori chosen minimum and maximum values. The optimal, time-varying duty cycle is computed using a gradient-search technique. The predicted concentrations are dependent upon the reaction time; for the conditions examined here, an improvement of twenty-five percent in the production and nine-hundred percent in the selectivity of p-aminophenol may be achieved by using the optimal, time-varying voltage in comparison to the best steady value. Since a chattering control is a mathematical construct, we illustrate that a rectangular, high-frequency waveform may be applied to yield results which are indistinguishable from those effected by a chattering cell voltage. The period of the waveform must be short enough so that surface concentrations are time invariant over it and yet, simultaneously, must be long enough so that double-layer charging does not account for a significant passage of coulombs.