noun
also sul·fon·phthalein |səlˌfōn, -fän+
Etymology: sulfone, sulfon- + phthalein
: any of a group of organic compounds (as phenolsulfonephthalein or bromocresol purple) that are analogous to the phthaleins and like them are intensely colored in alkaline solution and that are made by condensation of phenols with anhydrides or acid chlorides of ortho -sulfobenzoic acid or its derivatives