Instant Notes: Analytical Chemistry

Table 1. Value of the half-wave potential

Ion E^1 ⁄ 2 /V with respect to the SCE

in 0.1 M KCl in 1 M NH 3 , NH 4 Cl

Cd^2 + -0.60 -0.81

Cu^2 + +0.04 -0.24

• 0.22 -0.51
  Zn^2 + -1.00 -1.35
  K+ -2.14
  C 6 H 5 NO 2 - 0.22
  Note that copper(II) is reduced in two stages.

Instrumentation For most voltammetric and amperometric methods, the instrumentation
includes a working microelectrode, a reference electrode and an auxillary or
counter electrode, together with electronic equipment to control the voltage and
voltage sweep, plus a computer or recorder to collect data.
The earliest microelectrode used was the dropping mercury electrode
(DME), where pure mercury flows through a fine capillary, either due to gravi-
tational force, or by applied pressure. Drop times of a few seconds are usual.
This electrode has the advantages that:

● the surface area is small and is constantly refreshed so that products of

electrolysis do not accumulate;

● mercury has a high overpotential for hydrogen formation, which allows the

reduction of other species.

Other electrodes used are the static, orhanging mercury drop electrode,

where the drop is dislodged at a particular time and size, and solid micro-

electrodes, such as platinum and glassy carbon, which may be incorporated into

a rotating disc electrode.

Dissolved oxygen in the sample solution must be removed, since oxygen may

be reduced in two steps, giving waves that overlap with those of the sample.

O 2 +2H++2e-=H 2 O 2 E^1 ⁄ 2 =-0.05 V

H 2 O 2 +2H++2e-=H 2 O E^1 ⁄ 2 =-0.9 V

This is usually done by passing oxygen-free nitrogen through the sample

solution during the experiment.

Maxima on the waves are due to surface effects, and may be suppressed by

adding a small amount of surface-active agents, such as gelatin or Triton-X100.

Anodic stripping voltammetry is designed to measure trace amounts by

preconcentrating them onto a suitable electrode. The experiment has two stages:

(i) The sample is electrolyzed onto a hanging mercury drop, or a mercury film

deposited on a carbon electrode. By Faraday’s laws, (Topic C2), passing a

current of Iamps for t seconds will produce a concentration cRin a mercury

film of thickness l, area A:

cR=It/nF lA

Because the current is limited by diffusion:

I =mnFD cBA

where m is a mass transfer coefficient, D the diffusion coefficient and cB the

bulk concentration.

C9 – Voltammetry and amperometry 101