ELECTROCHEMICAL METHODS Fundamentals and Applications - Allen.J.Bard

8 • Chapter 1. Introduction and Overview of Electrode Processes
Cathodic
Hg/I-Г, ВГ(1 M)/AgBr/Ag
Onset of H +
reduction ,
0.5
-0.5 -1.5
Onset of Hg
oxidation
Anodic
Potential (V vs. NHE)
Figure 1.1.5 Schematic current-potential curve for the Hg electrode in the cell Hg/H + , Br (1
M)/AgBr/Ag, showing the limiting processes: proton reduction with a large negative overpotential
and mercury oxidation. The potential axis is defined through the process outlined in the caption to
Figure 1.1.4.
With Hg, the anodic background limit occurs when Hg is oxidized to Hg2Br2 at a potential
near 0.14 V vs. NHE (0.07 V vs. Ag/AgBr), characteristic of the half-reaction
Hg 2
Br 2
+ 2e«±2Hg
2Br" (1.1.10)
In general, the background limits depend upon both the electrode material and the solution
employed in the electrochemical cell.
Finally let us consider the same cell with the addition of a small amount of Cd 2+
to
the solution,
Hg/H + ,Br"(l M), Cd 2+ (10" 3 M)/AgBr/Ag (1.1.11)
The qualitative current-potential curve for this cell is shown in Figure 1.1.6. Note the
appearance of the reduction wave at about -0.4 V vs. NHE arising from the reduction
reaction
CdBr|~ + 2e S Cd(Hg) + 4Br~ (1.1.12)
where Cd(Hg) denotes cadmium amalgam. The shape and size of such waves will be covered
in Section 1.4.2. If Cd 2+
were added to the cell in Figure 1.1.3 and a current-potential
curve taken, it would resemble that in Figure 1.1.4, found in the absence of Cd 2+ . At a
Pt electrode, proton reduction occurs at less positive potentials than are required for the
reduction of Cd(II), so the cathodic background limit occurs in 1 M HBr before the cadmium
reduction wave can be seen.
In general, when the potential of an electrode is moved from its open-circuit value toward
more negative potentials, the substance that will be reduced first (assuming all possible
electrode reactions are rapid) is the oxidant in the couple with the least negative (or
most positive) E®. For example, for a platinum electrode immersed in an aqueous solution
containing 0.01 M each of Fe 3+ , Sn 4+ , and Ni 2+
in 1 M HC1, the first substance reduced
will be Fe 3+ , since the E° of this couple is most positive (Figure 1.1.7a). When the poten-