D₂O Isotope Effects on the Ionization of β-Naphthol and Boric Acid at Temperatures from 225 to 300 °C using UV-Visible Spectroscopy

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 H₂PO ⁻₄ (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 H₂PO ⁻₄ /HPO ²⁻₄ and D₂PO ⁻₄ /DPO ²⁻₄ , 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 $\log_{10}K_{\mathrm{H}_{2}\mathrm{O}}$ and $\log_{10}K_{\mathrm{D}_{2}\mathrm{O}}$ 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 H₂PO ⁻₄ (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.