Variation of the Apparent Dissociation Constants of Carbonic Acid With Magnesium and Calcium Concentrations in Seawater


The effects of calcium and magnesium ion concentrations in artificial seawater (ASW) on the equilibrium state of the system H2CO3-H2O have been tested. The measured first and second apparent dissociation constants of carbonic acid, K¡¦1 and K¡¦2, increase with an increase in the concentrations of calcium and magnesium on ASW. The total and free activity coefficients of HCO3- and CO23- at the ionic strength of 0.718 and temperature of 25¢J decrease as a function of Mg:Ca concentration ratios in ASW. The results also show that the calcium ion has a greater effect on the apparent dissociation constants of carbonic acid than magnesium, suggesting that the calcium ion associates more strongly with bicarbonate and carbonate than the magnesium ion does.

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