Electrochemical Methods for Environmental Analysis - Fundamentals of Environmental Sampling and Analysis - page 291

solution. These two separate compartments are called half-cells. To complete the
cell circuit, an external metal wire is connected between two electrodes and a salt
bridge connects two separating electrolyte solutions. The metal wire is needed to
allow electrons to flow from one electrode to the other and the salt bridge allows
charge (ions) transfer through the solutions but prevents from mixing the solution.
The oxidation and reduction reactions are always coupled. That is to say that no
reaction in a single cell can occur by itself. Electrons are also conservative, in a sense
that, electrons donated from the oxidation of a reducing agent in an anode will be
equal to the electrons accepted by an oxidizing agent through a reduction reaction in
a cathode. Two half-cell reactions described in Figure 11.1 are

Zn 2þ ðaqÞþ2e Zn ðsÞ ! ðanodeÞ ð11 : 2Þ

Cu 2þ ðaqÞþ2e

!

Cu ðsÞ ðcathodeÞ ð11 : 3Þ

Note that as the oxidation-reduction reaction occurs, the anode would become more
positive as Zn 2þ are produced and the cathode would become more negative as Cu 2þ
are removed from the solution. The salt bridge allows the migration of ions in both
directions to maintain electrical neutrality. That is, cations (Zn 2þ ) from the anode
migrate via the salt bridge to the cathode, whereas the anion, SO 2 4 , migrates in
the opposite direction. Combining the above two half-cell reactions to cancel out the
electrons will result in a balanced overall reaction

Cu 2þ ðaqÞþZn ðsÞ ! CuðsÞþZn 2þ ðaqÞ ð11 : 4Þ

The fact that the reaction occurs spontaneously indicates that once these half-cells
are connected, an electrical energy is produced. This kind of electrochemical cell is a
galvanic (voltaic) cell in contrast to an electrolytic cell where electrical energy is
used to force a nonspontaneous redox reaction to occur. The galvanic cells are used
in potentiometric analysis described in Section 11.2, whereas electrolytic cells are
used in voltammetric analysis described in Section 11.3.
An important characteristic of all electrochemical cells is the difference in
potential energy between two electrodes. The difference in potential energy is called
an electrode potential or electromotive force (emf) and is measured in terms of volts.
This potential is dependent on the concentrations of the redox species and varies
from the standard potential as described by the Nernst equation. For the redox
reaction in Eq.11.4, the Nernst equation is

nF ln ½ Zn 2þ

log ½ Zn 2þ
½Cu 2þ

RT 0 : 059

n

E ¼ E 0

½Cu 2þ ¼ E 0

ð11 : 5Þ

where E is the electrode potential in volts, E 0 is the standard potential for the
reaction under standard state conditions (298 K, 1 atm, and unity molar concentra-
tions for all redox species), R is the gas constant (8.3145 J/K/mol), T is the absolute
temperature in K, n is the number of moles of charges transferred in the reaction per