Section C – Analytical reactions in solution
C9 VOLTAMMETRY AND
AMPEROMETRY
Principles Voltammetric techniques involve the electrolysis of the solution to be analyzed
using a controlled external power source and measuring the resultant current-
potential or current time curves to obtain information about the solution.
The species to be determined undergoes oxidation or reduction at a
working electrode. The voltage between the working electrode and an auxil-
lary orcounter electrode is controlled by the external circuitry in order to
maintain a preselected potential difference at the working electrode, with
respect to the reference electrode, as a function of time. A typical voltam-
metric cell, shown in Figure 1, has a working electrode, reference electrode
and an auxillary electrode and contains the solution to be analyzed. Often the
solution is deaerated with nitrogen to prevent interference due to the reduc-
tion reactions of oxygen.
If there is no reaction at the working electrode, the potential changes greatly for
a very small increase in current. A mercury drop electrode, for example, has a
polarization range between +0.3 V and -2.7 V against the SCE and in the absence
of oxygen so that many reactions that occur in that range may be studied.
By controlling the potential of the working electrode, a particular reaction
may be selected. Suppose a cell has two inert, solid electrodes and a reference
electrode, which dip into an aqueous solution containing copper ions.
In order to cause any reaction, the applied potential must exceed the decom-
position potential. This may be calculated by considering the reactions at each
electrode, and adding the extra potentials or overpotentials due to polarization
effects at the electrodes and to the voltage needed to drive the current against
the resistance of the solution. In this example, in order to drive the cell reaction,
a voltage greater than about –2.54 V must be applied.
Key Notes
Voltammetry is the study of the variation of current with applied
potential in an electrolysis cell where the reactions are controlled by the
diffusion of the sample species. The current is proportional to the
concentration of the electroactive species and amperometric methods
involve current measurement.The cell uses a working microelectrode, a reference electrode, and a
counter electrode, and a controlled voltage supply.These methods are used qualitatively to determine the nature of metal
and organic species and their reactions, and quantitatively to measure
trace levels of metals and organic compounds.Related topics Other topics in Section C.PrinciplesInstrumentationApplications