Despite its well-documented limitations, the semiempirical Nicolsky-Eisenman equation is used throughout the existing analytical literature to describe the selectivity of modern polymer membrane-based ion-selective electrodes (ISEs). In this paper, a new quantitative description for the response/selectivity function based on ion-extraction equilibria at the sample/membrane interface is presented. The proposed selectivity formalism clearly illustrates the range of validity for the conventional Nicolsky-Eisenman formalism. Extended equations are derived describing the electrode response in an exact manner, particularly with respect to analyte and interfering ions of different charge. The expression obtained corresponds to the matched potential method proposed previously by Christian and co-workers on the basis of solely empirical observations. Selectivity coefficients required for a given analytical problem with a predefined maximum error can now be predicted more accurately. Such predictions with respect to analyte and interfering ions of varying charges differ by 1-2 orders of magnitude in comparison to the selectivity values required on the basis of the extended Nicolsky-Eisenman formalism.
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.
The electrochemical response mechanism of a previously reported potentiometric oxygen (0,) sensing system based on thin films of copper sputtered on single crystal Si(100) is examined. The potentiometric 0 2 response of such films is shown to depend on sample stirring rate as well as the pH, ionic strength and nature of the buffer salts within the test solution. XPS studies of the copper films exposed to solution for several days confirm the presence of compper corrosion products on the surface. These findings, in conjunction with cyclic voltammetric measurements, suggest that the potentiometric 0 2 response originates from slow corrosion of copper and simultaneous reduction of 0, -at the surface of the thin films. A steady-state situation exists when the rates of these two reactions are equal, resulting in a corrosion potentlal (also referred as rest potential E, for the system) that varies in a near Nernstian (1 e-) manner with the partial pressure of 0 2 in solution. A mathematical formulation for this type af response, based on the Butler-Volnier equation, is presented. The analytical implications of these findings with respect to devising useful potentiometric O2 sensors based on copper films are discussed.
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