Page 95 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

EQUlllBRlUM CONSTANTS OF DXIDATION-REDUCTION REACTIONS 2.33

the oxidant in being converted into the reductant, and aox and a,,, are the
activities of the oxidant and reductant respectively.
Since activities are often difficult to determine directly, they may be replaced
by concentrations; the error thereby introduced is usually of no great importance.
The equation therefore becomes:

Substituting the known values of R and F, and changing from natural to common
logarithms, at a temperature of 25 OC (T = 298K):
0.0591 [Ox]
E,, =EO+--- ---
n log [Red]
If the concentrations (or, more accurately, the activities) of the oxidant and
reductant are equal, E,,, = Ee, i.e. the standard reduction potential. It follows
from this expression that, for example, a ten-fold change in the ratio of the
concentrations of the oxidant to the reductant will produce a change in the
potential of the system of 0.0591/n volts.

2.33 EOUlllBRlUM CONSTANTS OF OXIDATION-REDUCTION REACTIONS
The general equation for the reaction at an oxidation-reduction electrode may
be written:
pA+qB+rC ... +ne=sX+tY+uZ+ ...

The potential is given by:

where a refers to activities, and n to the number of electrons involved in the
oxidation-reduction reaction. This expression reduces to the following for a
temperature of 25 OC (concentrations are substituted for activities to permit
ease of application in practice):

It is, of course, possible to calculate the influence of the change of concentration
of certain constituents of the system by the use of the latter equation. Consider,
for example, the permanganate reaction:

The concentration (or activity) of the water is taken as constant, since it is
assumed that the reaction takes place in dilute solution, and the concentration
of the water does not change appreciably as the result of the reaction. The