A second approach to coulometry is to use a constant current in place of a constant potential, which produces the current–time profile shown here. Controlled-current coulometry has two advantages over controlled-potential coulometry. First, the analysis time is shorter because the current does not decrease over time. A typical analysis time for controlled-current coulometry is less than 10 min, compared to approximately 30–60 min for controlled-potential coulometry. Second, because the total charge is simply the product of current and time, there is no need to integrate the current-time curve.
Using a constant current presents us with an important problem. During electrolysis the analyte’s concentration—and, therefore, the current due to the its oxidation or reduction—continuously decreases. To maintain a constant current we must allow the potential to change. If the change in potential allows another species to undergo oxidation or reduction, then the total charge no longer is a measure of the amount of analyte in our sample.
Consider, for example, an analysis for iron based on the oxidation of Fe2+ to Fe3+. As shown here, as the concentration of Fe2+ decreases, the working electrode’s potential shifts toward more positive values until the oxidation of H2O begins.
Because a portion of the total current comes from the oxidation of H2O, the current efficiency for the analysis is less than 100% and we cannot use the total charge to determine the amount of Fe2+ in the sample.
Although we cannot prevent the potential from drifting, we can maintain the relationship between charge and the concentration of Fe2+ if we add to the sample a species whose oxidized form reacts both rapidly and quantitatively with Fe2+. As shown here, we can accomplish this by adding an excess of Ce3+ to the analytical solution.
When the potential of the working electrode shifts to a more positive potential Ce3+ eventually begins to oxidize to Ce4+, which then reacts rapidly with Fe2+ to form Fe3+ and to reform Ce3+. In this way we maintain the quantitative relationship between charge and the amount of Fe2+ in the original sample. A species, such as Ce3+, which is used to maintain 100% current efficiency, is called a mediator.