This paper is dedicated to Professor Douglas E. Ryan on the occasion of his 65th birti~dayROBERT D. GUY and SARATH NAMARATNE. Can. J . Chem. 65, 1133 (1987). The properties of mercury-coated glassy carbon electrodes coated with a thin membrane of Nafion 1 17 were studied to evaluate the potential analytical applications in metal ion analysis. The Nafion membrane was found to contain two types of binding sites -a strong site that bound metal ions in a nonelectroactive form and cation exchange sites that were useful for preconcentrations prior to differential pulse voltammetry. The exchange constants for the weak sites ranged from 0.413 for copper to 12.5 for methyl viologen. The differential pulse currents were increased by a factor of 75 to 100 and a linear calibration curve was observed from about 25 nM to 1000 nM for copper, lead, cadmium, and zinc. The ion exchange reaction is very sensitive to ionic strength and significant enrichments occur only in solutions of ionic strength below 0.10. The Nafion-coated glassy carbon electrode in a flow cell can be used to determine the free copper ion concentration in samples by a combination of ion exchange and differential pulse voltammetry. The flow-through system required about 15 mL of sample solution and an analysis time of about 10 min. The ion exchange measurement gave a linear calibration from 40 nM to 10 p M for free copper.
IntroductionVoltammetric techniques provide a sensitive method for the detection and quantification of inorganic and organic species in aqueous and organic solutions. The modification of electrodes has recently provided methods to improve both the selectivity and sensitivity of voltammetric methods. Wang and co-workers (1, 2) have coated a mercury-coated glassy carbon electrode (MCGCE) or glassy carbon electrode with a cellulose acetate membrane. The membrane was hydrolyzed with base to form a porous structure. The membrane acts like a miniature dialysis membrane and reduced the adsorption interferences from organic substances. A second approach to altering the selectivity of voltammetric methods is to form a species that adsorbs onto the mercury surface. Wang and Mahmoud (3) have used adsorptive preconcentration to determine titanium in natural waters. A third approach is to form a permanent membrane on the electrode surface that chemically reacts with metal ions in solution. Polymer-coated electrodes with attached chelating groups have been shown (4) to selectively preconcentrate iron and copper.
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