Current pulse based ion-selective electrodes for chronopotentiometric determination of calcium in seawater

Calcium is closely related to the oceanic biosphere and carbonate system. Potentiometry based on a calcium-selective electrode (Ca-ISE) represents a promising tool for detection of calcium ion activities in seawater. This technique, however, generally is limited by the low sensitivity for the inherent Nernstian response (i.e. ca 30 mV/dec). A current pulse based ion-selective electrode with enhanced sensitivity for chronopotentiometric measurements of calcium in seawater is proposed. The Ca²⁺-selective membrane containing lipophilic salt ETH 500 instead of traditional ion-exchanger is galvanostatically controlled. An applied constant cathodic current pulse can leads to the extraction of the calcium ions into the membrane to produce a chronopotential response, which shows a stable and reproducible super-Nernstian response in a narrow calcium activity range. The super-Nernstian region of the electrode depends on not only the the magnitude and duration of the applied current pulse but also the interfering ions. Under optimal conditions, the proposed Ca-ISE exhibits a super-Nernstian response between the calcium concentrations of 10^-2.5-10^-1.5 M with a slope of ca 80 mV/dec. The current pulse based Ca-ISE has been applied to determination of calcium in seawater with satisfactory results.