After cooling to a temperature of 20° to 30° C., the suspension, which was present after the letting down, of the tetrasodium salt of naphthalene-1,4,5,8-tetracarboxylic acid (NTC) was worked up by a process analogous to that described in Example 3 c) , by a procedure in which the disodium salt of NTC was first formed by acidification to pH 4.8 to 4.5 and was isolated, this salt was then dissolved in water by conversion into the tetrasodium salt, insoluble impurities were separated off by filtration, and the 1,8-monoanhydride of NTC was then precipitated by acidification with hydrochloric acid and was isolated. 13.1 kg of naphthalene-1,4,5,8-tetracarboxylic acid 1,8-monoanhydride of 96% purity, corresponding to a yield of 97% of theory, were obtained.
