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.

[1] It thus can indicate the endpoint in the titration of thiols using o-hydroxymercuribenzoic acid[2] or its sodium salt.

[3] 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.

[4] Unlike fluorescein and other related fluoran dyes that have oxygen substituents on the benzene rings, dithiofluorescein, which has sulfur substituents, is not fluorescent.