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 mechanism of the jalpaite copper(II) ion-selective electrode (ISE) using electrochemical impedance spectroscopy (EIS) under conditions of minimal overpotential stimulation (i.e., ±10 mV). The EIS spectra of the jalpaite ISE in seawater and other ionic media revealed a high-frequency surface film impedance that is attributable to the modified hydrated surface layers of the ISE, and a low-frequency charge-transfer impedance that can be ascribed to the reductive ion-exchange of Cu(II) at the membrane/electrolyte interface. EIS data for a rotating disk electrode have shown that the charge-transfer process is under diffusion control. EIS and potentiometric response data in UV-oxidized seawater spiked with humic acid imply that organic complexation of Cu(I) at the electrode diffusion layer is responsible for a weak interference effect. The classical chloride interference effect of the jalpaite Cu(II) ISE occurring at high concentrations of Cu(II) (i.e., > 10- 6 M) yields an EIS response similar to the one observed with organic ligands. An EIS experiment for a jalpaite Cu(II) ISE aged in seawater for 72 h demonstrated that the charge-transfer impedance is constant until electrode release of Cu(II) exceeds the complexing capacity of seawater, and this mechanistic information is consistent with the hypothesis that electrode dissolution can be used to autotitrate the natural ligands in seawater