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Isotopic fractionation factors ϕa and ϕb for the hydroxy hydrogen and hydrogen-bonded water hydrogens of the solvated hydroxide ion are important in controlling solvent isotope effects upon reactions involving the hydroxide ion, and were required for a vibrational analysis of the solvated ion. The product ϕaϕb3= 0.434 is accurately available from e.m.f. measurements corrected for free energies of ion transfer between H2O and D2O, but separation of ϕa and ϕb, which requires measurements in H2O–D2O mixtures, is known to be subject to considerable uncertainty. Thus careful measurements with 1,8-bis(dimethylamino)naphthalene gave KD2O/KH2O= 0.420 ± 0.006 for the ratio of basic ionisation constants in H2O and D2O and ϕP= 0.901 ± 0.014 for the fractionation factor of the protonated base, but measurements in 1 : 1 H2O–D2O gave an ionisation constant too small to be consistent with any reasonable fractionation factor model for the hydroxide ion. Alternative methods of dissecting ϕa and ϕb, from measurements of isotope separation factors between hydroxide solutions and water vapour and autoprotolysis constants of H2O–D2O mixtures, are critically reviewed and optimum values are assessed. The results are shown to be sensitive to experimental error and to medium effects but not to departures from the Rule of the Geometric Mean.
Chiang et al. (Tue,) studied this question.