A chemical equilibrium model for natural waters

This paper reviews the present status of the Pitzer chemical equilibrium mo del, which can be used to characterize the one-atmosphere activity coeffici ents of ionic and non-ionic solutes in natural waters as a function of temp erature and ionic strength. The model considers the ionic interactions of t he major seasalt ions (H, Na, K, Mg, Ca, Sr, Cl, Br, OH, HCO3, B(OH)(4), HS O4, SO4, CO3, CO2, B(OH)(3), H2O) and is based on the 25 degrees C model of Weare and co-workers. The model has been extended by a number of workers s o that reasonable estimates can be made of the activity coefficients of mos t of the major seasalt ions from 0 to 250 degrees C. Recently coefficients for a number of solutes that are needed to determine the dissociation const ants of the acids from 0 to 50 degrees C (H3CO3, B(OH)(3), H2O, HF, HSO4-, H3PO4, H2S, NH4+ etc.) have been added to the model. These results have bee n used to examine the carbonate system in natural waters and determine the activity of inorganic anions that can complex trace metals. The activity an d osmotic coefficients determined from the model are shown to be in good ag reement with measured values in seawater. This model can serve as the found ation for future expansions that can examine the activity coefficient and s peciation of trace metals in natural waters. At present this is only possib le from 0 to 50 degrees C over a limited range of ionic strengths (<1.0) du e to the limited stability constants for the formation of the metal complex es. The future work needed to extend the Pitzer model to trace metals is di scussed.