Page 98 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

2 RNDAHENTAL THEORETICAL PRINCIPLES OF REACTIONS IN SOLUTION

for their possible use in quantitative analysis. It must be emphasised, however,
that these calculations provide no information as to the speed of the reaction,
upon which the application of that reaction in practice will ultimately depend.
This question must form the basis of a separate experimental study, which may
include the investigation of the influence of temperature, variation of pH and
of the concentrations of the reactants, and the influence of catalysts. Thus,
theoretically, potassium permanganate should quantitatively oxidise oxalic acid
in aqueous solution. It is found, however, that the reaction is extremely slow
at the ordinary temperature, but is more rapid at about 80 OC, and also increases
in velocity when a little manganese(I1) ion has been formed, the latter apparently
acting as a catalyst.
It is of interest to consider the calculation of the equilibrium constant of the
general redox reaction, viz.:
a Ox, + b Red, e b Ox, + a Red,
The complete reaction may be regarded as composed of two oxidation-reduction
electrodes. a Ox,, a Red, and b Ox,,, b Red, combined together into a cell; at
equilibrium, the potentials of both electrodes are the same:
0.0591 [Ox,]"
El = Et+--- -
n log [~ed,]"

At equilibrium, El = E,, and hence:
0.0591 [Ox,]" 0.0591 [Ox,Ib
Et + --- log- = EF+---log---
n [Red,]" n C Red,]
[Ox,Ib x [Red,]" n
or log = logK = ---
o.o591 (Et - EF)
[Red,Ib x [Ox,]"
This equation may be employed to calculate the equilibrium constant of any
redox reaction, provided the two standard potentials E? and EF are known;
from the value of K thus obtained, the feasibility of the reaction in analysis may
be ascertained.
It can readily be shown that the concentrations at the equivalence point,
when equivalent quantities of the two substances Ox, and Red, are allowed to
react, are given by:

This expression enables us to calculate the exact concentration at the equivalence
point in any redox reaction of the general type given above, and therefore the
feasibility of a titration in quantitative analysis.

For References and Bibliography see Sections 3.38 and 3.39.