Instant Notes: Analytical Chemistry

(ii) The reduced species (that is, the metal) is then oxidized out of the film by

making the electrode increasingly anodic. A peak appears on the current-

potential plot, and the peak current can be shown to be:

Ip=k(cBnt)

where the constant kincludes the diffusion and other constants, and nis the

rate of increase of the anodic potential. The peak potential at which an

active species is oxidized is characteristic of that species, and is close to its

half-wave potential.

Applications Polarographic techniques may be used in both qualitative andquantitative modes.
Since the half-wave potential is characteristic of the particular reaction that is
occurring at that potential, it is possible to identify the species involved. A
simple case is shown in Figure 3where a mixture of metal ions was analyzed.
The two reduction waves for copper occur at –0.1 and –0.35 V, cadmium at
–0.69, nickel at -1.10 and zinc at –1.35 V. This illustrates an analysis that may
identify the species qualitatively and, by using a standard addition method, can
also determine the ions quantitatively.
Organicsubstances may be determined either in an aqueous or a nonaqueous
medium. For example, the concentration of nitrobenzene in commercial aniline
may be found by studying the reaction:
C 6 H 5 NO 2 +4H++4e-=C 6 H 5 NHOH +H 2 O
The oxygen electrode is based on voltammetric principles and depends on
the diffusion and reduction of oxygen. It is also called the Clark sensor.
The cell has a lead anode and a silver cathode set close together in an alkaline
solution, often 1M KOH. At the anode, the reaction is
Pb(s) +4OH-(aq) =PbO 22 - (aq) +2H 2 O +2e-
The silver cathode is inert, unless oxygen or another reducible species can
diffuse to it. A semipermeable membrane through which only oxygen can
diffuse surrounds the electrodes, and then the reduction reaction takes place.
O 2 (aq) +2H 2 O +4e-=4OH-

102 Section C – Analytical reactions in solution

E (applied) (V)

Current,

I

0 0.3 0.6 0.9 1.2 1.5

Fig. 3. Polarogram of Cu^2 +, Cd^2 +, Ni^2 +and Zn^2 +ions at ~ 10 -^4 M.