In a 16-cm. evaporating dish are placed 42.2 g. (0.2 mole) of 3-nitrophthalic acid,¹ 50 ml. of water, and a few drops of phenolphthalein indicator solution. The mixture is made faintly alkaline to phenolphthalein with about 66 ml. (0.4 mole) of 6 N sodium hydroxide solution (Note 1), the last portions of which are added slowly with good stirring so that the end point may be observed. The color of the phenolphthalein is discharged by the addition of 0.2–0.3 g. of the 3-nitrophthalic acid, and 26.0 g. (0.2 mole) of hydrazine sulfate² is added. The solution is evaporated to dryness over a sand bath, with stirring at the latter part of the evaporation (Note 2), and the residual solid is cooled, ground in a mortar to a fine powder, and placed in a 200-ml. conical flask with 25 ml. of tetralin (Note 3). The mixture is heated at 160–170° for 3 hours and allowed to cool. After the addition of 50 ml. of benzene the solid is collected on a Büchner funnel and pressed down well, and most of the benzene is removed by suction. The solid is then washed with two 25-ml. portions of ether and allowed to stand in the air until the odor of ether is no longer detectable. The resulting material, weight 62–68 g., is an equimolar mixture of sodium sulfate and 5-nitro-2,3-dihydro-1,4-phthalazinedione, and it may be used directly for preparing 5-amino-2,3-dihydro-1,4-phthalazinedione (luminol, p. 69).

For purification, the crude material is suspended in 600–700 ml. of boiling water, and solid sodium carbonate is added in portions until the nitro compound has dissolved. Decolorizing carbon is added cautiously, and the suspension is boiled for a few minutes and filtered. The clear red-brown filtrate is acidified with concentrated hydrochloric acid, and the 5-nitro-2,3-dihydro-1,4-phthalazinedione is precipitated as a cream-colored flocculent solid. The solution is allowed to cool to room temperature, and the solid is separated by filtration and dried. The product thus obtained weighs 31–32 g. (75–78%) and has a melting point of 315–316° (dec.) when determined with the Kullman copper block (Note 4).