Coulometric Methods of Analysis
In potentiometry, the potential of an electrochemical cell under static conditions is used to determine an analyte’s concentration. As seen in the preceding section, po- tentiometry is an important and frequently used quantitative method of analysis. Dynamic electrochemical methods, such as coulometry, voltammetry, and amper- ometry, in which current passes through the electrochemical cell, also are important analytical techniques. In this section we consider coulometric methods of analysis.
Coulometric methods of analysis are based on an exhaustive electrolysis of the analyte. By exhaustive we mean that the analyte is quantitatively oxidized or re- duced at the working electrode or reacts quantitatively with a reagent generated at the working electrode. There are two forms of coulometry: controlled-potential coulometry, in which a constant potential is applied to the electrochemical cell, and controlled-current coulometry, in which a constant current is passed through the electrochemical cell.
The total charge, Q, in coulombs, passed during an electrolysis is related to the absolute amount of analyte by Faraday’s law
Q = nFN ………………….. 11.23
where n is the number of electrons transferred per mole of analyte, F is Faraday’s constant (96487 C mol–1), and N is the moles of analyte. A coulomb is also equiva- lent to an A.s; thus, for a constant current, i, the charge is given as
Q = ite ………………….. 11.24
where te is the electrolysis time. If current varies with time, as it does in controlled- potential coulometry, then the total charge is given by
In coulometry, current and time are measured, and equation 11.24 or equation 11.25 is used to calculate Q. Equation 11.23 is then used to determine the moles of analyte. To obtain an accurate value for N, therefore, all the current must result in the analyte’s oxidation or reduction. In other words, coulometry requires 100% current efficiency (or an accurately measured current efficiency established using a standard), a factor that must be considered in designing a coulometric method of analysis.