The Effect of the Pressure on Proton Jumps in Ethanol–Water Mixtures at 25 °C

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Abstract
The limiting molar conductances of hydrochloric acid in 0, 5, 10, 20, 30, and 80 mol% ethanol–water mixtures, and those of potassium chloride in 80 mol% of ethanol, were determined at 25 °C as a function of the pressure up to 2000 kgf cm−2 (1 kgf cm−2=0.9807×105 Pa) from the conductances measured in a dilute concentration range. In each solvent except for 80 mol% of ethanol, the limiting molar conductances of the proton, λ°(H+), are about four to six times larger than those of the potassium ion, λ°(K+), at all the pressures studied; in 80 mol% of ethanol, the value of λ°(H+) is very close to that of λ°(K+) at atmospheric pressure. The results suggest that the proton-jump mechanism acts in the water-rich region, but not in the ethanol-rich region. The excess proton conductances at an infinite dilution, λ°E, as estimated by [λ°E=λ°(H+)−λ°(K+)], were found to increase with an increase in the pressure. The pressure dependence of the excess proton conductances normalized to their values at 1 atm, λ°E(P)⁄λ°E(1), is most prominent in 5 mol% of ethanol; its magnitude decreases in this order; 5>0>10>>20>>30 mol% of ethanol. These results are discussed in relation to the effect of ethanol molecules on the water structure in the mixtures; a small addition of ethanol to water stabilizes the three-dimensional networks of water, while a further addition of ethanol makes it difficult to form them.