Fuming sulphuric acid: Dependence of the concentration of sulphur trioxide and its activity coefficient on the composition of oleum

Abstract UV absorption measurements of fuming sulphuric acid solutions of varying composition (3‐60% sulphur trioxide) were made at 25±2°. From these measurements the concentration dependence of sulphur trioxide on acid composition was estimated and compared with calculated sulphur trioxide concentrations assuming the simple equilibrium H2SO4 + SO3 ⇌ H2S2O7 [6]. From this comparison it was deduced that K6 ≥ 5 mole−1 ·l. The sulphur trioxide concentration in 100% sulphuric acid at 25 ± 2° was calculated with this value for K6 to be 8 × 10−5 mole·l−1 at the most. Evidence is put forward that the activity coefficient of sulphur trioxide is almost independent of the composition of oleum over the acid range studied.