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 ^O_E (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 ^O_E (D ⁺) and λ ^O_E (H ⁺). With respect to the isotope effect on the excess conductance, λ ^O_E (OH ^-)/λ ^O_E (D ⁺) decreases with presure as in the case of λ ^O_E (H ⁺)/λ ^O_E (D ⁺), but the value of λ ^O_E (OH ^-)/λ ^O_E (OD ^-) itself is much larger than that of λ ^O_E (H ⁺)/λ ^O_E (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.