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Application of Cu(II) ion-selective electrode (Cu- ISE) for Cu speciation in coastal waters

Copper (Cu) is a micronutrient required in a number of cellular processes that are key for phytoplankton growth. However, at physiologically high concentrations, copper is toxic and may affects both planktonic abundance and diversity in coastal waters. Range of copper concentrations between these two extremes is relatively narrow [1]. In natural waters Cu is mostly found as inorganic and organic complexes. A classical model for estimation of the bioavailability/toxicity assumes free Cu (Cu2+) as an indicator for potential Cu toxicity. Due to very low concentration of Cu in natural waters, the measurement of Cu is very challenging. The most used analytical techniques are electrochemical ones, mainly anodic and adsorptive cathodic stripping voltammetry (ASV and AdCSV, respectively [2]). Potentiometry based on Cu(II) ion-selective electrode was also found promising (measurements down to pM levels), despite a common opinion that it is not sufficiently sensitive to measure Cu concentrations below 1 µM, especially in marine waters due to the chloride influence [3, 4]. In this work we examined the applicability of Cu- ISE (based on jalpaite membrane) for the measurement of free Cu in model solution (0.5 M NaCl) and seawater. The calibrations of Cu-ISE were performed at different levels of total Cu (0.3 mM - 0.01 µM) and ethylenediamine (EN: 1 mM - 5 µM) concentrations. At high total Cu, calibrations were linear down to pM/fM level of free Cu, with the obtained slopes very close to theoretical Nernstian one (29.6 mV per decade). However, the decrease of the slope was observed by lowering the total Cu concentration, so that as low as ~10 mV per decade was found at 10 nM Cu concentration. Calibrations performed only with additions of Cu showed "super-Nernstian" response with linear range down to 10 nM of Cu in 0.5 M NaCl and 100 nM Cu in seawater. Several aspects of the practical use of Cu-ISE will be discussed, along with the examples of Cu-ISE application in real seawater samples. [1] R. Zitoun, et al., Sci. Total Environ. 653 (2019) 300-314. [2] I. Pižeta, et al. Mar. Chem. 173 (2015) 3-24. [3] T.N. Tait, et al. Environ Chem 13 (2016) 140- 148. [4] R. De Marco, G. Clarke, and B. Pejcic. Electroanalysis 19 (2007) 1987-2001.

Copper speciation ; Ion-selective electrodes ; Marine environment

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