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 (CO2,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.