A process for the preparation of 2-phenylbenzimidazole-5-sulphonic acid, characterized
in that 3,4-diaminobenzenesulphonic acid is reacted at a pH between 4 and 7 in
aqueous solution with 0.9 to 1.5 mol of benzaldehyde per mole of 3,4-diaminobenzenesulphonic
acid and with 1.0 to 3.0 mol of SO2 per mole of 3,4-diaminobenzenesulphonic
acid, or an agent which comprises 1.0 to 3.0 mol of SO2 per mole of
3.4-diaminobenzenesulphonic acid, is described.
