Coulometry and Electrogravimetry

Coulometry and electrogravimetry determine an analyte by exhaustively electrolyzing it and measuring the charge consumed or the mass deposited.

Definition

Coulometry and electrogravimetry are electroanalytical methods that quantify an analyte by completely converting it at an electrode and measuring, respectively, the total electrical charge passed or the mass of material deposited.

Scope

This topic covers the bulk-electrolysis methods of electroanalysis: controlled-potential and controlled-current coulometry, coulometric titrations in which a titrant is generated electrically, and electrogravimetry in which an analyte is deposited quantitatively on an electrode and weighed. It treats the requirement of complete and current-efficient electrolysis and the appeal of these methods as primary, calibration-free techniques.

Core questions

• How do Faraday's laws make charge a direct measure of analyte amount?
• Why can coulometry and electrogravimetry serve as primary, calibration-free methods?
• What is required for complete and current-efficient electrolysis?
• How does coulometric titration generate a titrant in situ with high precision?

Key theories

Faraday's laws of electrolysis

The amount of substance reacted at an electrode is proportional to the charge passed, with the proportionality fixed by the Faraday constant and the number of electrons in the reaction; this makes a measured charge or deposited mass an absolute measure of analyte quantity, requiring no external calibration.

Mechanisms

The analyte is electrolyzed to completion at a working electrode. In coulometry, the total charge required—measured directly at controlled potential or as current multiplied by time at controlled current—gives the amount of analyte through Faraday's laws, provided the current efficiency is essentially one hundred percent. In coulometric titration, a constant current generates a titrant that reacts with the analyte, and the time to an endpoint sets the charge. In electrogravimetry, the analyte deposits as a solid on a weighed electrode whose mass increase quantifies it.

Clinical relevance

Coulometric methods underpin precise determinations such as Karl Fischer water analysis and chloride measurement, and serve as reference and primary methods for certifying standards, while electrogravimetry is used for accurate determination of metals such as copper.

History

The quantitative basis is Faraday's 19th-century laws of electrolysis. Electrogravimetry was among the earliest instrumental methods for metal determination. Controlled-potential and constant-current coulometry, developed and systematized in the mid-20th century notably by Lingane, established coulometric titration and analysis as precise, primary techniques.

Key figures

• Michael Faraday
• Karl Fischer
• James J. Lingane

Seminal works

• harris2020
• skoog2017
• bard2001

Frequently asked questions

Why are coulometry and electrogravimetry called primary methods?

They rely on Faraday's laws, which tie analyte amount directly to measured charge or mass through fundamental constants, so they need no calibration against standards of the analyte and can themselves certify other measurements.

What does current efficiency mean in coulometry?

It is the fraction of the passed charge that goes into the intended analyte reaction; for an accurate result essentially all the current must drive that reaction, with no side reactions consuming charge.