Electrochemical Methods for Environmental Analysis - Fundamentals of Environmental Sampling and Analysis - page 290

11.1 INTRODUCTION TO ELECTROCHEMICAL
THEORIES

To fully appreciate the electroanalytical techniques that follow, one requires a good
understanding of the basic theories and nomenclatures of electrochemistry. To this
end, the following section is devoted to an introduction to these subjects followed by
the general principles of electrochemical methods. Readers are advised of further
readings listed in this chapter for more details (e.g., Christian, 2003; Rubinson and
Rubinson, 2000).

11.1.1 Review of Redox Chemistry
and Electrochemical Cells

All oxidation-reduction (redox) reactions involve electron transfers. As a result of
electron transfer, electric current and potential can be generated by the use of a
device called an electrochemical cell. A familiar example is perhaps the lead-acid
batteryusedinautomobilesinwhichbothleadmetal(Pb)andleaddioxide(PbO 2 )
areimmersedinasolutionofsulfuricacid.Thereactionoccursasfollows:

Pb ðsÞþPbO 2 ðsÞþ2H 2 SO 4 !

2PbSO 4 þ2H 2 O ð11 : 1Þ

The above reaction occurs spontaneously, from which chemical energy is released
and converted to electrical energy. Since each cell produces about 2 V, a system
consisting of six cells in series constitutes a 12-V battery found in most cars today. In
this example, Pb 0 in its metallic form is oxidized, whereas the ionic form Pb 4þ in

PbO 2 is reduced.
Perhaps a better example is an electrochemical cell shown in Figure 11.1. This
cell consists of two electrical conductors called electrodes. The zinc electrode is
immersed in ZnSO 4 solution and the copper electrode is immersed in CuSO 4

Electrical device

μ A

e -

Salt b ridge

e -

2-

SO 4

Zn 2+

2e -

2e -

Zn 2+ Cu 2 +

Zn Cu

SO 4 2-

2-
SO 4

Figure 11.1 An electrochemical cell
with a salt bridge

Anode Cathode