Electrode charge measurements at dropping electrodes Part I. Back-pressure effect and measurement of changes in charge by a potential-step method

Abstract

The dynamics of growth of ideally polarized dropping mercury electrodes were determined by estimating the drop area at time intervals of 5 ms by means of a computer-based data acquisition system. The drop area was estimated by measuring the step changes in electrode charge that resulted when the applied potential was stepped between two fixed values at different times during the drop life. An approximate expression for the rate of drop growth, accounting for the back-pressure effect, was found to hold satisfactorily. The error made in electrode-charge measurements by a potential-step method (and, indirectly, in differential-capacity measurements) when assuming that the flow rate of mercury is constant and equal to its average value in air, was both calculated and determined experimentally for three different electrolytic solutions and for different capillaries. Rather than accounting for this error, due to the back-pressure effect, its complete elimination is proposed by means of a convenient pressurized dropping electrode, upon whose mercury reservoir pressures of 8–12 atm were applied.