coulometrie

Coulométrie is an electrochemical technique used for quantitative analysis in which the amount of a substance is determined from the total electric charge passed during an electrochemical reaction. It rests on Faraday’s laws: the moles of substance transformed are proportional to the charge Q delivered (or withdrawn). If the reaction involves the transfer of z electrons per molecule, the number of moles n reacting equals Q/(zF), where F is the Faraday constant (approximately 96485 C/mol).

Two common modes are coulometric titration, where a titrant is generated in situ by electrolyzing a reagent at an electrode under controlled current, and complete electrolysis (coulometry) in which the analyte itself is oxidized or reduced until none remains. The endpoint, or equivalence point, is reached when the analyte is consumed; the measured charge Q then relates to the amount of analyte by n = Q/(zF).

Instrumentation consists of an electrochemical cell with a working electrode, a counter electrode, and a reference electrode, connected to a constant-current source or potentiostat that measures accumulated charge. In coulometric titration a coulometer records Q in real time and can directly provide concentration without external standards.

Applications include determination of redox-active species, halides, sulfites, cyanide, and the water content by Karl Fischer coulometry. The method offers high accuracy with very small sample and reagent consumption and is particularly well suited for trace analysis and for samples where standard solutions are difficult to prepare. Limitations include side reactions, gas evolution at the electrodes, and the need for well-characterized reaction stoichiometry.

Coulométrie is named after Charles-Augustin de Coulomb and builds on Faraday’s laws of electrolysis. It has become a standard tool in analytical chemistry, chemical manufacturing, and quality control.