Conductance titrations in dilute oleum and the dissociation constant of disulphuric acid in solvent sulphuric acid

Abstract

Conductance titrations of oleums with potassium hydrogen (or deuterium) sulphate are reported for the temperatures 25 and 69° in both the hydrogen and the deuterium sulphuric acid systems. A single experiment was also carried out at 11° with the hydrogen system. The conductance at the minimum in such titrations is greater than that of the pure acid and indicates that the hydrogen (deuterium) disulphate ion has an appreciable mobility, which must be taken into account in the analysis of the results to determine K_a. In the H₂SO₄ system, the K_a of H₂S₂O₇ was 0.025 mole (kg solv.)^–1 at 25° and 0.029 at 69°, implying ΔH_a= 0.7 kcal mole^–1: the single experiment at 11° gave 0.022, confirming the small temperature dependence. In the D₂SO₄ system, the K_a of D₂S₂O₇ is 0.0060 (same units) at 25° and 0.0074 at 69°, implying ΔH_a= 1.0 kcal mole^–1. The sensitivity of these numbers to the choice of other constants is examined, and simple rules are given for estimating the effects of any subsequent adjustments in these constants.