Analytical Chemistry

can act as its own supporting electrolyte. Reducible anions such as can also
be determined provided solutions are well buffered. Organic applications are restricted by an inability to
distinguish between members of a homologous series and by the irreversibility of many reductions at
the DME. Solutions must be well buffered as many electrode reactions are pH dependent. The limited
aqueous solubility of most organic compounds necessitates the use of polar solvents such as glycols,
dioxane, formamide and methyl cellosolve with lithium or tetraalkylammonium salts as the supporting
electrolyte. Reducible organic functional groups include ketones, aldehydes, alkenes, nitriles, azo,
peroxy, disulphide, nitro, nitroso and nitrite. Examples of the applications of polarography are given in
Table 6.4.

Table 6.4 Applications of polarography

Element or compound determined Type of sample

Cu, Pb, Sn, Zn foodstuffs

Ga, Zn, Cd, Ni high-purity aluminium

Cu, Pb, Ni, Co steels

Mo, Ge, As, Sb minerals and ores

Sn, Pb beer and soft drinks

transition metals high-purity salts

free sulphur petroleum fractions

antioxidants fuels

riboflavin milk, pharmaceuticals

antibiotics, steroids body fluids

vitamin C fruit and vegetables

oxygen seawater, gases

Amperometric titrations have an even wider range of application than polarography. Although the
titrant may be added from a burette, in many applications it is electrically generated in a coulometric
cell (p. 261). Such an arrangement lends itself to complete automation and is particularly valuable for
the titration of very small quantities. For examples of coulometric titrations with amperometric
equivalence point detection see Table 6.5.

6.3—

Electrogravimetry and Coulometry

Electrogravimetry, which is the oldest electroanalytical technique, involves the plating of a metal onto
one electrode of an electrolysis cell and weighing the deposit. Conditions are controlled so as to
produce a uniformly smooth and adherent deposit in as short a time as possible. In practice, solutions
are usually stirred and heated and the metal is often complexed to improve the quality of the deposit.
The simplest and most rapid procedures are those in which a fixed applied potential or a constant cell
current is employed, but in both cases selectivity is poor and they are generally used when there are