pH of CO₂ saturated water and CO₂ saturated brines: Experimental measurements and modelling

In this work, both spectroscopic and electrometric methods were employed to measure the pH of water saturated with carbon dioxide at pressures up to 6 MPa, temperature ranges from 293.15 to 353.15 K and salinities up to 3 mol kg⁻¹. Furthermore, a model was developed to predict the changes in the pH due to the solubility of CO₂ in the aqueous phase at high pressure and high temperature conditions as well as the effect of NaCl. The pH model was developed by coupling the Cubic-Plus-Association Equation of State (CPA EoS) and the Pitzer equations. The former was used to determine the solubility of acid gases in aqueous solutions while the latter was employed to calculate activity coefficients for each ion species. The predictive capability of the pH model was evaluated against the data gathered from the literature and data measured in this work. The model allowed a prediction of the pH with an overall average absolute deviation (AAD) to measured data of 0.03 and 0.06 pH units in the CO₂-H₂O system using electrometric and spectroscopic techniques, respectively, and between 0.04 and 0.10 pH units in the CO₂-H₂O-NaCl systems by employing the spectroscopic technique.