Polarographic current-time curves and the Ilkovic Equation.

Abstract

The Ilkovic equation for the limiting diffusion current obtained with a dropping mercury electrode predicts that the instantaneous current grows during the life of the mercury drop as the one-sixth power of the time, and that the ratio of the instantaneous current at the end of the drop life (the maximum current) to the average current is 1.17. McKenzie (1948) showed in a preliminary study that these relations are not obeyed. The present paper is concerned with a more detailed study of current-time curves for cadmium(II), lead(II), and thallium(I) ions and oxygen. Measurements are made both in the presence and absence of maximum suppressor (gelatin) in two supporting electrolytes (potassium chloride and potassium nitrate). It is found that the rate of growth of the instantaneous current is not in accordance with the Ilkovic equation. Also, it does not accurately follow the modified equations, such as the Lingane-Loveridge equation, particularly during the early stages of drop life. The ratio of maximum to average current varies for the different electroactive substances, but in all cases examined 1.23<i_max./i_av.<1.30. An interesting observation is also made on the current-time curves for cadmium(II) in potassium nitrate in the presence of gelatin. At pH values appreciably below the isoelectric point (_?pH 5) the current-time curves and the current-voltage curves are distorted. The implications of these results in the measurement of polarographic waves, both in theoretical and analytical applications, are discussed.