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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.

1. customhad formed
2. filtrationThe solid was then rapidly filtered off with suction
3. additionThe filter cake was introduced into 1500 g of water
4. temperatureThe mixture was heated to a temperature of 70° to 80° C.
5. additiona 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
6. dissolutionDuring this operation, the NTC dissolved as the tetrasodium salt
7. additionAfter addition of about 5 g of active charcoal and about 5 g of kieselguhr
8. filtrationthe solid was filtered off hot
9. washwith suction, and rinsed with a little water
10. temperatureThe clear filtrate was heated to 80° to 100° C
11. additionA 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
12. stirringThe mixture was subsequently stirred at 80° to 100° C. for one hour
13. customThe coarsely crystalline product which had precipitated
14. filtrationwas then filtered off with suction
15. washthe filter cake was washed with about 500 g of a 1% strength hydrochloric acid
16. customThe product was dried at 100° C