Second Dissociation Constant of Carbonic Acid in H₂O and D₂O from 150 to 325 °C at p = 21 MPa Using Raman...

Solvent-corrected reduced isotropic spectra of carbonate and bicarbonate in light and heavy water have been measured from 150 to 325 °C at 21 MPa using a confocal Raman microscope and a custom-built titanium flow cell with sapphire windows. The positions of the symmetric vibrational modes of CO₃^2- and HCO₃^-/DCO₃^- were compared to density functional theory (DFT) calculations with a polarizable continuum model in light and heavy water. The experimental Raman peak positions shifted linearly toward lower wavenumbers with increasing temperatures. Raman scattering coefficients, measured relative to a perchlorate internal standard, were used to determine equilibrium molalities of the carbonate and bicarbonate species. These yielded quantitative thermodynamic equilibrium quotients for the reaction CO₃^2- + H₂O ⇌ HCO₃^- + OH^- and its deuterium counterpart. Ionization constants for HCO₃^- and DCO₃^-, K_2a,H,m and K_2a,D,m, calculated in their standard states using the Meissner-Tester activity coefficient model, were combined with critically evaluated literature data to derive expressions for their dependence on temperature and pressure, expressed as solvent molar volume, over the range 25 to 325 °C from p_sat to 21 MPa. These are the first experimental values to be reported for this reaction in light water above 250 °C and in heavy water above 25 °C. The value of the deuterium isotope effect on the chemical equilibrium constant, ΔpK_2a,m = pK_2a,D,m - pK_2a,H,m, decreased from ΔpK_2a,m = 0.67 ± 0.07 at 25 °C to ΔpK_2a,m = 0.17 ± 0.13 at 325 °C and p_sat.