Amperometric Electrodes—Measurement of Oxygen

An advantage of carbon electrodes is that they are not troubled by oxide formation on the surface, as platinum electrodes are. While carbon electrodes can be used at fairly negative potentials, a dropping mercury electrode (DME) is often preferred because better reproducibility can be achieved. This is because the electrode surface is constantly renewed (small mercury drops fall from a capillary attached to a mercury reservoir). Voltammetric techniques using a dropping mercury electrode are called polarography. 

Solid electrode voltammetry isHlsed largely for the oxidation of substances at fairly positive potentials, although for very easily reducible substances, it is also useful. However, reproducibility frequently suffers because the surface characteristics of the electrodes are not reproducible and the surface becomes contaminated. For this reason, the related technique of polarography is preferred when apphcable. 

Water or protons ai e easily reduced at a platinum electrode, limiting tlie available negative potential range to about —0.1 V versus SCE. 

Potentials of — 1 V versus SCE can be reached with a carbon electrode and —2 V with a mercury electrode. Oxygen must be removed for measurements more negative than —0.1 V. This is done by bubbling with nitrogen. 

Amperometry is the application of voltammetric measurements at a fixed potential to detect changes in currents as a function of concentration of electroactive species. Electrochemical sensors can be designed based on amperometric measurement. An important example is the oxygen electrode. 

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