Application of the Ilkovic equation to quantitative polarography

Abstract

Conditions of applicability of the Ilkovic equation to polarographic analysis were investigated for representative ions singly and in mixtures. Gelatin was used to suppress maxima found in current-voltage curves.Results obtained with a capillary of usual characteristics showed that a suppresser is necessary to secure agreement with the llkovic equation over any considerable drop-time range, and that the need increases with the drop-rate of the electrode and the dilution of reducible ion. A criterion of diffusion control satisfactory for classification of polarograms by visual examination was found. The application of the Ilkovic equation to quantitative analysis was shown to be inadvisable unless it is definitely established that the experimental conditions are well within the range for insuring diffusion control. A logarithmic relation between critical drop-time and suppresser concentration was found.

Examination of Absolute and Comparative Methods of Polarographic Analysis

“…The suppression of these maxima is imperative if the absolute method is to be employed. It has been shown that errors of many per cent in the values of diffusion-current constants can result from the presence of maxima which are not readily recognizable (3). Such is the case, for example, when cadmium is electrolyzed in neutral solutions of potassium chloride, where a study of diffusion-current constants as a function of the drop time of the electrode was required to show the presence of maxima.…”

“…If the logarithm of the minimum drop time above which constant values for the diffusion-current constant are obtained, designated as the critical drop time, is plotted with respect to the logarithm of the concentration of gelatin in the solution, a linear relationship is found to exist which is independent of the concentration or nature of the reducible ion. Preliminary experiments have shown that the relationship depends on the size of the drops produced by the electrode, but the quantitative relationship has not been determined as yet (3).…”

“…Large amounts of suppressor should not be used indiscriminately, since an excess may decrease the size of waves, especially in dilute solutions of reducible ion (3,15). Although there is no completely reliable simple test for determining when maxima have been suppressed, two methods have been suggested which may have practical value under certain conditions.…”

Polarographic Theory, Instrumentation, and Methodology

“…Cr(0H)(H20)5 + + + 3e = Cr + 5H20 + OH' (13) as well as by the reversal of the hydrolytic equilibrium 10 in the diffusion layer, there would be no contribution at all of hydrogen ion to the total diffusion current if Dc, were equal to Dh. The observed hydrolysis current results from the fact that the diffusion coefficient of hydrogen ion actually is much larger than that of the hydrolyzed chromic ion.…”

Device for Estimating Height of Polarographic Waves

“…One solution was 0.45 millimolar in cadmium, 1 " in hydrochloric acid, and contained 0.01% gelatin to suppress maxima. The other solution contained the same amount of cadmium and gelatin but was 1 ,V in nitric acid.…”