The polarographic maxima of the third kind arise during adsorption of some organic substances on the surface of a mercury drop. They were first described by Doss and Venkatesan 1. The possibility of the appearance of polarographic maxima in the presence of a concentration gradient of a surfactant was considered in refs. 2 and 3. Sathyanarayana 4' 5 showed that well-defined maxima of the third kind are observed in camphor solutions. The authors of refs. 1, 4 and 5 believed that the potentials of the maxima of the third kind must coincide with the potentials of the desorption peaks of the organic substance on the C-q~ curve (C= differential capacity, (p= electrode potential). In refs. 6-9 it was shown, by correlation of the results of measurements of the differential capacity and of the current I on a drop, that the maxima can be observed at any potential within the adsorption range of the organic substance if the coverage 0--0.3-0.5. It was also established that the appearance of these maxima is not associated with the presence of an electric field gradient.
So far typical maxima of the third kind have been observed during adsorption of some organic substances belonging to different classes: camphor, borneol, adamantanol (AdOH) 6-9, guanine*. All these substances are sparingly soluble in water, show a large surface activity and, at sufficient concentration, form two-dimensional condensed layers on a mercury surface 5" 10,1
The tangential motions of the drop surface accompanying the appearance of the maxima of the third kind could be revealed by the motion of carbon particles suspended in solution, which was observed with the help of a cathetometer ~2"23 The carbon particles were first cleansed by the technique described in ref. 14. In order to avoid a change in the organic substance concentration upon introduction of carbon into solution, the carbon had previously been brought into equilibrium with the solution, e.g. 0.01 g of the carbon was kept in 5 ml of 1 M Na2SO4 + 10 -3 M AdOH solution for a day. The observation of the suspended carbon particles showed that over the whole potential range of the appearance of the polarographic maxima of the third kind the solution at the drop surface was in motion. However, when the streaming of the solution for maxima of the third and that of the second kind * Ill-defined maxima of tlae third kind are observed at the desorption potentials of nonyl alcohol and tribenzylmethylammonium sulfate. We leave open the question whether various anomalies of the I ~o curves at a DME occurring in the presence of surfactants and described in the polarographic literature should be identified with typical maxima of the third kind. * The theoretical investigation of the stability of the liquid/gas interface inthe presence of insoluble films of surfactants carried out in ref. 27 has proved the possibility of the existence of a periodic pattern of circulation flows at the interface. This treatment applies to the liquid/liquid interface as well.