1 pH of CO 2 saturated water and CO 2 saturated brines : 1 Experimental measurements and modelling 2

8 In this work, both spectroscopic and electrometric methods were employed to measure the pH 9 of water saturated with carbon dioxide at pressures up to 6 MPa, temperature ranges from 10 293.15 to 353.15 K and salinities up to 3 mol·kg. 11 Furthermore, a model was developed to predict the changes in the pH due to the solubility of 12 CO2 in the aqueous phase at high pressure and high temperature conditions as well as the effect 13 of NaCl. The pH model was developed by coupling the Cubic-Plus-Association Equation of 14 State (CPA EoS) and the Pitzer equations. The former was used to determine the solubility of 15 acid gases in aqueous solutions while the latter was employed to calculate activity coefficients 16 for each ion species. The predictive capability of the pH model was evaluated against the data 17 gathered from the literature and data measured in this work. The model allowed a prediction 18 of the pH with an overall average absolute deviation (AAD) to measured data of 0.03 and 0.06 19 pH units in the CO2-H2O system using electrometric and spectroscopic techniques, 20 respectively, and between 0.04 and 0.10 pH units in the CO2-H2O-NaCl systems by employing 21 the spectroscopic technique. 22