Coulometry is based on an exhaustive electrolysis of the analyte. By exhaustive we mean that the analyte is completely oxidized or reduced at the working electrode or that it reacts completely with a reagent generated at the working electrode. During an electrolysis, the total charge, Q, in coulombs, passing through the electrochemical cell is proportional to the absolute amount of analyte by Faraday’s law
Q = nFNA
where n is the number of electrons per mole of analyte, F is Faraday’s constant (96,487 C mol–1), and NA is the moles of analyte. Measuring the total charge during the electrolysis allows us to determine the amount of analyte in our sample.
In constant-potential coulometry we apply a fixed potential and monitor current as a function of time. The resulting current–time profile is shown here, where Q is determined by integrating the current between t = 0 and t = te.
A typical Pt-gauze working electrode for constant-potential coulometry is shown here. The electrode has a diameter of 13 mm and a height of 48 mm, and is fashioned using Pt wire with a diameter of approximately 0.15 mm. The electrode’s surface has 360 openings/cm2 and a total surface area of approximately 40 cm2. A large surface area allows for a shorter analysis time.