Abstract: [Objective] The injection of gases containing CO? into deep aquifers will dissolve to form carbonic acid, leading to a reduction in the pH value of the aquifer. This, in turn, will cause metal corrosion of pipelines and dissolution or precipitation of minerals, which will have an impact on the safety and efficacy of CO? geological utilisation and storage. The combination of experimental measurements of the pH of pure/impure CO? saturated solution systems with model predictions allows for the assessment of chemical changes under conditions of CO? geological sequestration. [Methods] In this study, the pH of pure CO?-water systems and impure CO?-water systems were measured under in situ conditions using potential and spectroscopic methods at temperatures between 35 and 93°C and pressures between 0.38 and 18 MPa. A chemical equilibrium model based on solubility calibration was then established to calculate and predict the pH values of pure/impure CO?–water systems for evaluation purposes. [Results] The findings indicate that N? and CH? exerts a detrimental impact on the CO?-saturated system, resulting in the reduction of CO? solubility in water and the subsequent elevation in pH. Notably, the influence exerted by CH? is more pronounced than N?. The model demonstrats superior performance in predicting the pH of the pure CO2-water system, with a maximum deviation of only 0.05 pH. However, in the impure CO2-water system, some deviation is observed, particularly at 50°C and a CO2-impurity gas ratio of 1:9, with a maximum deviation of 0.15 pH. [Conclusion] The findings of this study offer valuable insights into the chemical changes associated with impure CO2 injection into the formation, which is crucial for enhancing the safety and efficacy of carbon sequestration.