In a 2.5-l. beaker, 100 g. (0.91 mole) of hydroquinone is dissolved in 2 l. of water heated to about 50°. After the solid is completely dissolved, the solution is cooled to 20°, 100 g. (54.4 cc.) of concentrated sulfuric acid is slowly poured in, and the mixture is again cooled to 20° (Note 1). A concentrated solution of technical sodium dichromate is prepared by dissolving 140 g. (0.47 mole) in 65 cc. of water (Note 2). This solution is then added gradually to the hydroquinone solution, with the use of a mechanical stirrer (Note 3), the mixture being cooled so that the temperature never rises above 30°. At first a greenish-black precipitate forms, but, upon further addition of the sodium dichromate solution, the color changes to yellowish green. As soon as this color remains permanent (a slight excess of sodium dichromate does no harm) the reaction is complete. This requires about one-half to three-quarters of an hour; 90–110 cc. of sodium dichromate solution is necessary. The mixture is then cooled to about 10° and filtered with suction. As much water as possible is pressed out of the crystals.

The filtrate is extracted twice, 150 cc. of benzene being used for each extraction. The precipitate of quinone is transferred to a 1-l. beaker, and 500 cc. of benzene, including the 300 cc. used to extract the filtrate, is added. The mixture is now heated with stirring on a steam bath, and as soon as most of the quinone has dissolved the benzene layer is decanted into another beaker. It is dried while hot by stirring a short time with a little calcium chloride, and then filtered through an ordinary funnel into a 1-l. distilling flask before it cools. There is a certain amount of quinone which does not go into the 500 cc. of benzene, so that the residue is extracted a second time with about 100 cc. of benzene, which is dried and filtered with the first extract. During these extractions (Note 4), the benzene should not be at the boiling point, as this will cause a considerable volatilization of the quinone.

The distilling flask is now attached to a condenser set for downward distillation, and the benzene is distilled. As soon as the quinone starts to separate, the residue in the flask is transferred to a beaker and cooled in an ice bath. The precipitate is filtered off with suction and the product spread out for a short time to dry. The product is yellow in color and weighs 75 to 80 g. (76–81 per cent of the theoretical amount). Material made in this way will hold its yellow color over long periods of time, provided it is protected from light.

The benzene distillate is yellow and contains some quinone. This, as well as the benzene from the final filtration of the quinone crystals, may be used in a subsequent run and thus raises the yield of the subsequent runs to about 85–90 g. (86–92 per cent of the theoretical amount) (Note 5).