Controlled-Potential Coulometry: Electrolytic Methods

Controlled-potential coulometry, also called potentiostatic coulometry, uses a three-electrode system where a potentiostat precisely regulates the working electrode's potential. The analysis records electrolysis current and integrates it over time to determine charge and analyte amount using Faraday's law. This method ensures quantitative oxidation or reduction of the analyte without interference from other sample components.

Platinum working electrodes are used for positive potentials because they resist oxidation, while mercury pool electrodes are preferred for very negative potentials due to their wider potential window. The platinum counter electrode is separated from the analyte using a membrane or salt bridge to prevent interference with the analysis and ensure accurate results.

The chosen potential maintains 100% current efficiency by enabling selective oxidation or reduction of the analyte. However, water decomposition becomes a competing process at high potentials, which can interfere with results. Selecting the appropriate potential prevents unwanted side reactions and ensures only the target analyte is quantitatively converted.

Prolonged electrolysis time is a major source of inaccuracy. To minimize electrolysis duration and improve accuracy, use electrodes with large surface areas, small-volume electrochemical cells, and rapid stirring. These modifications increase mass transfer rates and reduce the time needed to complete the analysis.

Controlled-potential coulometry maintains a constant electrode potential using a potentiostat, allowing selective analysis of specific analytes. In contrast, controlled current coulometry coulometric titration applies constant current. Controlled-potential methods offer better selectivity because potential controls which species oxidize or reduce, while current-based methods rely on titration endpoints.

The three-electrode system consists of a working electrode where the analyte reacts, a counter electrode separated by a membrane or salt bridge, and a reference electrode that monitors potential. This configuration allows the potentiostat to precisely control and maintain the working electrode potential, ensuring selective and quantitative analysis without interference from counter electrode reactions.

Faraday's law relates the amount of substance oxidized or reduced to the total charge passed through the electrochemical cell. By integrating the electrolysis current over time, analysts calculate the total charge and use Faraday's law to determine the exact quantity of analyte present. This direct relationship between charge and moles makes coulometry a quantitative analytical technique.