| Abstrakt: |
Abstract The deuterium-isotope effects on the ionization constants of β-naphthol (2-naphthol) and boric acid, Δlog 10 K=[log 10 K D2O−log 10 K H2O], have been determined from measurements in light and heavy water at temperatures from 225 °C≤t≤300 °C and pressures near steam saturation. β-Naphthol is a thermally-stable colorimetric pH indicator, whose ionization constant lies close to that of H2PO4− (aq), the only acid for which Δlog 10 K is accurately known at elevated temperatures. A newly designed platinum flow cell was used to measure UV-visible spectra of β-naphthol in acid, base, and buffer solutions of H2PO4−/HPO42− and D2PO4−/DPO42−, from which the degree of ionization at known values of pH and pD was determined. Values of the ionization constants of β-naphthol in light and heavy water were calculated from these results, and used to derive a model for and over the experimental temperature range with an estimated precision of ±0.02 in log 10 K. The new values of K H2O and K D2O allowed us to use β-naphthol as a colorimetric indicator, to measure the equilibrium pH and pD of the buffer solutions B(OH)3/B(OH)4− and B(OD)3/B(OD)4− up to 300 °C, from which the ionization constants of boric acid were calculated. The magnitude of the deuterium isotope effect for H2PO4− (aq) is known to fall from Δlog 10 K=−0.62 to Δlog 10 K=−0.47, on the “aquamolal” concentration scale, as the temperature rises above 125 °C, but then remains almost constant. Although the temperature range is more limited, the new results for β-naphthol and boric acid appear to show a similar trend. [ABSTRACT FROM AUTHOR] |
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