Nafion-coated mercury-coated glassy carbon electrodes for metals analysis and speciation

The properties of mercury-coated glassy carbon electrodes coated with a thin membrane of Nation 117 were studied to evaluate the potential analytical applications in metal ion analysis. The Nation 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 n M to 1000 n M 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 n M to 10 μ M for free copper.