Analytical Chemistry

Half-wave Potentials

Electroactive species are characterized by their E1/2 values which are constants related to the standard

electrode potentials and given by the equation

where k and kr are proportionality constants relating cell current to the rates of diffusion of oxidized and

reduced forms of the electroactive species. Values are independent of the bulk concentration C but

depend on the composition of the supporting electrolyte which can affect if complexes are formed.
The components of a mixture will give separate polarographic waves if the E1/2 values differ by at least

0.1 V. Often, if two waves overlap, by careful choice of a complexing agent, the and hence the E1/2

value of one component can be changed by up to a volt or more. For example, in a potassium chloride

solution Fe(III) and Cu(II) waves overlap whereas in fluoride medium, the value of E1/2 for the
anion is 0.5 V more negative, the Cu(II) wave being unaffected. Alkali metals which are reduced at
potentials around –2.0 V can be determined using a tetraalkylammonium salt as the supporting
electrolyte. Some other examples of the effect of complexing are given in Table 6.3.

Table 6.3 The effect of complexation on half-wave potentials (volts)

Ion No complexing agent

E1/2/V

KCN

E1/2/V

NH 3

E1/2/V

Cu2+ +0.02 * –0.24 and –0.51

Zn2+ –1.00 * –1.35

Cd2+ –0.59 –1.18 –0.81

Pb2+ –0.40 –0.72 —

* Not reducible

Characteristics of the DME

A diagrammatic representation of a DME is shown in Figure 6.10. Mercury from a reservoir is forced
through a narrow-bore glass capillary (~ 0.06 mm bore) by gravity. A succession of identical drops is
formed which are detached at regular intervals (3–8s), timing and reproducibility of drop dimensions
being ensured by electrical control. This characteristic of the DME results in an oscillating cell current,
the average of which is given by the Ilkovic equation

when n denotes the number of farads, D is the diffusion coefficient for the electroactive species, m is
the rate of flow of mercury, t is the drop time, C is the concentration in the bulk solution, and A is a
constant. The equation is useful in comparing diffusion currents from electrodes with different capillary

characteristics, i.e. different values of. The diffusion coefficient D is temperature sensitive to the
extent of about 2.5% per kelvin; so