In a 12-l. flask fitted with a stirrer and reflux condenser are placed 188 g. (1 mole) of o-sulfobenzoic imide ("saccharin insoluble"), 565 cc. of distilled water and 188 g. (158 cc.) of concentrated hydrochloric acid (sp. gr. 1.19). The mixture is boiled over a free flame with continual stirring (Note 1) until all the solid is in solution; this requires two and one-half to three hours. A second quantity of 188 g. of o-sulfobenzoic imide is then added, and the mixture again heated with stirring until a clear solution results, which requires one and one-half to two hours. Heating is then continued for one hour longer, whereupon the solution is poured into a crock and allowed to cool.

The crystals which separate are collected on a suction funnel, washed as free from hydrochloric acid as possible (Note 2) with ice-cold distilled water, and dried. The mother liquor and washings are concentrated on a steam bath under reduced pressure until the separation of crystals causes bumping, when the solution is again allowed to crystallize. This procedure is repeated, the final mother liquor being evaporated nearly to dryness. The main product, together with that from the mother liquors, weighs 410–427 g. (91–95 per cent of the theoretical amount) and is sufficiently pure for conversion into o-sulfobenzoic anhydride (p. 495). In order to obtain a purer product the material may be recrystallized from an equal weight of distilled water; the yield on recrystallizing is about 90 per cent.