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

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 H 2 SO 4 + SO 3 ⇌ H 2 S 2 O 7 [6]. From this comparison it was deduced that K 6 ≥ 5 mole −1 · l . The sulphur trioxide concentration in 100% sulphuric acid at 25 ± 2° was calculated with this value for K 6 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.