constant potential, in which case the current diminishes to zero as the reaction goes to completion, or at
constant current. The quantity of electricity involved in the former is measured by means of a chemical
coulometer or by integrating the area under the current-time curve. Constant current methods involve
the generation of a titrant for a measured length of time, the completion of the reaction with the species
to be determined being indicated by any of the methods used in titrimetric analysis. For this reason,
such procedures are described as coulometric titrations. In both constant potential and constant current
coulometry, the current efficiency must be 100%, i.e. all the electricity passing through the cell must be
utilized in a reaction involving the species to be determined, either directly or indirectly.
Coulometry at Constant Potential
The technique is similar to electrogravimetry at constant cathode potential (p. 260), but is much more
versatile because the substance to be determined need not form a stable, adherent deposit on the
electrode. The quantity of electricity consumed in the reaction is determined by connecting a chemical
coulometer in series with the cell or by using an electronic or electro-mechanical integrator. Chemical
coulometers take the form of electrolysis cells where the total amount of product(s) liberated at the
electrode(s) can readily be measured. The simplest to use are gas coulometers, in which the amount of
hydrogen and oxygen (or nitrogen) liberated from a suitable electrolyte solution (e.g. sodium sulphate)
is measured in a gas burette. During the analysis close control of the cathode (or anode) potential with a
potentiostat (p. 261) is essential to avoid electrode reactions other than that related to the species of
interest. A convenient working-electrode is a mercury cathode at which numerous metals and organic
compounds can be reduced. Examples include the determination of lead in cadmium, nickel in cobalt
and trichloroacetic acid in the presence of the mono- and dichloro-derivatives. Precisions of better than
1% are easily attained, but analyses can take up to an hour or more.
Coulometric Titrations
The titrant is generated at a working electrode by the passage of a constant current until the equivalence
point is indicated by potentiometric, amperometric or, less commonly, visual or photometric means. An
accurate timer is required to enable the total quantity of electricity used to be calculated. A schematic
diagram of a coulometric titrator is shown in Figure 6.17. Constant-current sources, often just a battery,
should be capable of delivering currents of 10–100 mA with an accuracy of 0.5% or better. Electric
timers with solenoid-operated brakes or electronic timers should be used to eliminate cumulative errors
arising from starting and stopping the motor, a sequence which may be repeated many times as the
equivalence point is approached.