Abstract
A review is given of the characteristics of the limiting current, reversible and irreversible waves, criterion of reversiblity of the wave form, the kinetic current and the adsorption waves (prewave and postwave) at the rotated dropping mercury electrode (RDME). The limiting current at the RDME in the presence of a suitable curl tce active substance is hardly affected by variation of the mercury pressure, while the etcct of drop time and hence of potential is much greater at the RDME (t1/2) than at the DME (t1/6). The large drop time effect is particularly important when a mixture of two or more electroactive species is analyzed. Reversible waves at the RDME cars be analyzed in the same way as those at the DME. The half-wave potential of reversible reduction waves at the RDME of simple ions of metals which are soluble in mercury is depcnclcrlt on characteristics of the electrode and is more negative than that at the DME. When oxidized and reduced species are both soluble in the solution, the half-wave potential at the RDME is independent of characteristics of the electrode. The half-wave potential of totally irreversible waves is much more negative at the RDME than at the DME. An expression for the difference in half wave potential at the two electrodes is derived and illustrated by comparing the waves of nickel ion in sodium perchlorate solution obtained at the DME and at the RDME. Criteria of reversibility of waves obtained with the RDME and with the DME are compared. Because of the greater rate of mass transfer at the RDME, the specific rate constant must be greater for the wave to be reversible at the RDME than at the DME. When the rate constant is not sufficiently large, a species yielding a reversible wave at the DME may give an irreversible wave at the RDME. This situation is illustrated by composite waves obtained with a mixture of vanadic and vanadous ions in sulfuric acid solution. Kinetic currents at the RDME are treated on the basis of the concept of reaction layer. When the current is entirely controlled by the rate of the chemical step involved, the expression for the kinetic current is the same at the DME and at the RDME. Since the kinetic current and the mass-transfer controlled limiting current are both practically independent of the mercury pressure, the dependence of current on the mercury pressure cannot be used to detect kinetic nature of the current at the RDME. On the other hand, the kinetic current at the RDME decreases with increasing speed of rotation of the electrode, the decrease corresponding to the decrease in drop time. Thus kinetic currents can be identified at the RDME by determining the effect of speed of rotation upon the value of the limiting current. The RDME is particularly useful in the study of polarograms involving adsorption owing to the fact that maxima of the second kind always appear in the absence of adsorption. This is demonstrated by the application of the RDME to the study of the potwave observed with copper (ll)-thiocyanate system and the anodic prewave observed with solutions containing iodide ions.
