Patent ID: 6888005

Claim:
Process for the preparation of 2-phenylbenzimidazole-5-sulphonic acid, comprising reacting 3,4-diaminobenzenesulphonic acid 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 SO 2 per mole of 3,4-diaminobenzenesulphonic acid, or an agent which comprises 1.0 to 3.0 mol of SO 2 per mole of 3.4-diaminobenzenesulphonic acid, wherein the agent is alkali metal salts selected from the group consisting of Na 2 HSO 3 , Na 2 S 2 O 5 and Na 2 SO 3.