The pH of CO2-saturated aqueous NaCl and NaHCO3 solutions at temperatures between 308Â K and 373Â KÂ at pressures up to 15Â MPa

The pH is a critical variable for carbon storage in saline aquifers because it affects the reaction rate and equilibrium state of the reservoir rocks, thus influencing the rates of mineral dissolution or precipitation and the integrity of caprocks. In this work, high-pressure pH and Ag/AgCl-reference electrodes were used to measure the pH of CO2-saturated aqueous solutions of NaCl and NaHCO3. The expanded uncertainty of the pH measurements is 0.20 at 95% probability. For CO2-saturated NaCl(aq), measurements were carried out at total pressures from (0.37 to 15.3) MPa and temperatures from (308 to 373) K with NaCl molalities of (1, 3 and 5) mol·kg−1. For CO2-saturated NaHCO3(aq), the pH was measured at total pressures from (0.2 to 15.3) MPa and temperatures from (308 to 353) K with NaHCO3 molalities of (0.01, 0.1 and 1) mol·kg−1. The pH was found to decrease with increase in pressure and with decrease in temperature for both CO2-saturated NaCl and NaHCO3 solutions. For CO2-saturated NaCl(aq), the pH was observed to decrease with increase of salt molality, while for CO2-saturated NaHCO3, the opposite behaviour was observed. The results have been compared with predictions obtained from the PHREEQC geochemical simulator (version 3.3.9) incorporating the Pitzer activity-coefficient model with parameters taken from the literature. For CO2-saturated NaCl(aq), agreement to within ±0.2 pH units was observed in most cases, although deviations of up to 0.3 were found at the highest molality. In the case of CO2-saturated NaHCO3(aq), the experimental data were found to deviate increasingly from the model with increasing salt molality and, at 1 mol·kg−1, the model underestimated the pH by between 0.3 and 0.7 units.