Quantification of the Dissolved Inorganic Carbon Species and of the pH of Alkaline Solutions Exposed to CO₂ under Pressure: A Novel Approach by Raman Scattering

Dissolved inorganic carbon (DIC) content of aqueous systems is a key function of the pH, of the total alkanility (TA), and of the partial pressure of CO₂. However, common analytical techniques used to determine the DIC content in water are unable to operate under high CO₂ pressure. Here, we propose to use Raman spectroscopy as a novel alternative to discriminate and quantitatively monitor the three dissolved inorganic carbon species CO₂(aq), HCO₃^–, and CO₃^2– of alkaline solutions under high CO₂ pressure (from P = 0 to 250 bar at T = 40 °C). In addition, we demonstrate that the pH values can be extracted from the molalities of CO₂(aq) and HCO₃^–. The results are in very good agreement with those obtained from direct spectrophotometric measurements using colored indicators. This novel method presents the great advantage over high pressure conventional techniques of not using breakable electrodes or reference additives and appears of great interest especially in marine biogeochemistry, in carbon capture and storage and in material engineering under high CO₂ pressure.