Second Dissociation Constant of Carbonic Acid in H2O and D2O from 150 to 325 °C at p = 21 MPa Using Raman Spectroscopy and a Sapphire-Windowed Flow Cell.

Abstract

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 CO32- 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 CO32- + H2O ⇌ HCO3- + OH- and its deuterium counterpart. Ionization constants for HCO3- and DCO3-, K2a,H,m and K2a,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 psat 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, ΔpK2a,m = pK2a,D,m - pK2a,H,m, decreased from ΔpK2a,m = 0.67 ± 0.07 at 25 °C to ΔpK2a,m = 0.17 ± 0.13 at 325 °C and psat.