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nonhomogeneous potential on the working electrode. This effect leads to deviations from the applied potential which are larger for large currents and would cause negative deviations from theory for higher R values. In this case, however, the current decreases greatly after the initial electrolysis, to the value required to maintain the ox/red ratio as the chemical reaction proceeds. This current is generally less than 10 µ A, usually on the order of 1-3 µ A, values which should result in negligible error in aqueous solutions. Further evidence that ohmic losses are negligible is provided by the fact that halving the PAP concentration, resulting in smaller total current, caused no observable change in rate constants at the half-wave potential. The third source of error is more difficult to avoid, that of a slow charge transfer rate. If the chemical reaction becomes fast relative to the charge transfer rate, the ox/red ratio will not be maintained, and the result will be a drawn-out &obsd vs. Eapp plot, similar to a quasi-reversible polarogram. A likely means to remove this problem is the addition of a redox mediator to the solution. If the mediator interacts rapidly with both the electrode and the couple of interest, the solution potential will be maintained at the proper value, and the ox/red ratio will be a constant. Finally, it should be noted that all these effects can cause uncertainty in the starting time for the reaction. Although not important for first-order reactions, the uncertain starting time will be more critical for higher order systems. In these cases, care must be taken to assure that the kinetic run is long relative to the time required to establish the ox/red ratio.It should be emphasized that the strengths of this thin-layer kinetic method all result from the fact that one is working with a homogeneous solution in equilibrium with an electrode. The