PREPARATION OF SALT-FREE, CHLORINE-SUBSTITUTED, CRYSTALLINE 3-NITROBENZENESULFONIC ACID HYDRATE

The present invention relates to a process for the preparation of crystallised salt-free chloro-substituted 3-nitrobenzenesulphonic acid hydrates from sulphonation mixtures containing chloro-substituted 3-nitrobenzenesulphonic acid, by mixing the sulphonation mixture with 8 to 25% by weight of aqueous sulphuric acid at 40 to 100@C, cooling the resulting suspension to 5 to 20@C, filtering off the precipitate, dissolving the precipitate using water at 30 to 100@C, optionally filtering, cooling the solution to 5 to 40@C, filtering and optionally recycling the sulphuric acid-containing filtrate.