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.