Second Dissociation Constant of Carbonic Acid in H<sub>2</sub>O and D<sub>2</sub>O from 150 to 325 °C at <i>p</i> = 21 MPa Using Raman Spectroscopy and a Sapphire-Windowed Flow Cell
Bibliographic record
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 CO 3 2– and HCO 3 – /DCO 3 – 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 CO 3 2– + H 2 O ⇌ HCO 3 – + OH – and its deuterium counterpart. Ionization constants for HCO 3 – and DCO 3 –, 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, Δp K 2a,m = p K 2a,D,m – p K 2a,H,m, decreased from Δp K 2a,m = 0.67 ± 0.07 at 25 °C to Δp K 2a,m = 0.17 ± 0.13 at 325 °C and p sat .
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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.001 | 0.000 |
| Research integrity | 0.000 | 0.001 |
| Insufficient payload (model declined to judge) | 0.003 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one source (direct Gemma or distilled Codex), not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".