Meta toluene sulphonic acid and related compounds

Theoretically toluene should yield three isomeric mono sulphonic acids, in which the entering sulphonic acid group occupies the positions ortho, meta, or para to the methyl group.
The action of sulphuric acid on toluene was first investigated by Jaworsky, (Ztschr. Chem. 1,272), who described one toluene sulphonic acid as the resulting compound. In 1869 Engelhardt and Latschinoff (Ibid. 617), heated toluene with sulphuric acid and made and studied the potassium salt of the resulting acid. They obtained two forms, and identified them by fusion with caustic potash which converted them into the ortho and para cresols, showing that sulphonation had taken place in the positions ortho and para to the methyl group.
Barth (Ann. chem. (Liebig) 152,91), working similarly but fusing under different conditions which also caused oxidation of the methyl group to a carboxyl group obtained para hydroxy benzoic acid and salicylic acid (ortho hydroxy benzoic acid.) This also proved that the sulphonic acid group had entered in the ortho and para positions.
Anna Wolkow (Ztschr. Chem (1870) 321), confirmed this work but ascribed the meta position to the sulphonic acid that passed into salicylic acid. She also made two acid chlorides and amides and recorded their melting points.
Fittig and Ramsay (Ann. chem. (Liebig) 168,242) studied this result of Anna Wolkow, and to determine and settle the constitutions of the sulphonic acids, carefully prepared and purified their sulphonic acids; they proved definitely that no meta acid was formed, and that the products were only the ortho and para toluene sulphonic acids.
Hubner and Post (Ann. chem. (Liebig) 169,47) worked on pure para brom toluene, and obtained two para brom toluene mono-sulphonic acids, one of which they proved to be the ortho acid by removal of the bromine by metallic sodium, and oxidation to salicylic acid. By exclusion they decided that the other acid must be one containing the sulphonic acid group in meta position to the methyl group.