PREDICTION OF PH IN NATURAL HYDROTHERMAL FLUIDS AT ELEVATED P-T CONDITIONS

The pH of fluids at elevated temperature-pressure conditions are routi nely calculated from fluid compositions measured in the laboratory. Nu merical errors, uncertainties in thermodynamic data, and analytical er rors can affect these computed values. An analysis of pH values comput ed at elevated temperature and pressure conditions for natural fluids reveals that the amount of error in computed pH values can be, but is not always, dependent on fluid composition. Not all reactions occurrin g in a fluid affect computed pH values. Therefore, inadequacies in the thermodynamic properties of a species may affect computed pH values i n some fluid compositions but not others. When a single reaction contr ols the production/consumption of H+, the amount by which the pH chang es as a function of temperature and pressure is independent of the mea sured pH. However, when two or more reactions produce/consume sufficie nt H+ to change the pH, the amount by which the pH changes as a functi on of temperature and pressure is dependent on the measured pH. Uncert ainties in the measured pH are magnified as temperature and pressure i ncrease for some fluid compositions, while the uncertainty decreases i n others. When small uncertainties in the measured pH are magnified, t he activities of aqueous species calculated at elevated temperature an d pressure conditions cannot be used to predict fluid-mineral equilibr ia. (C) 1998 Elsevier Science B.V. All rights reserved.