Controlled-Current Coulometry

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.

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David Harvey

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