Proton concentration of supercritical water and high-concentrated carbon dioxide mixture using UV-vis spectroscopy

High-temperature and high-pressure UV-vis spectroscopy was applied to evaluate the proton concentration in supercritical water (SCW)-CO₂ (3 and 7 mol%) mixture at 380 °C and a water density of 400 kg m^-3 using 4-nitrophenol as a probe molecule. From the absorption spectra measured at various NaOH concentrations, the first dissociation constant of aqueous carbon dioxide (CO_2,aq + H₂O = H⁺ + HCO₃ ^-) was determined to be 3.3 × 10^-13. The determined dissociation constant was one order of magnitude smaller than that estimated from the revised HKF equation of state (2.5 × 10^-12). Using the determined dissociation constant, proton concentration in SCW-CO₂ mixture was estimated at various CO₂ concentrations (1-7 mol%) and was shown to be 6-19 times higher than that in pure SCW. The compound, 4-nitrophenol, was found to be a reliable probe for estimating proton concentrations in supercritical aqueous solutions. The estimated proton concentration in SCW-CO₂ mixture can be used for detailed analysis of SCW reactions, especially for acid-catalyzed ones.