24.1: Introduction to Coulometry

Coulometry is based on an exhaustive electrolysis of the analyte. By exhaustive we mean the analyte is oxidized or reduced completely at the working electrode, or reacts completely with a reagent generated at the working electrode. There are two forms of coulometry: controlled-potential coulometry, in which we apply a constant potential to the electrochemical cell, and controlled-current coulometry, in which we pass a constant current through the electrochemical cell.

During an electrolysis, the total charge, Q, in coulombs, that passes through the electrochemical cell is proportional to the absolute amount of analyte by Faraday’s law

\[Q=n F N_{A} \label{intro1} \]

where n is the number of electrons per mole of analyte, F is Faraday’s constant (96 487 C mol^–1), and N_A is the moles of analyte. A coulomb is equivalent to an A•sec; thus, for a constant current, i, the total charge is

\[Q=i t_{e} \label{intro2} \]

where t_e is the electrolysis time. If the current varies with time, as it does in controlled-potential coulometry, then the total charge is

\[Q=\int_{0}^{t_e} i(t) d t \label{intro3} \]

In coulometry, we monitor current as a function of time and use either Equation \ref{intro2} or Equation \ref{intro3} to calculate Q. Knowing the total charge, we then use Equation \ref{intro1} to determine the moles of analyte. To obtain an accurate value for N_A, all the current must oxidize or reduce the analyte; that is, coulometry requires 100% current efficiency or an accurate measurement of the current efficiency using a standard.

Current efficiency is the percentage of current that actually leads to the analyte’s oxidation or reduction.

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