Page 92 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

2 FUNDAMENTAL THEORETICAL PRINCIPLES OF REACTIONS IN SOLUTION

2.30 CALCULATION OF THE e.m.f. OF A VOLTAIC CELL
An interesting application of electrode potentials is to the calculation of the
e.m.f. of a voltaic cell. One of the simplest of gaivanic cells is the Daniell cell.
It consists of a rod of zinc dipping into zinc sulphate solution and a strip of
copper in copper sulphate solution; the two solutions are generally separated
by placing one inside a porous pot and the other in the surrounding vessel. The
ce11 may be represented as:

At the zinc electrode, zinc ions pass into solution, leaving an equivalent negative
charge on the metal. Copper ions are deposited at the copper electrode, rendering
it positively charged. By completing the external circuit, the current (electrons)
passes from the zinc to the copper. The chemical reactions in the ce11 are as
follows:
(a) zinc electrode: Zn = ZnZ+ + 2e;
(b) copper electrode: CuZ+ + 2e e Cu.
The net chemical reaction is:

The potential difference at each electrode may be calculated by the formula
given above, and the e.m.f. of the ce11 is the algebraic difference of the two
potentials, the correct sign being applied to each.
As an example we may calculate the e.m.f. of the Daniell ce11 with molar
concentrations of zinc ions and copper(I1) ions:

The small potential difference produced at the contact between the two solutions
(the so-called liquid-junction potential) is neglected.

2.31 OXIDATION-REDUCTION CELLS
Reduction is accompanied by a gain of electrons, and oxidation by a loss of
electrons. In a system containing both an oxidising agent and its reduction
product, there will be an equilibrium between them and electrons. If an inert
electrode, such as platinum, is placed in a redox system, for example, one
containing Fe(II1) and Fe(I1) ions, it will assume a definite potential indicative
of the position of equilibrium. If the system tends to act as an oxidising agent,
then Fe3+ + FeZ+ and it will take electrons from the platinum, leaving the latter
+ + Fe3+),
positively charged; if, however, the system has reducing properties (FeZ
electrons will be given up to the metal, which will then acquire a negative charge.
The magnitude of the potential will thus be a measure of the oxidising or
reducing properties of the system.
To obtain comparative values of the 'strengths' of oxidising agents, it is
necessary, as in the case of the electrode potentials of the metals, to measure
under standard experimental conditions the potential difference between the
platinum and the solution relative to a standard of reference. The primary
standard is the standard or normal hydrogen electrode (Section 2.28) and its
potential is taken as zero. The standard experimental conditions for the redox