In a classical reaction, phenolphthalein is synthetised from phthalic anhydride and phenol with acid catalyst. When one uses substituted phenols, one obtains similar compounds (e. g. cresolphthalein, thymohlphthalein).
However, some acid-base indicators contain a sulfo- group instead of a carboxy- group of phthaleins. Does that mean that they are synthetised from phenols and the anhydride of o-sulfobenzoic acid? Phthalic anhydride is fairly cheap, but how is o-sulfobenzoic anhydride prepared?