Page 91 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

CONCENTRATION CELLS 2.29

determined indirectly by measuring the e.m.f. of a ce11 formed from the electrode
in question and a convenient reference electrode whose potential with respect
to the hydrogen electrode is accurately known. The reference electrodes generally
used are the calomel electrode and the silver-silver chloride electrode (see
Sections 15.3-4).
When metals are arranged in the order of their standard electrode potentials,
the so-called electrochemical series of the metals is obtained. The greater the
negative value of the potential, the greater is the tendency of the metal to pass
into the ionic state. A metal will normally displace any other metal below it in
the series from solutions of its salts. Thus magnesium, aluminium, zinc, or iron
will displace copper from solutions of its salts; lead will displace copper, mercury,
or silver; copper will displace silver.
The standard electrode potential is a quantitative measure of the readiness
of the element to lose electrons. It is therefore a measure of the strength of the
element as a reducing agent in aqueous solution; the more negative the potential
of the element. the more ~owerful is its action as a reductant.
It must be emphasised that standard electrode potential values relate to an
equilibrium condition between the metal electrode and the solution. Potentials
determined under, or calculated for, such conditions are often referred to as
'reversible electrode ~otentials'. and it must be remembered that the Nernst
equation is only stri;ly applicable under such conditions.

2.29 CONCENTRATION CELLS
An electrode potential varies with the concentration of the ions in the solution.
Hence two electrodes of the same metal, but immersed in solutions containing
different concentrations of its ions, may form a cell. Such a ce11 is termed a
concentration cell. The e.m.f. of the ce11 will be the algebraic difference of the
two potentials, if a Salt bridge be inserted to eliminate the liquid-liquid junction
potential. It may be calculated as follows. At 25 OC:
0.0591
E=- log c, + Ee
n
0.0591 cl
- log -, where cl > c,
--
n Cz
As an example consider the cell:

AgNO, aq. AgNO, aq.
Ag 1 [Agt] = 0.00475 M ii [A~'] = 0.043 M

C +
E2 El
Assuming that there is no potential difference at the liquid junction:
0.0591 0.043
E = El - E, = -log- = 0.056 volt
1 0.004 75