Page 93 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

CALCULATION OF THE STANDARD REDUCTION POTENTIAL 2.32

system are those in which the ratio of the activity of the oxidant to that of the
reductant is unity. Thus for the Fe3+ - Fe2+ electrode, the redox ce11 would be:

The potential measured in this way is called the standard reduction potential.
A selection of standard reduction potentials is given in Table 2.6.
The standard potentials enable us to predict which ions will oxidise or reduce
other ions at unit activity (or molar concentration). The most powerful oxidising
agents are those at the upper end of the table, and the most powerful reducing
agents at the lower end. Thus permanganate ion can oxidise Cl-, Br-, 1-, Fe2+
and [Fe(CN),I4-; Fe3+ can oxidise H3As03 and 1- but not Cr20:- or Cl-.
It must be emphasised that for many oxidants the pH of the medium is of great
importance, since they are generally used in acidic media. Thus in measuring
the standard potential of the Mn04-Mn2+ system; Mn04 + 8H+ + 5e =
Mn2+ + 4H20, it is necessary to state that the hydrogen-ion activity is unity;
this leads to Ee = + 1.52 volts. Similarly, the value of Eefor the Cr20:--Cr3+
system is + 1.33 volts. This means that the Mn04-Mn2+ system is a better
oxidising agent than the Cr20:--Cr3+ system. Since the standard potentials
for Cl2-2C1- and Fe3+-Fe2+ systems are + 1.36 and 0.77 volt respectively,
permanganate and dichromate will oxidise Fe(I1) ions but only permanganate
will oxidise chloride ions; this explains why dichromate but not permanganate
(except under very special conditions) can be used for the titration of Fe(I1) in
hydrochloric acid solution. Standard potentials do not give any information as
to the speed of the reaction: in some cases a catalyst is necessary in order that
the reaction may proceed with reasonable velocity.
Standard potentials are determined with full consideration of activity effects,
and are really limiting values. They are rarely, if ever, observed directly in a
potentiometric measurement. In practice, measured potentials determined
under defined concentration conditions (forma1 potentials) are very useful for
predicting the possibilities of redox processes. Further details are given in
Section 10.90.

2.32 CALCULATION OF THE STANDARD REDUCTION POTENTIAL

A reversible oxidation-reduction system may be written in the form
Oxidant + ne e Reductant

Ox + ne = Red
(oxidant = substance in oxidised state, reductant = substance in reduced state).
The electrode potential which is established when an inert or unattackable
electrode is immersed in a solution containing both oxidant and reductant is
given by the expression:

where ET is the observed potential of the redox electrode at temperature T