In the collection of data with the rotating disk electrode (RDE), the functional variation of interest is that of limiting current (ilim) VS. square root of rotation rate (co~). Typically, a plot or tabulation of ilim VS. (~0 + is built up point by point from a series of measurements of i~im, each made at a different, fixed rotation rate. Depending on the number of points involved, such a process may require from 10 to 30 min to complete. However, a continuous plot of ilim VS. O9 3 spanning the rotation rate range of 300-10,000 rev. min-1 can be automatically plotted in less than a minute by the use of the technique described in brief previously 1 and more fully herein.
In a typical experiment, a voltage proportional to inr " is applied to the Y-axis input of an XY recorder, while a voltage proportional to co ~ is applied to the recorder X-axis input. The potential of the electrode is stepped to a value which is sufficient to cause the desired electrode reaction welt up on the limiting current plateau so chargetransfer rates are not prohibitively slow, and after the current transient has decayed, the electrode rotation is swept through the required range, producing the desired plot.
Measurement of ilim while electrode rotation rate is varied was originated by Prater 2, who called the technique "rotoamperometry". In his technique, co is varied linearly with time to produce parabolic plots of ilim US. 60. Although the basic information gathered by the two techniques is similar, the technique developed in the present study appears to offer two advantages. First, if the system under study departs from ideal electrochemical behavior, such departure produces deviations from a straight-line rather than a parabolic plot, so that the deviations, if small, are more apparent. Second, as discussed later, short experiment times are often desirable, in which case a non-linear variation of co with time is useful.
II. THE TECHNIQUE
In Fig. a plot is shown of ilim VS. (D ~ obtained, by the technique developed in the present study, for the electrolysis of a 8.0 x 10 -4 F solution of 5,10-dihydro-5,10dimethylphenazine (DMPZ) in acetonitrile (MeCN)/0.1 F tetraethylammonium perchlorate (TEAP). Points from fixed rotation rate experiments are included for comparison ; the points and plot are coincident to within ca. 1 ~o over the rotation rate range of 300-7000 rev. min-1.
The DMPZ system is ideal for testing a technique of this sort, since the oxidation is known to be a reversible one-electron process 3 and the oxidation products do