On the Theory of the Galvanostatic Double Pulse Technique for the Investigation of Fast Electrode Reactions

Abstract

A general theory of the galvanostatic double pulse technique is presented in which the minimum in the overpotential‐time curve can be shifted to any arbitrary time after the end of the prepulse. Such a theory is of interest because the overpotential‐time trace on the oscilloscope is always distorted immediately after the end of the prepulse by inevitable ringing which can mask the minimum. Two modifications of the original technique are investigated. In the first modification the minimum occurs at fixed time duration after the end of the prepulse , while in the second modification the ratio of to is fixed . All previous applications of the technique tacitly used the first modification. The systematic errors of the technique, caused by inadequacies in the mathematical approximations, are examined in detail and are compared with random experimental errors. The systematic errors are presented graphically as a function of the time constant of the double layer, and of which is called the rate constant parameter because, under simplified conditions, it is equal to 632 . For a maximum 20% error in the determination of the exchange current density, the limits of the technique can be summarized as follows. The first modification is limited by systematic errors and it can be utilized only up to rate constant parameters of 750 , but only if the time constant is larger than . For smaller rate constant parameters the usable range of time constants increases until, at a value of 350 or less, this modification can be used for any value of the time constant. The second modification can be used for faster reactions. It is limited by the random experimental errors to a rate constant parameter of about 6000 at small time constants . For smaller rate constant parameters this modification becomes applicable for both small and large time constants: for example, at a value of 1300 it is applicable for time constants larger than or smaller than . Finally, at a rate constant parameter value of 140 it can be used for any value of the time constant.