High Temperature D2O Isotope Effects on Hydrolysis and Ionization Equilibria in Water

Erickson, Kristy M.
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University of Guelph

This thesis is an investigation of the relative differences of acid ionization constants and ion mobility in D2O versus H2O under hydrothermal conditions, for acetic acid and phosphoric acid. Values of specific conductivity were measured for each acid in H2O and D2O, as well as for a series of strong electrolytes using a high-temperature high-pressure AC conductivity flow cell that was originally designed at the University of Delaware. The Fuoss-Hsai-Fernández-Prini (FHFP) equation was fitted to the experimental values of molar conductivity, Λ, to obtain molar conductivities at infinite dilution, Λo. The molar conductivities at infinite dilution for each acid were used to calculate degrees of dissociation and ionization constants in D2O versus H2O from 298 K to 571 K. Measured values of differences in pKaq in D2O vs H2O, ΔpKaq = [pKaq,D2O – pKaq,H2O], become relatively independent of temperature above 423 K, with values of: ΔpKaq 0.45 for acetic acid and ΔpKaq 0.35 for phosphoric acid. The Density Model was then fitted to the values of pKaq in H2O and D2O to represent their temperature dependence to a precision of ± 0.01 in ΔpKaq. Comparisons of the molar conductivities and ionic molar conductivities at infinite dilution for the strong electrolytes in H2O and D2O as a function of temperature have also been made, based on Walden’s rule correlations, (λ°•η)D2O / (λ°•η)H2O. Changes in values of (λ°•η)D2O / (λ°•η)H2O as a function of temperature are consistent with a change in the relative hydration behavior of ions, where the effective Stokes’ radii of the ions in D2O versus H2O changes at temperatures above ~ 450 K. Changes in values of (λ°•η)D2O / (λ°•η)H2O for D+/H+ and OD-/OH- suggest that proton hopping “Grotthuss” mechanisms become more efficient in D2O versus H2O with increasing temperature.

Deuterium isotope effects, Hydrothermal Chemistry, Solution Conductivity, Ionization Constants