Highly pure para-toluene-sulphonic acid prepn. by sulphonating toluene with conc. sulphuric acid at atmos. pressure

Toluene is sulphonated with conc. 92-98 % H2SO4 at atmos., pressure in H2SO4 : toluene ratio 1.27-2.5
(1.8), by stirring for 1-7 hrs. Stirring is stopped when 50 % of the H2SO4 is converted to toluene-sulphonic acid. The layers of toluene and sulpho-mass are sepd. The toluene is syphoned off and recycled entirely
to the new charge. The sulpho-mass is stirred with water in wt. ratio water : sulpho-mass 0.1-0.5.
The reaction mixt., at 70-85 deg. C, is then cooled to 0-30 deg. C for 15 min. The p-toluenesulphonic acid mono-hydrate crystals are sepd. in 80-90 % yield by centrifuging. The dried crystals, having purity over 43 % p-toluenesulphonic acid and 3 % H2SO4, are dissolved in water, in water : crystal wt. ratio 0.1-0 (sic) at 60-100 deg. C. The soln. is hot-filtered to remove any mechanical contaminants and cooled to 0-30 deg. C. The p-toluenesulphonic acid mono-hydrate crystallizes and is sepd. by centrifuging. The dried, over 98 %-pure crystals of p-toluenesulphonic acid are dried in vacuo at 50-100 deg. C. The filtrate, contg. 65 % p-toluenesulphonic acid, is recycled for the dilution and crystallization of p-toluenesulphonic acid in the subsequent charge.