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<imax./iav.<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.
