Patent ID: 8038975

Claim:
A process for preparing crystalline hydroxylamine-O-sulfonic acid, wherein a. hydroxylammonium sulfate is dissolved in sulfuric acid and oleum is metered in at a temperature in the range of from 105 to 115° C. within a period of from 0.2 to 4 hours to form a reaction mixture, b. the reaction mixture, after the oleum addition has ended, is stirred at a temperature in the range of from 105 to 115° C. for a further period in the range of from 6 to 10 hours, c. the reaction mixture is subsequently cooled to a temperature in the range of from 20 to 30° C. within a period of from 6 to 9 hours to form hydroxylamine-O-sulfonic acid crystals and d. the hydroxylamine-O-sulfonic acid crystals formed are filtered off and washed with methyl benzoate and then with ethyl acetate.