tion with positive feedback and doublepulse instrumentation. Of course, the use of better amplifiers will improve the effectiveness of all the methods for interpretation of the data taken at short times.Even with advanced instrumentation, full advantage of the potentiostatic technique will be realized only by application of correction methods.

Results are presented of an extensive theoretical study of the second harmonic ac polarographic wave for systems involving rate control by diffusion and charge transfer kinetics (the quasi-reversible case). The expanding plane model of the dropping mercury electrode is employed. Emphasis is placed on aspects of the second harmonic response which have been considered only briefly in previous work such as the effects of a non-Nernstian dc process, the phase angle behavior, and the data analysis problem. The results make apparent the potentialities of second harmonic ac polarography in the study of quasi-reversible processes, particularly as a measurement which complements the more conventional dc and fundamental harmonic ac data.

Results are presented of experimental measurements of the second harmonic ac polarographic response with the systems Cr(CN)e3_/Cr(CN)e4_ in 1A1 KCN and Cd2+/ Cd(Hg) in 1M Na2S04. The data encompass an applied alternating potential frequency range of 23 Hz to 4.4 KHz. Agreement with theoretical predictions for the second harmonic ac polarographic response of quasireversible systems is excellent. The best fit of theory with experimental data was obtained for k, = 0.25 cm sec-1 and a = 0.55 in the case of the Cr(CN)63-/Cr-(CN)e4-redox system. The corresponding values for the cadmium system were 0.15 cm sec-1 and 0.30, respectively. In both cases, these rate parameter values are essentially identical to those obtained by fundamental harmonic measurements.

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