Determination of copper complexation in sea water by cathodic stripping voltammetry and ligand competition with salicylaldoxime

Abstract

Copper complexing ligands in sea water were determined by cathodic stripping voltammetry (CSV) with ligand competition using salicylaldoxime (SA) at a detection window intermediate to that and overlapping with those currently available using other electroactive ligands. The optimised condition to determine total dissolved copper in sea water by CSV using SA entails an SA concentration of 25 μM, a solution pH of 8.0–8.4, and a deposition potential of -1.1 V; the voltammetric scans were initiated at -0.15 V. The conditional stability constants for copper complexation by SA were calibrated by ligand competition against EDTA in sea water of salinities between 1 and 35. The following empirical relationships were found to hold between these conditional stability constants and the salinity (S, in psu): log K′CuSA = (10.12 ± 0.03) - (0.37+-0.02) log S, and log β′Cu(SA)2 = (15.78 +- 0.08) - (0.53 +- 0.07) log S. The centre of the detection window for detection of copper complexation in sea water using SA can be varied in the range of 3.6–5.8 by varying the SA concentration between 1 and 25 μM; this range lies between that covered by tropolone (2.5–4.5) and 8-quinolinol (5.0–8.4). Use of SA in CSV has the analytical advantage of high sensitivity for copper (3–4 fold greater than using catechol, 8-quinolinol or tropolone), reflected in a limit of detection of 0.1 nM copper at a deposition time of 1 min which can be lowered further by extending the deposition time. Comparative complexing ligand titrations indicated that an equilibration period of 6 h is required to attain equilibrium between the added copper, the natural comlexing ligands and the added SA. The method was tested by determining copper complexation at several windows in samples from the North Sea, the NW Mediterranean Sea and the NE Atlantic Ocean, revealing the presence of several complexing ligands. At a constant detection window the ligand concentration in the upper water column of a station in the NE Atlantic was found to vary between 3 and 8 nM, with a maximum at 78 m depth.