Disadvantages
Titrations are slow and time-consuming unless automated.
Potentiometry is the most widely used electroanalytical technique. It involves the measurement of the
potential of a galvanic cell, usually under conditions of zero current, for which purpose potentiometers
are used. Measurements may be 'direct' whereby the response of samples and standards are compared,
or the change in cell potential during a titration can be monitored.
Electrode Systems
The cell consists of an indicator and a reference electrode, the latter usually being the calomel or silver-
silver chloride type. The potential of the indicator electrode is related to the activities of one or more of
the components of the solution and it therefore determines the overall cell potential. Ideally, its response
to changes of activity should be rapid, reversible and governed by the Nernst equation. There are two
types of indicator electrode which possess the desired characteristics – metallic and membrane.
Metallic Indicator Electrodes
Metals such as silver, copper, mercury, lead and cadmium respond to variations in the activities of their
own ions in a Nernstian and reproducible manner, e.g. for silver, the electrode reaction is Ag+ + e– =
Ag, and the electrode potential is given by
Iron, nickel, cobalt, tungsten and chromium do not behave reproducibly due to crystal strain or oxide
coatings. Metal electrodes which respond directly to solutions of their own ions are called 'Class I' or
'first order'.
Metals which form sparingly soluble salts will also respond to changes in the activity of the relevant
anion provided the solution is saturated with the salt, e.g. for silver in contact with a saturated solution
of silver chloride and containing solid silver chloride the electrode reaction is AgCl + e– = Ag + Cl–,
and the electrode potential is given by:
Such electrodes are described as 'Class II' or 'second order'.
For titrations involving a change in oxidation state (redox systems) an inert electrode material such as
platinum is used. The electrode potential is determined by the proportions of oxidized and reduced
forms present, e.g.
and