Measurement of free cupric ion concentration in seawater by a ligand competition technique involving copper sorption onto C18 SEP‐PAK cartridges1

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Abstract
Knowledge of ion speciation and free ion concentrations is important in predicting the biological availability and geochemical behavior of trace metals. Here we describe a new technique for measuring free cupric ion concentration in seawater based on sorption of copper onto SEP‐PAK C18 cartridges and internal free cupric ion calibration. Calibration was achieved by adding cupric ion buffers, consisting of copper and excess concentrations of synthetic chelator, ethylenediaminetetraacetic acid (EDTA). EDTA competes with natural organic ligands for copper complexation, but its copper chelate is not retained by the C18 cartridges. This latter property allows EDTA to be used for free cupric ion calibration.Titrations in seawater utilizing the new technique revealed that copper was highly complexed. Free cupric ion concentrations in three samples of seawater from lower Narragansett Bay, Rhode Island, and one from off the coast of Peru were measured to be 10−12.3, 10−12.0, 10 12.1, and 10−11.5 M at total dissolved copper concentrations of 26.7, 16.5, 19.4, and 3.7 nM. From these values and from computed and experimental values for inorganic complexation, we compute that only 0.03–2% of the dissolved copper can be accounted for as inorganic species (Cu2+, CuCO3, CuOH +, etc.). The remainder, 99.97–98%, appears to be organically complexed. These results are in general agreement with several other recent studies, using several techniques, of copper complexation in seawater.