coulometry

Coulometry is a method of quantitative chemical analysis based on measuring the total electric charge passed during an electrochemical reaction. By Faraday’s laws, the amount of substance that is transformed is proportional to the total charge, with moles of reaction participants equal to the total charge divided by the product of the number of electrons transferred per molecule and the Faraday constant (n = Q/(zF)). In practice, coulometry relates measured charge to analyte content or concentration with high accuracy, assuming near 100% current efficiency and complete reaction.

There are two principal approaches. In constant-current (galvanostatic) coulometry, a fixed current is applied to drive the electrochemical reaction, and the time required to reach completion is used with Q = It to calculate the amount of analyte that reacted. In coulometric titration, the titrant is generated electrochemically in situ by controlled electrolysis of an electrolyte; the quantity of titrant produced is governed by the total charge passed, allowing determination of the analyte when the generated species reacts with it. End points can be detected by sensors such as pH or by monitoring current changes.

Applications are widespread in inorganic and organic analysis. Coulometry is used to determine oxidizable or reducible species, to quantify halides, metals, and acid–base equivalents, and to assess water content or sample purity in certain matrices. It is valued for high accuracy, independence from standard solutions, and small sample requirements, though it requires rigorous control of current efficiency and complete reaction, and careful electrode design to minimize side reactions.

Instrumentation typically includes a stable current source or potentiostat, a coulometer cell with inert electrodes, and means to measure total charge or time, sometimes with an endpoint detector in titrimetric variants.