Optimised ligand exchange solvent extraction methodology for the determination of labile and inert forms of copper in...

A recent study using a competitive ligand exchange solvent extraction (CLE-SE) method indicates that oceanic copper (Cu) exists in two distinct pools: a labile fraction dominating surface waters, likely produced by photochemical degradation of an inert pool, and a more stable inert fraction prevailing in the ocean interior. This finding challenges earlier studies and suggests that electrochemical characterisation may overestimate both the concentration and binding strength of natural Cu chelators. Here, we optimised the CLE-SE method using the artificial Cu ligand salicylaldoxime (SA). Assuming SA was present in sufficient excess to bind all exchangeable Cu, toluene was used to selectively extract Cu-SA complexes, presumably as the neutral Cu-SA bis-complex (Cu(HSA)₂). Extracted Cu was back-extracted with 10% nitric acid (HNO₃) and quantified by differential pulse anodic stripping voltammetry (DP-ASV). Method sensitivity of 100% (quantitative extraction of Cu) at SA concentrations ([SA]_TOT) above 10 μM was assessed using model solutions and an estuarine sample in the absence of organic ligands. Solution speciation was evaluated using thermodynamic modelling, spectroscopic observations, and extraction behaviour, supporting the dominance of Cu(HSA)₂ at [SA]_TOT above 10 μM. The formation constant of Cu(HSA)₂ (log K''_CuSA) was determined by spectrophotometric reverse titration with ethylenediaminetetraacetic acid (EDTA). Application to estuarine samples revealed an inert Cu fraction representing 15-25% of total dissolved Cu ([Cu]_TOT), which did not exchange with SA, even at [SA]_TOT up to 100 µM. Redox speciation excluded Cu(I) as the source of inert behaviour, while ultrafiltration identified colloids as the dominant form of inert Cu.