New Liquid-Membrane Electrodes used for the Determination of Copper and Nickel

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Abstract
Four electrodes with a liquid membrane, Cu−2 -selective and Ni2+ -selective, respectively, not described previously in the literature, were prepared and characterized. From these electrodes 1 and 2 are based on simple complexes of Cu(II) and Ni(II) with 6-nitro-quinoxaline-2,3-dithiol (6 - nitro - Q.D.T.) as ligand. Electrodes 3 and 4 are based on the corresponding mixed complexes in which the ammonia molecule acts as the second ligand along with 6-nitro-quinoxaline-2,3-dithiol as the first ligand. Both the Cu2+ -selective electrode and Ni2+ -selective electrode have been used to determine the Cu2+ and Ni2+ ions in aqueous solution both by direct potentiometry and by potentiometric titration with EDTA. The primary and secondary characteristics of these electrodes, as well as their selectivity constants for practical use in the potentiometric assay of copper and nickel, were determined. By comparison with the electrodes based on the simple complexes (1 and 2) the electrodes based on mixed complexes(3 and 4) present a better selectivity over the interfering ions and also they have greater potential breaks in the titration curves. For that reason the electrodes based on mixed complexes are better suited for titrations of dilute solutions, down to 10−5M. All these electrodes present a Nemstian response that is 29 mV at 25°, in a concentration range between 10−1 - 5.10−5M for the electrodes 1 and 2 and 10−1 - 10−6M for the last two electrodes. Direct potentiometric methods for the determination of Cu2- and Ni2+ ions in aqueous solutions and in industrial waters were also studied. The experimental analytical results were checked by using the method of standard addition and also by comparison with atomic absorption spectrometric determinations.

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