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 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.

Nuclear Magnetic Resonance Study of CO2 Capture by Fluoroalkylamines: Ammonium Ion pKa Depression via Fluorine Modification and Thermochemistry of Carbamylation

“…The equilibrium constants K 2 and K 3 of reactions and were reported in the literature (p K 2 = 6.352 24 or 6.18 and p K 3 = 10.3). The acidity constants p K 4 (R 1 R 2 NH 2 + ⇌ R 1 R 2 NH + H + ) were experimentally determined by strong acid/base titration ( vide supra ), and they are p K 4 ( 2a ) = 6.05 and p K 4 ( 3a ) = 7.68.…”

Second ionization constant of sulfuric acid in H2O and D2O from 150 to 300 °C at p = 11.5 MPa using flow AC conductivity

“…The deuterium isotope effect on chemical equilibrium constants has been reported in the literature for very few chemical systems at high temperatures. 5–12 As was described in the review by Laughton and Robertson, 2 a typical deuterium isotope effect is 0.2 > Δp K a,m > 1.0 at 25 °C with the general trend that the deuterium isotope effect increases with the acid p K a value. 4,76 As the temperature increases, the deuterium isotope effect on an acid ionization constant may increase or decrease, and the relative order of the effect between different acids is not preserved.…”

“…19 The potentiometric work by Dickson et al 20 was not included because the experiments were conducted in NaCl media over a wide concentration range of 0.1-5 mol kg À1 and this study relied on a Pitzer treatment to extrapolate the equilibrium constants to infinite dilution. We speculate based on previous work 11 that using a Pitzer model in this manner with only four adjustable fit parameters will lead to a systematic underestimation of the equilibrium constants with increasing temperature. Table 8 lists the values of the parameters resulting from the equation of state fit (eqn ( 40)).…”

“…The equilibrium constant in light water, K 2a,H,m , was modeled as a function of temperature and molar volume by fitting with an equation of state that is a modified form 11 of the ‘density’ model reported by Anderson et al : 60 where p 1 , p 2 , and p 3 are adjustable fit parameters, T is the temperature in Kelvin, and the solvent density ρ H 2 O has been replaced by its molar volume, in cm 3 mol −1 to remove the effects of solvent molar mass. Values of the molar volumes of light and heavy waters are obtained from the NIST REFPROP database.…”

“…1,2 The dissociation constants of weak acids are known to be larger in H 2 O than in D 2 O with a general trend of the difference being larger for weaker acids. 3,4 It is not well understood how these deuterium isotope effects change at high temperature and pressure because very few studies [5][6][7][8][9][10][11][12] exist in heavy water above B100 1C. In order to develop accurate thermodynamic models of heavy water systems at high temperatures, more experimental equilibrium constants and transport property data under these conditions are required.…”