I’ve seen that pink color develop in commercial PTSA. I haven’t seen any definitive identification - but the color looks like a diphenyl- or triphenylmethane dye. Oxidation of the toluene methyl by air, followed by Friedel-Crafts in the acidic environment would make this type of impurity.

Toluene p-sulfonic acid monohydrate can either be white or pink. The highly pure compound can be expected to be white; the pink colour seems to be chiefly caused by (ferric) iron or manganese impurities, perhaps even if just at trace level. Another common impurity is benzenesulfonic acid, besides the other (o-; m-) isomers of toluene p-sulfonic acid. Sulfuric acid can be present in toluene solutions of toluene p-sulfonic acid. Eventually, the ferric p-toluenesulfonate salt can precipitate or co-precipitate from toluene, perhaps along with the sodium salt. The colour of isolated ferric p-toluenesulfonate can vary from yellow to dark orange. Sulfonation side reactions may also contribute to the colour of the solution.

Hi,

Benzenesulfonic acid and sulfuric acid.

You can remove these impurities by recrystallization from its concentrated aqueous solution followed by azeotropic drying with toluene.

With my best regards