The ionization of acids in estuarine waters

The stoichiometric, K_HA^*, and apparent, K'_HA, constants for the ionization of a number of weak acids (NH₄⁺, HSO₄^-, HF, H₂O, B(OH)₃, H₂CO₃, HCO₃^-, H₃PO₄, H₂PO₄^-, HPO₄², H₃AsO₄ H₂AsO₄^- and HAsO₄^2-) in seawater at 25°C diluted with water have been fitted to equations of the form (Millero, 1979). In K_HA^* = In K_HA + AS ^{1 2} + BS where In K_HA is the thermodynamic constant in water, S is the salinity, A and B are adjustable parameters. The validity of this equation in estuarine waters has been examined by using an ion pairing model (Millero and Schreiber, 1981). The calculated values of K_HA^* and K'_HA at S = 35%. are in good agreement with the measured values for all the systems examined. The equation used to extrapolate the measured values to pure water K_HA predicted values that agreed with those determined by using the ion pairing model. The exception was the ionization of HPO₄^2- due to the strong interactions of Ca²⁺ and Mg²⁺ with PO₄^3-. The differences in the predicted values of K_HA^* in seawater diluted with pure water and average river water were very small for all the acids except HPO₄^2- (the maximum ΔpK = 0.96 in average river water). The larger difference in the K_HA^* for HPO₄^2- in river waters is due to the strong interactions of Ca²⁺ and PO₄^3-.