10.14 : Controlled-Potential Coulometry: Electrolytic Methods

Controlled-potential coulometry, also known as potentiostatic coulometry, uses a three-electrode system where the working electrode's potential is carefully controlled using a potentiostat.

Platinum working electrodes are used when positive potentials are required, while mercury pool electrodes are preferred for very negative potentials.

The platinum counter electrode is separated from the analyte using a membrane or salt bridge to prevent any interference with the analysis.

The potential is chosen to maintain a 100% current efficiency by allowing quantitative oxidation or reduction of the analyte without interference from other components of the sample.

Notably, decomposition of water becomes a competing process at high potentials.

The analysis is completed by recording the electrolysis current and integrating it over time to determine the charge and the amount of analyte from Faraday's law.

However, a longer electrolysis time may lead to inaccurate results. This can be avoided by using electrodes with a large surface area, small-volume electrochemical cells, and rapid stirring to shorten the duration of the electrolysis.