Pressure and isotope effects on the proton jump of the hydroxide ion at 25°C

Abstract

The limiting molar conductances Λ0 of potassium deuteroxide KOD in D2O and potassium hydroxide KOH in H2O were determined at 25°C as a function of pressure to disclose the difference in the proton-jump mechanism between an OH− (OD−) and a H3O+ (D3O+) ion. The excess conductance of the OD− ion in D2O λ E O (OD -), as estimated by the equation $$\lambda _E^O (OD^ - ) = \Lambda ^O (KOD/D_2 O) - \Lambda ^O (KCl/D_2 O)$$ increases a little with pressure as well as the excess conductance of the OH− ion in H2O $$\lambda _E^O (OH^ - ) = \Lambda ^O (KOH/H_2 O) - \Lambda ^O (KCl/H_2 O)$$ However, their rates of increase with pressure are much smaller than those of the excess deuteron and proton conductances, λ E O (D +) and λ E O (H +). With respect to the isotope effect on the excess conductance, λ E O (OH -)/λ E O (D +) decreases with presure as in the case of λ E O (H +)/λ E O (D +), but the value of λ E O (OH -)/λ E O (OD -) itself is much larger than that of λ E O (H +)/λ E O (D +) at each pressure. These results are ascribed to the difference in the pre-rotation of water molecules, which is brought about by the difference in the intial orientation of the rotating water molecule adjacent to the OH− (OD−) or the H3O+ (D3O+) ion.