One hundred fifteen grams (0.9 mole) of p-chlorophenol, 300 g. (2.0 moles) of phthalic anhydride, 50 g. of crystallized boric acid, and 2 kg. (1090 cc.) of 95 per cent sulfuric acid are thoroughly mixed in a 1.5-l. round-bottomed flask. The flask is then immersed to the neck in an oil bath and the temperature gradually raised to 200° during the course of one-half to three-quarters of an hour (Note 1). It is then kept constant at 200° for three and one-half hours. Owing to the tendency of the phthalic anhydride to sublime, the mouth of the flask is covered with a piece of porous plate. When cool, the colored melt is poured slowly into 5 l. of cold water with continuous stirring, and filtered. The precipitate is boiled with 10 l. of water and filtered hot to remove the excess of phthalic anhydride which goes into solution. The residue is then suspended in 10 l. of boiling water, and to this suspension is added a sufficient quantity of 10 N potassium hydroxide solution to produce a purple color (about 30 cc.), followed by 300 cc. more (Note 2). The alkaline solution is filtered hot, and the residue (about 5–10 g.) is washed with 200 cc. of hot 2 per cent potassium hydroxide solution and discarded.

The purple quinizarin solution is next saturated with carbon dioxide and the precipitated quinizarin again filtered. The product is then boiled with 5 l. of a 10 per cent solution of sodium carbonate until it appears black (probably the monosodium salt) to dissolve the last traces of purpurin. The mixture is cooled to room temperature and filtered, and the precipitate is boiled with 5 l. of 5 per cent hydrochloric acid to liberate the quinizarin. The mixture is again cooled to room temperature, and the final product is filtered, washed with cold water, and dried at 100°.

The yield is 147–160 g. (68–74 per cent of the theoretical amount). It sinters at 190–191° and melts at 199–200° (corr.). A sample twice recrystallized from glacial acetic acid melted at 200–202° (corr.) (Note 3). The crystal form of this product compares very favorably with that of quinizarin of the highest purity, as observed under the microscope.