In a 2-l. flask fitted with a mechanical stirrer, a separatory funnel, and an efficient reflux condenser are placed 219 g. (1 mole) of finely powdered acid ammonium o-sulfobenzoate (p. 14) and 200 cc. of dry benzene (Note 1). To this is added with stirring 145 g. (86.3 cc., 1.22 moles) of thionyl chloride (b.p. 76–78°). A tube is connected to the upper end of the reflux condenser which passes into an ice-cooled flask; from this another tube is provided to carry off the hydrogen chloride and sulfur dioxide generated during the reaction (Fig. 7, p. 97). The mixture is gently warmed on a steam bath in such a way that gases are evolved fairly briskly (Note 2); continuous stirring is essential. The bulk of the entrained benzene and thionyl chloride is condensed in the ice-cooled receiver; the condensate is returned periodically to the reaction mixture through the separatory funnel. After about fifteen hours' heating, the evolution of gases slackens; an additional 400 cc. of dry benzene is then added and the heating continued until the evolution of gases ceases; this requires about five hours longer.

The hot mixture is then filtered (Note 3) with suction into a 2-l. flask, and the solid material, consisting of ammonium chloride, is washed with 100 cc. of hot benzene. This solid is then returned to the 2-l. flask and boiled with stirring for one hour with 300 cc. of dry benzene; the mixture while hot is then filtered with suction into the same 2-l. flask. The combined filtrates and washings are then distilled until about 300 cc. of benzene has been removed, after which the solution is cooled in an ice bath; this causes the greater part of the sulfobenzoic anhydride to crystallize. The benzene mother liquor is decanted from these crystals and the bulk of the solvent removed by distillation.

The flask containing the crystals is attached to a condenser and heated until the solid has completely melted; the adhering solvent is then removed from the liquid by slightly reducing the pressure. The clear residue is poured into a dish and allowed to cool. The product thus obtained melts at 121–123°. The residue from the mother liquor, being as a rule somewhat dark in color, is preferably distilled under reduced pressure (Note 4), the flask being heated in a metal bath. The anhydride distils at 184–186°/18 mm. The second crop may have a slightly lower melting point. The total yield of o-sulfobenzoic anhydride, which is of sufficient purity for the preparation of sulfonephthaleins, amounts to 118–121 g. (64–66 per cent of the theoretical amount) (Note 5). In order to convert this into a product melting at 126–127° (Note 4), it is recrystallized from three times its weight of dry benzene.