C.
Dithizone. The crude
diphenylthiocarbazide is added to a solution of
60 g. of potassium hydroxide in 600 ml. of methanol in a
1-l. round-bottomed flask. The flask is immersed in a boiling water bath, and the mixture is
refluxed for exactly 5 minutes (Note
8). The red solution is cooled with ice water and filtered by gravity. Ice-cold
1 N sulfuric acid (900–1100 ml.) is added to the filtrate, which is stirred vigorously by means of a mechanical stirrer, until the solution is just acid to Congo red paper (Note
9). The blue-black precipitate is filtered with suction and washed with 50 ml. of cold water (Note
10). The crude carbazone is dissolved in
500 ml. of 5% sodium hydroxide solution, the mixture is filtered with suction, and the filtrate is cooled in an
ice bath and acidified immediately with ice-cold
1 N sulfuric acid (about 650 ml. is required) until it is just acid to Congo red paper. The precipitate is filtered with suction and then washed by transferring it to a
2-l. beaker and stirring it thoroughly with 1.6–1.8 l. of cold water. The mixture is filtered, and the process of washing is repeated until there is no trace of sulfate in the washings (Note
11). After air has been drawn through the precipitate on the Büchner funnel for 20–30 minutes, the solid is dried in an
oven at 40°. The product at this stage weighs
63–85 g. For purification, a portion of
5–10 g. of the carbazone is placed in the thimble of a Soxhlet extractor (Note
12), covered with
ether, allowed to stand for 1 hour, and then extracted for 1.5 hours. The material in the thimble is transferred immediately to a beaker, stirred with
50 ml. of ether, and the mixture is filtered with suction until most, but not all, of the liquid is removed. The wet product is then dried by pressing it between filter papers. The yield of pure
dithizone (diphenylthiocarbazone) which decomposes sharply at a temperature between 165° and 169° is
43–54.8 g. (
50–64% based on phenylhydrazine) (Note
13).