coulometer

A coulometer is an instrument that measures the quantity of electric charge (in coulombs) by directing an electrochemical reaction to completion and integrating the current that passes during the reaction. By Faraday's laws, the amount of substance transformed is proportional to the total charge, enabling highly quantitative analyses.

In practice, the instrument is formed as an electrochemical cell in which the analyte reacts stoichiometrically with a generated titrant or is deposited/dissolved at an electrode. A current is passed for a measured time or until a predefined endpoint, and the charge Q = ∫ I dt is recorded. The measured charge is then related to moles of substance via n = Q/(zF), where z is the electrons transferred and F is Faraday's constant.

Types include coulometric titration, where a titrant is generated electrochemically until the analyte is consumed; or electrogravimetric coulometry, where the analyte or the product is deposited or dissolved, and mass change is used alongside charge to determine quantities. Another widely used variant is Karl Fischer coulometry for water content determination.

Coulometry is valued for its high accuracy, direct traceability to fundamental constants, and automation potential. Limitations include the need for clean, well-behaved electrochemistry and careful control of side reactions, electrolyte composition, and cell geometry.