Dithiofluorescein (sometimes generically called thiofluorescein) is a complexometric indicator used in analytical chemistry. It changes from blue to colorless when it binds to mercury(2+) ions. It thus can indicate the endpoint in the titration of thiols using o-hydroxymercuribenzoic acid or its sodium salt. The reagent can be immobilized t in a polymer on a fiber optic, which might allow development of a detector for sulfide ions in a flow cell. Unlike fluorescein and other related fluoran dyes that have oxygen substituents on the benzene rings, dithiofluorescein, which has sulfur substituents, is not fluorescent.