Marine Science Faculty Publications

Inorganic Complexation of Zinc (II) in Seawater

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Formation constants for zinc complexation by bicarbonate, carbonate, and oxalate, on the molal concentration scale, were determined through observation of aqueous/tributyl phosphate distribution equilibria. At 25°C in our 0.68 molal ionic strength mixtures (0.55 m NaCl plus 0.13 m NaClO4-NaHCO3) we obtained the following results: Hβ'1= [ZnHCO3+][Zn2+]−1[HCO3−]t−1logHβ'1= 0.85 ± 0.04 β'1= [ZnCO30][Zn2+]−1[CO32−]t−1log β'1= 3.30 ± 0.08 Oxβ'1= [ZnC2O40][Zn2+]−1[C2O42−]t−1logOxβ'1= 3.58 ± 0.03 where [ ] denotes the concentration of each indicated chemical species, and [HCO3−]t, [CO32−]t, and [C2O42−]t are the total (free plus ion paired) concentrations of bicarbonate, carbonate, and oxalate. Our analyses in chloride media provide additionally an assessment of the conditional complexation constant of zinc with chloride ion: Clβ1= [ZnCl+][Zn2+]−1[Cl−]−1; logClβ1= −0.4 ±0.1 Our results indicate that in seawater (S = 35%., 25°C, carbonate alkalinity = 2.09 × 10−3 eq 1−1, pH 8.2) approximately 64% of total inorganic zinc exists as the free ion, 14% is complexed with chloride, 10% is complexed with carbonate, and complexation with hydroxide, sulfate, and bicarbonate account for approximately 6, 5, and 1% of the total inorganic zinc, respectively.

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Geochimica et Cosmochimica Acta, v. 54, issue 3, p. 753-760