Lecture Notes Monday February 26th 2007

Answers to quiz:

When current is measured for a fixed voltage the technique is called amperometry. It is quite widely used for the measurement of dissolved oxygen in environmental water samples, but a device called a Clark "electrode". Just as for a pH "electrode" , the Clark device is actually two electrodes forming, in this case, an electrolysis cell. The pH cell is actually a battery--the voltage produced is related to the hydrogen ion concentration. At one electrode the oxygen is reduced to water and at the other silver is oxidized to Ag^+^. As this happens in the presence of chloride than silver precipitates at AgCl. Amperometry is also used in devices for sensing glucose in blood. The glucose is selectively oxidized (over any other sugars present) by dissolved oxygen (or another electron acceptor) in the presence of an enzyme, glucose oxidase. The resulting hydrogen peroxide (or reduced electron acceptor) is detected by oxidation at an indicator electrode to regenerate the oxygen (or electron acceptor).

Amperometry is also quite widely used as a detector for HPLC, as the chromatographic separation means that the detector does not have to be selective. As an example, sugars can be oxidized at a copper electrode as they emerge from the column. Water is reduced at a stainless steel flow through electrode to complete the circuit. The potential of the copper electrode is held constant versus that of a third, reference, electrode inserted in the flow system.

This type of three-electrode cell is quite common in electrochemistry. It allows the potential of the working electrode to be controlled but allows the current to flow through an auxiliary electrode. If current flows through the reference electrode (especially if it is small), the potential can change.