The suspension, present after the letting down, of the tetrasodium salt of naphthalene-1,4,5,8-tetracarboxylic acid (called NTC below) was brought to a pH of 4.8-4.5 with about 150 g of a 31% strength hydrochloric acid at a temperature of 20° to 30° C. The mixture was subsequently stirred at 20° to 30° C. and pH 4.8 to 4.5 for 3 hours, until the disodium salt of NTC, which is sparingly soluble in water, had formed. The solid was then rapidly filtered off with suction. The filter cake was introduced into 1500 g of water. The mixture was heated to a temperature of 70° to 80° C., and a pH of 10 to 10.5 was established at this temperature by slow addition of about 46 g of a 33% strength aqueous sodium hydroxide solution. During this operation, the NTC dissolved as the tetrasodium salt. After addition of about 5 g of active charcoal and about 5 g of kieselguhr, the solid was filtered off hot, with suction, and rinsed with a little water. The clear filtrate was heated to 80° to 100° C. A pH of 0.5 to 1 was then established at 80° to 100° C. by slow addition of about 130 g of a 31% strength hydrochloric acid. The mixture was subsequently stirred at 80° to 100° C. for one hour. The coarsely crystalline product which had precipitated was then filtered off with suction and the filter cake was washed with about 500 g of a 1% strength hydrochloric acid. The product was dried at 100° C. 52 g of naphthalene-1,4,5,8-tetracarboxylic acid 1,8-monoanhydride of 96% purity, corresponding to a yield of 97% of theory, were obtained. The waste water obtained during the filtration was free from halogenohydrocarbons.
