sulfonic acid (CAS NO. ) is purified by dissolving in a small volume of distd H₂O and stirring with slightly less than the theoretical amount of BaCO₃. When effervescence is complete and the solution is still acidic, filter off the insoluble barium benzenesulfonate. The salt is collected and dried to constant weight in vacuo, then suspended in H₂O and stirred with a little less than the equivalent (half mol.) of sulfuric acid. The insoluble BaSO₄ (containing a little barium benzenesulfonate) is filtd off and the filtrate containing the free acid is evapd in a high vacuum. The oily residue will eventually crystallise when completely anhydrous. A 32% commercial acid was caused to fractionally cryst at room temp over P₂O₅ in a vac desiccator giving finally colorless deliquescent plates m 52.5 °C.

The anhydrous crystn acid is deliquescent and should be stored over anhyd Na₂SO₄ in the dark and should be used in subdued sunlight as it darkens under sunlight. The main impurifty is Fe which readily separates as the Fe salt in the early fractions. It is an irritant to the skin and eyes.