Para-­toluene-­sulphonic acid prepn. ­ by treating di:para-­tolyl­-tri:chloro­ethane prodn. residue, contg. sulphuric acid and toluene­-sulphonic acid, with toluene

Para-­toluene-­sulphonic acid is prepd. by using the residue from di-­p-­tolyl-­trichloroethane prodn. which contains 60% H2SO4 and 27% toluene-­sulphonic acid. The residue is treated with toluene in ratio 1 : 1 to 1 : 4 (1 :1 :8), with stirring and heating, at 84­-140 (110­-114) deg.C. Unreacted­ and excess toluene distil azeotropically with water. After sepg. the water ­toluene layers, the water is discharged from process. Toluene is recirculated. Re-fluxing is continued 5­-24 hrs., when the sulpho-­mass contains 1-­6% H2SO4. After cooling to 50-­100 (70) deg.C, the vacuum distn. of excess toluene is continued. The excess toluene is reused in process. The resulting sulpho-­mass contains 85% p-­toluene-­sulphonic acid.