The speciation of cobalt in seawater and fresh waters at 25°C

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Cobalt(II) speciation in seawater and fresh waters was calculated using recent spectrophotometrically determined stability data for cobalt(II) chloride and bromide complexes in aqueous solutions, and a theoretically estimated stability constant for the cobalt(II) bicarbonate complex. Calculations for seawater suggest that 56.25% of total dissolved cobalt(II) occurs as the free ion, Co²⁺, 39.53% as CoCl⁺, 2.76% as CoCl₂⁰, 1.02% as CoSO₄⁰, and less than 1.0% as CoNH₃²⁺, CoCO₃⁰, CoHCO₃⁺, CoCl₃^–, CoF⁺ and CoBr⁺. In neutral to acidic fresh waters, 90% or more of the dissolved cobalt(II) occurs as the free ion. CoCO₃⁰ becomes important in basic fresh waters (pH > 8.5), and predominant in very basic fresh waters (pH > 9.5). Co(II) organic complexes are not included in the calculations, discussions suggest that they are unlikely to be abundant in seawater. Co(II) organic complexes may, however, be important in fresh waters, and future work should investigate this possibility.