Standard potentials Eo are evaluated with full regard to
activity effects and with all ions present in simple form: they
are really limiting or ideal values and are rarely observed
in a potentiometric measurement. In practice, the solutions
may
Standard potentials Eo are evaluated with full regard to
activity effects and with all ions present in simple form: they
are really limiting or ideal values and are rarely observed
in a potentiometric measurement. In practice, the solutions
may be quite concentrated and frequently contain other
electrolytes; under these conditions the activities of the
pertinent species are much smaller than the concentrations,
and consequently the use of the latter may lead to unreliable
Also, the actual active species present (see example below)
may differ from those to which the ideal standard potentials
apply. For these reasons 'formal potentials' have been
proposed to supplement standard potentials
The concept of formal potential was introduced by Swift and he
defined it as the experimentally determined potential of a
solution containing both the oxidised and reduced species
each at a concentration of 1 formal together with other
substances at a given concentration.
1F ≡ 1g formula weight per litre solution
The std. Potential or E0 (normal potential) is a limiting or ideal value
assuming that the reacting species are in their simple forms. Since in
practice such ideal solutions are rarely found. The term formal
potential has been proposed and found to be more practical one.
[ Fe +3 ] Where the ionic sp. are aqueous forms
E = E0 log
[ Fe +2 ]
Fe+3 aq or Fe+3(H2O)6 ; Fe+2 aq or Fe+2 (H2O) simple form of Fe+3 or Fe+2
In IM HCl soln, however part of Fe+3 exists as various sp.like
FeCl+2, FeCl2+, FeCl4-2 (all in Feric state).
Similarly Fe+2 as FeCl+, FeCl4-2 etc.
Let, α be the fraction of total Fe+3 irons, β is the fraction of
total Fe+2 iron ex