Revised application of copper ion selective electrode (Cu-ISE) in marine waters: A new meta-calibration approach

Copper (Cu) is a bio-essential trace element that is of concerns due to its potential toxicity at concentrations commonly encountered in coastal waters. Here, we revisit the applicability of Cu(II) ion selective electrode (Cu-ISE) based on a jalpaite membrane for the measurement of Cu_free in seawater. At high total Cu concentration (>0.1 mM), (near)Nernstian slope was obtained and determination of Cu_free down to fM levels was possible. However, this slope decreases with decreasing total Cu concentration (e.g. 7 mV/decade at 15 nM total Cu) making the use of a common single calibration approach unreliable. To solve this problem, we carried out several calibrations at different levels of total Cu (15 nM - 1 mM) and ethylenediamine (EN: 5 μM - 15 mM) and fitted the calibration parameters (slope and intercept) as a function of total Cu using the Gompertz function (a meta-calibration approach). The derived empirical equations allowed the determination of Cu_free at any total Cu concentration above 20 nM (determination of Cu_free at lower total Cu levels is prevented by the dissolution of the electrode). We successfully tested this meta-calibration approach in UV digested seawater in presence of a synthetic ligand (EN), isolated natural organic matter (humic acid, HA) and in a natural estuarine sample. In each case, our meta-calibration approach provided a good agreement with modeled speciation data (Visual MINTEQ), while standard single approach failed. We provide here a new method for the direct determination of the free Cu ion concentration in seawater at levels relevant for coastal waters.