Coulometry
Also known as: coulometric titration, electrochemical coulometry, amperes titration
Coulometry is an electrochemical analytical method that determines the concentration of an analyte by measuring the total electric charge (in coulombs) required to oxidize or reduce the analyte completely at an electrode. Developed by James J. Lingane in the 1940s, coulometry is highly accurate because it is based on fundamental constants (Faraday's law) and does not require external standards or calibration curves. This method is particularly valuable for trace analysis, water determination, and analysis of reactive species.
Read the full method
Sign in with a free account to read this section.
Method map
The neighbourhood of related methods — select a node to explore.
When to use it
Coulometry is ideal for determining the concentration of species that can be oxidized or reduced at an electrode, such as water (Karl Fischer coulometry), active halogens, phenolic compounds, and metal ions. Use coulometry when absolute accuracy is critical (primary method for validation), when sample volume is very small, when analyte concentration is extremely low, or when external standards are unavailable. Coulometry is particularly valuable in standardization of reagents and for establishing reference values.
Strengths & limitations
- Inherently accurate (based on fundamental Faraday's law and physical constants)
- Does not require calibration against external standards
- Primary method for determining water content (Karl Fischer coulometry)
- Excellent for trace analysis (detection limits in the micromole range)
- Can be highly selective by choosing appropriate electrode potential
- Absolute quantification without reference materials
- Limited to electrochemically active analytes that can be oxidized or reduced
- Requires complete electrode reaction; partial oxidation/reduction compromises accuracy
- Interference from other electroactive species in sample matrix
- Requires oxygen-free environment (deoxygenation step needed)
- Slower than many instrumental techniques (total time can be minutes per sample)
Frequently asked
Why is coulometry considered a primary analytical method?
Coulometry is based on Faraday's law, a fundamental law of electrolysis relating the amount of chemical reaction to the charge transferred. Faraday's constant is one of the most precisely known physical constants. Because coulometry measures fundamental quantities (charge and time) related by well-established stoichiometry (electron transfer), it does not require calibration against external standards and is classified as a primary method, like gravimetry.
What is Karl Fischer coulometry, and why is it important?
Karl Fischer coulometry is a specialized coulometric method for determining trace water content. The method uses a reagent (iodine in pyridine or similar solvent) that reacts with water in a 1:1 molar ratio. Coulometric titration generates iodine electrochemically at controlled potential, and the charge required is directly proportional to water content. This method is the official ASTM standard for moisture measurement and achieves parts-per-million (ppm) precision in oils, solvents, and pharmaceuticals.
How does constant potential coulometry differ from constant current coulometry?
Constant potential (coulostatic) coulometry maintains a fixed electrode potential, ensuring selectivity by favoring only the desired oxidation/reduction reaction. Constant current (galvanostatic) coulometry applies a fixed current, which may be faster but is less selective. Constant potential is preferred for analyses requiring high selectivity; constant current is used when selectivity is less critical.
What is the role of the counter electrode in coulometry?
The counter electrode (or auxiliary electrode) completes the electrical circuit and allows current to flow through the cell. However, it does not participate in the analytical reaction; it typically oxidizes or reduces the supporting electrolyte. Good cell design ensures that counter electrode reactions do not interfere with the working electrode reaction.
Why must the sample be deoxygenated before coulometric analysis?
Dissolved oxygen is easily reduced at the electrode, consuming electrical charge that would otherwise be used to oxidize/reduce the analyte. This background oxygen reaction produces spurious signals and causes errors in the measured coulometry. Deoxygenation (via bubbling inert nitrogen gas or other methods) removes oxygen before the analysis begins.
Sources
- Lingane, J. J. (1974). Electroanalytical Chemistry (2nd ed.). Interscience Publishers. ISBN: 978-0486409023
- Skoog, D. A., West, D. M., Holler, F. J., & Crouch, S. R. (2014). Fundamentals of Analytical Chemistry (9th ed.). Cengage Learning. ISBN: 978-1133170960
- Simonsen, K. B., Larsen, K. L., Frandsen, H., & Andersen, J. E. T. (2012). Coulometric titration for determination of peroxide compounds. Journal of Pharmaceutical and Biomedical Analysis, 71, 22–28. link ↗
How to cite this page
ScholarGate. (2026, June 3). Coulometry. ScholarGate. https://scholargate.app/en/analytical-chemistry/coulometry
Which method?
Set this method beside its closest kin and read them side by side — the library lays the books on the table; the choice is yours.
- Atomic Absorption SpectroscopyAnalytical Chemistry↔ compare
- Ion ChromatographyAnalytical Chemistry↔ compare
- Potentiometric TitrationAnalytical Chemistry↔ compare
- UV-Vis SpectrophotometryAnalytical Chemistry↔ compare
- VoltammetryAnalytical Chemistry↔ compare