Coulometry
Definition and meaning of Coulometry in chemistry.
Coulometry is an analytical technique that determines the amount of a substance by measuring the total electric charge (in coulombs) consumed or produced during a quantitative electrochemical reaction at an electrode.
In more detail
The method rests on Faraday's laws of electrolysis: the charge passed, Q, is directly proportional to the moles of substance reacted, related by Q = nFN, where n is the number of electrons transferred per molecule, F is the Faraday constant, and N is moles of analyte. In controlled-potential coulometry, the working electrode potential is held constant until current decays to zero, ensuring complete conversion; in coulometric titration, a constant current generates a titrant electrochemically until an endpoint (visual, potentiometric, or amperometric) is reached, and the charge equals current times elapsed time. Because charge can be measured with extremely high precision, coulometry serves as an absolute method requiring no external calibration standard, provided the electrode reaction proceeds with 100% current efficiency.
Key facts
| Field | Analytical Chemistry |
|---|---|
| Governing Law | Faraday's laws of electrolysis (Q = nFN) |
| Key Types | Controlled-potential coulometry; coulometric titration (constant-current) |
| Unit Measured | Charge, in coulombs (C) |
Karl Fischer coulometric titration determines trace water content in a sample: iodine is generated electrochemically at a constant current from iodide in the Karl Fischer reagent, reacting stoichiometrically with water; the total charge passed to reach the endpoint gives the exact mass of water present via Faraday's law.
Frequently asked questions
Why is coulometry considered an absolute method?
Because charge is measured directly and related to moles of analyte through fundamental constants (Faraday's constant) rather than through comparison to a calibration curve or standard, so no reference standard of the analyte itself is needed, only accurate current and time (or total charge) measurement.
What is current efficiency and why does it matter?
Current efficiency is the fraction of passed charge that goes toward the intended electrode reaction rather than side reactions; coulometry requires current efficiency at or near 100% for the charge-to-mass relationship to give an accurate result.