The stoichiometric, KHA*, 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 HAsO42-) in seawater at 25°C diluted with water have been fitted to equations of the form (Millero, 1979). In KHA* = In KHA + AS 1 2 + BS where In KHA 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 KHA* 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 KHA predicted values that agreed with those determined by using the ion pairing model. The exception was the ionization of HPO42- due to the strong interactions of Ca2+ and Mg2+ with PO43-. The differences in the predicted values of KHA* in seawater diluted with pure water and average river water were very small for all the acids except HPO42- (the maximum ΔpK = 0.96 in average river water). The larger difference in the KHA* for HPO42- in river waters is due to the strong interactions of Ca2+ and PO43-.
ASJC Scopus subject areas
- Geochemistry and Petrology