Mixed Potential Response Mechanism of Cobalt Electrodes toward Inorganic Phosphate

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Abstract
The potentiometric response mechanism of a previously reported phosphate ion-sensitive electrode based on a surface-oxidized cobalt metal is examined. Beyond response to phosphate, the cobalt electrode is found to respond to changes in the partial pressure of oxygen in the sample solution. The potentiometric response toward phosphate ions and molecular oxygen is shown to depend on the sample stirring rate as well as the pH, ionic strength, and nature of the buffer salts present within the test solution. X-ray photoelectron spectroscopy studies of the cobalt electrodes, in conjunction with cyclic voltammetric measurements, suggest that the potentiometric response originates from a mixed potential resulting from the slow oxidation of cobalt and simultaneous reduction of both oxygen and Co2+ at the surface of the electrode. In contrast to an originally proposed host−guest mechanism, the present mixed potential mechanism more accurately explains behavior of oxidized cobalt electrodes in the presence of phosphate and oxygen species.