The only long-term systematic source of error was the evaporation of water. The first cell described here was operated for more than 2000 cycles, but required addition of 0.1 ml of distilled water after about 1500 cycles in order to replenish the solution for evaporated water.The question of accuracy and possible changes in cell capacity also arises. Although solutions can be prepared accurately, it is simpler to adjust the constant current level and so obtain the desired cycling regime.In this work the solution concentration ratio Co/C~ was arbitrarily chosen to be 4: 1. Any other ratio is suitable if it conforms to the cell mechanism. Likewise, ratios of 4: 1, in one half cell, and another ratio, such as 8:2 in the other, would be feasible but would not offer any advantage in timing applications. ConclusionsI. The redox coulometer is suitable for measuring periods of time continuously or cumulatively.2. The cell performance is reliable and reversible over a very long lifetime provided only that the cell is hermetically sealed to prevent evaporation of solvent water.3. The voltage limits of operation are determined by the concentration of the redox couple according to the Nernst relation. Deviation (increased voltage at constant current) occurs near the end of a half cycle due to transport phenomena.4. Within the voltage limits, the redox couple is chosen for the most stable voltage region with regard to solvent decomposition.5. The nature of the ion in limiting supply determines the choice of the ion exchange membrane to be used.

This is an integrated polarographic and potentiostatic study on the mechanism of the cathodic reduction of m‐dinitrobenzene for various pH values of aqueous electrolytes. The polarographic results confirmed previous results of a two‐stage reduction involving four electrons in each step. The supporting coulometric work was performed potentiostatically at a graphite cathode under a nitrogen atmosphere at a controlled potential of −0.48v on a solution of pH 6.00. The product of the first step, 3‐nitrophenylhydroxylamine, was quantitatively isolated and identified for coulometric equivalencies of 25, 50, 75, and 90% reduction.