ELECTROCHEMICAL IMPEDANCE SPECTROSCOPY STUDY OF THE RESPONSE MECHANISM OF THE JALPAITE CU-II ION-SELECTIVE ELECTRODE IN SEAWATER

This paper presents the results of a novel study into the response mec hanism of the jalpaite copper(II) ion-selective electrode(ISE) using e lectrochemical impedance spectroscopy (EIS) under conditions of minima l overpotential stimulation (i.e., +/-10 mV). The EIS spectra of the j alpaite ISE in seawater and other ionic media revealed a high-frequenc y surface film impedance that is attributable to the modified hydrated surface layers of the ISE, and a low-frequency charge-transfer impeda nce that can be ascribed to the reductive ion-exchange of Cu-II at the membrane/electrolyte interface. EIS data for a rotating disk electrod e have shown that the charge-transfer process is under diffusion contr ol. MS and potentiometric response data in UV-oxidized seawater spiked with humic acid imply that organic complexation of Cu-I at the electr ode diffusion layer is responsible for a weak interference effect. The classical chloride interference effect of the jalpaite Cu-II ISE occu rring at high concentrations of Cu-II (i.e., > 10(-6) M) yields an MS response similar to the one observed with organic ligands. An EIS expe riment for a jalpaite Cu-II ISE aged in seawater for 72 h demonstrated that the charge-transfer impedance is constant until electrode releas e of Cu-II exceeds the complexing capacity of seawater, and this mecha nistic information is consistent with the hypothesis that electrode di ssolution can be used to autotitrate the natural ligands in seawater.