Modern inorganic chemistry

ACIDS AND BASES: OXIDATION AND REDUCTION 105

At equilibrium at 298 K the electrode potential of the half-reaction
for copper, given approximately by

must equal the electrode potential for the half-reaction for zinc, given
approximately by

Thus,

Efn + ^- log 10 [Zn^2 + (aq)] = Eg, -f ^- log 10 [Cu2+(aq)]

— z.

Hence,

loglo[Zn^2 + (aq)] - loglo[Cu^2 + (aq)] = (Eg, - EfJ x -|-

Substituting for Eg, = + 0.34, and Efn=- 0.76 we have:

Hence

2 +

=

This is in fact the equilibrium constant for the reaction

Cu^2 + (aq) + Zn(s) -> Cu(s) + Zn^2 + (aq)

and its high value indicates that the reaction goes effectively to
completion.
Similar calculations enable the equilibrium constants for other
reactions to be calculated.

Potentiometrie titrations

The problem in any quantitative volumetric analysis for ions in
solution is to determine accurately the equivalence point. This is
often found by using an indicator, but in redox reactions it can often