Coulometric response characteristics of solid contact ion-selective electrodes for divalent cations

The chronoamperometric and coulometric response of solid contact ion-selective electrodes (SCISEs) for the detection of divalent cations was investigated in order to provide a more complete description of the mechanism of the recently introduced coulometric transduction method for SCISEs. The coulometric transduction method has earlier been employed only for SCISEs that were selective to monovalent ions. The SCISEs utilized poly(3,4-ethylenedioxythiophene) (PEDOT) doped with poly(styrene sulfonate) (PSS ⁻) as the solid contact (ion-to-electron transducer). PEDOT(PSS) was electrodeposited on glassy carbon and covered with plasticized PVC-based ion-selective membranes (ISMs) that were selective towards divalent cations (Ca ²⁺, Pb ²⁺). In contrast to earlier studies, the results obtained in this work show that the coulometric response for the Pb ²⁺-SCISE was limited mainly by ion transport in the PEDOT(PSS) layer, which was not the case for the Ca ²⁺-SCISE, nor was it observed earlier for the monovalent ions. The exceptional behavior of the Pb ²⁺-SCISE was explored further by electrochemical impedance spectroscopy, and it was shown that the effective redox capacitance of PEDOT(PSS) was significantly higher for the Pb ²⁺-SCISE than for the Ca ²⁺-SCISE although the polymerization charge of PEDOT(PSS) was the same. The slow transport of Pb ²⁺ in PEDOT(PSS) was tentatively related to complexation between Pb ²⁺ and PEDOT(PSS).