A Chemical Equilibrium Model for the Carbonate System in Natural Waters

This paper describes a chemical equilibrium model which can be used to characterize the carbonate system in natural waters from 0 to 50°C and high ionic strengths (6 mol L^-1). The model considers the ionic interactions in solutions of the major sea salts (H-Na-K-Mg-Ca-Sr-Cl-Br-OH-HCO₃-B(OH)₄-HSO ₄-SO₄-CO₃-CO₂-B(OH) ₃-H₂O). The estimated activity coefficients and infinite dilution constants have been used to determine the dissociation constants of all the acids (H₂CO₃, B(OH)₃, H₂O, HF, HSO₄^-, H₃PO₄, H₂S, NH₄⁺ etc.) needed to examine the carbon dioxide system in natural waters. The model is largely based on measurements of dissociation constants in NaCl solutions with small amounts of Mg²⁺. The model predicts the activity coefficients of HCl in seawater that agree with the measured values to 0.002 from S = 5 to 45 and t = 0 to 50°C. The model has also been used to examine the dissociation constants of acids in seawater to test its reliability. The calculated values of the dissociation constants for the ionization of carbonic and boric acids were found to be in good agreement (± 0.02 to ± 0.03 in pK) with the experimental measurements in NaCl and seawater solutions from 5 to 45°C and S = 10 to 45. The model predicts the dissociation constants of the minor acids with sufficient accuracy (± 0.03 to ± 0.06) to characterize the carbonate system in brines over a wide range of temperatures (0 to 50°C) and ionic strength (0 to 6 mol L^-1). These constants can be used to characterize the carbonate systems in natural brines using measurements of two carbonate parameters (pH-TA, pH, TCO₂ etc.).