B.
2-Hydroxycinchoninic acid. To a solution of
160 g. (about 3.8 moles) of U.S.P. sodium hydroxide in 9.6 l. of water in a
12-l. flask is added
310 g. (1.64 moles) of crude N-acetylisatin. The mixture is stirred mechanically and heated to boiling; the flame is adjusted to maintain gentle boiling. After the mixture has boiled 1 hour (Note
5) the flame is removed,
50 g. of decolorizing carbon is added cautiously (vigorous boiling and frothing may occur if the solution is too hot), and the suspension is stirred for 30 minutes. The charcoal is allowed to settle for a few minutes, and the solution is siphoned into a
filter funnel containing a layer of Supercel. The filtrate is allowed to cool overnight in a
5-gal. crock. The solution is stirred with a
hook-type stirrer driven by a powerful motor (Note
6), and
6 N hydrochloric acid is added until the mixture is neutral to Congo red. The suspension is filtered immediately (Note
7) through a
12-in. Büchner funnel, and the solid is washed with 2 l. of water. The filtrate is made distinctly acid to Congo red and set aside for a few hours. The solid acid is dried at 90° for 2 days; the anhydrous acid, m.p.
346–347° (cor.), so obtained has the correct neutral equivalent (Note
8) and weighs
200 to 205 g. (
70–73% over-all yield, based on
isatin consumed). From the acidified filtrate, after precipitation is complete, about
60 g. of
isatin is recovered.