only one or two metals present. Selective deposition of metals from multi-component mixtures can be
achieved by controlling the cathode potential automatically with a potentiostat. This device
automatically monitors and maintains the cathode potential at a predetermined value by means of a
reference electrode and servo-driven potential-divider. The value chosen for the cathode potential is
such that only the metal of interest is deposited and no gaseous products are formed.
Separations of transition and non-transition metals by deposition of the former at a mercury-pool
cathode can be used as a preliminary step in an analytical procedure. In such cases, the reduced metal
cannot be weighed, although quantitative separations may be achieved.
Coulometry
Summary
Principles
Measurement of the quantity of electricity used in an electrochemical reaction at constant potential or
constant current.
Instrumentation
Working and counter-electrodes; source of constant current and timer, or a potentiostat and integrator;
equivalence point detection system.
Applications
Quantitative determination of many species in solution, especially at trace levels (micrograms or less);
relative precision 0.2–5%. Useful for unstable titrants and easy to automate.
Disadvantages
Constant potential methods can be lengthy; constant current methods have similar disadvantages to
other titrimetric techniques.
Coulometric methods of analysis involve measuring the quantity of electricity required to effect a
quantitative chemical or electrochemical reaction and are based on Faraday's laws of electrolysis:
(1) The quantity of a substance liberated at an electrode during electrolysis is proportional to the
quantity of electricity passing through the solution.
(2) A given quantity of electricity liberates different substances in the ratio of their molar masses
divided by the number of electrons involved in the corresponding electrode reactions.
The electrolysis cell consists of a working-electrode, at which the species to be determined is reduced or
oxidized or at which a chemically reactive species is formed, and a counter-electrode. In practice
electrolysis may be at