Voltammetry is an electrochemical technique based on the measurement of the current produced by an analyte as a function of the potential applied to a working electrode. In many cases the applied potential is varied or the current is monitored over a period of time. The applied potential forces a change in the concentration of an electroactive species at the electrode surface by electrochemically reducing or oxidising it. Analytical chemists routinely use voltammetric techniques for the quantitative determination of a variety of dissolved inorganic and organic substances. The analytical advantages of the various voltammetric methods include excellent sensitivity with a very large useful linear concentration range for both inorganic and organic species. There are also many other uses including fundamental studies of oxidation and reduction processes in various media, adsorption processes on surfaces, and electron transfer and reaction mechanisms. Voltammetric methods are also applied to the determination of compounds of pharmaceutical interest and, when coupled with HPLC, they are effective tools for the analysis of complex mixtures.

The electrochemical cell, where the voltammetric process is carried out, consists of a working electrode, a reference electrode, and usually a counter electrode. In general, the working electrode provides the interface across which a charge can be transferred or its effects felt. In practice, it can be important to have a working electrode with known dimensions and surface characteristics. The auxiliary electrode can be almost anything as long as it doesn’t react with the bulk of the analyte solution and conducts well.

The reference electrode should provide a reversible half-reaction with Nernstian behaviour, be constant over time, and be easy to assemble and maintain. The most commonly used reference electrodes for aqueous solutions are the calomel electrode, and the silver/silver chloride electrode.

Voltammetry has developed very rapidly, and several types have been introduced with high efficiency, sensitivity, and selectivity. Of the numerous types of voltammetric techniques the more common ones are:

Linear sweep voltammetry – in which the current at the working electrode is measured while the potential between the working electrode and the reference electrode is swept linearly with time. The potential at which oxidation or reduction of the analyte occurs at the working electrode is shown as a peak or trough in the current.

Cyclic voltammetry – in which the current at the working electrode is measured while the potential between the working electrode and the reference electrode is swept linearly with time. Unlike with linear sweep voltammetry when a set potential is reached the potential at the working electrode is swept in the opposite direction to return to the initial potential. A cyclic voltammogram is obtained by plotting the current at the working electrode against the applied voltage. The sweeps maybe repeated many times.

Stripping voltammetry – in which there is pre-concentration of an analyte on an electrode, followed by a potential sweep to selectively oxidize or reduce the analyte, with the current generated proportional to the amount of analyte present on the electrode.

The original dropping mercury electrode was originally the working electrode of choice, however, the use of mercury electrodes has fallen out of favour because of the toxicity of mercury. Other electrodes that have gained favour for use in voltammetric methods include carbon paste, glassy carbon, platinum and gold. In addition, with a glassy carbon electrode it is possible to create a thin film of mercury on the electrode by reducing Hg2+. This provides the advantages of mercury electrodes described earlier without the necessity for using large quantities of mercury.

Sentek produces a range of working electrodes as well as calomel and silver/silver chloride reference electrodes. Sentek can also supply customised electrodes to meet many requirements.