Polarographic Adsorption Wave

Abstract

The present review deals with the so-called anomalous wave which is caused by the adsorption of depolarizing species or its electrolysis product. Adsorption current of such a type was first observed 1941 by Brdicka and Knobloch in the reduction of riboflavin and 1942 by Brdicka in the reduction of methylene blue. The physicochemical elucidation of adsorption wave was mainly given by Brdicka on the basis of Langmuir adsorption isotherm. It is clarified that in the reduction wave a prewave appears if the reduction product is easily adsorbable on the dropping mercury electrode surface, since the adsorbed reduction product is more stabilized than the unadsorbed product. On the contrary, if the oxidized form is adsorbable, the adsorbed oxidant is more stabilized than the unadsorbed oxidant, a postwave appears in the reversible reduction process. The most important property of the adsorption pre- or postwave consists in the fact that its wave height increases with concentration lower than the critical concentration corresponding to the adsorption saturation. When the concentration is higher than the critical one, the current intensity of the adsorption prewave remains constant, and the second step called the main wave appears. The sum of main and adsorption wave heights is proportional to the depolarizer concentration. There have been found some criteria for distinguishing whether a given polarographic wave is a normal diffusion current wave or an adsorption wave. It is emphasized that in the experimental work one should pay attention to the following properties of polarographic wave. These are:(1) dependence of wave height on concentration, (2) dependence of wave height on the mercury head h, (3) current-time curve during one drop-life, (4) measurement of the electrocapillary curve, (5) dependence of wave height on temperature. As to the analytical application it is shown that the depolarizers which give an ad-sorption wave can well be determined when the a.c. or high-frequency polarographic method is applied. Since the adsorption provokes a prominent increase in the capacitative component of the electrode surface resulting in an enhanced signal in the a.c. or high-frequency polarogram, the microdetermination of the depolarizers exhibiting adsorption wave, such as methylene blue, riboflavin, diethyldithiocarbamate and others, can comfortably carried out.