S-5-Amino-2-(dimethyl­ammonio)phenyl sulfothio­ate

The title compound, C(8)H(12)N(2)O(3)S(2), has been isolated as a by-product in the synthesis of methyl­ene blue dye. The compound crystallizes with four independent mol­ecules in the unit cell (Z′= 4). The zwitterionic form of the mol­ecule was established on the basis of the hydrogen atom located at the dimethyl­amino group. The crystal structure is dominated by inter­molecular hydrogen bonds of the N—H⋯O type formed between amino and ammonio N—H groups and O atoms from the sulfothio­ate group. There are in addition two weak inter­molecular N—H⋯N inter­actions and some non-conventional C—H⋯O hydrogen bonds.