14 октября, 2015 
Pokraskin OH
но/N OH
(NH4),SO, + NHS COOH
OH
HO /N OH
CO-NH,
The most important starting materials for the preparation of gallamide and gallic acid are gallnuts and sumac (Rhus coriaria). The tannins from these are split either into sugar and gallic acid by the action of sodium hydroxide, or into sugar, gallamide, and gallic acid by the action of ammonium sulfite. In the latter case, about equal parts of the amide and the acid are obtained.
200 grams of the tannin, 200 cc. water, 400 grams of 20 per cent ammonia, and 100 grams of sodium bisulfite solution (25 per cent S02) are heated for 12 hours at 50° (water bath) in a pressure bottle with a rubber stopper. The mixture must be shaken from time to time in order to get complete solution. The resulting solution is transferred to a large glass flask and concentrated to 400 cc. under reduced pressure.
After the residue has cooled, hydrochloric acid is added until the solution is just faintly acid to litmus. The gallamide separates completely in the course of 24 hours. (In laboratory preparations, it is — often necessary to cool a small portion of the solution and scratch the container in order to start the crystallization.) The product is filtered off and washed well with water. To the mother liquor is added 100 grams of 30 per cent sodium hydroxide and ammonia is removed in vacuum. The solution is then concentrated to 300 cc. and enough concentrated hydrochloric acid is added to make the solution just acid to Congo red. The sodium salt of gallic acid separates, in the course of a few days, as a finely crystalline precipitate which is filtered off and pressed out without washing. The sodium salt is dissolved in 100 cc. water and gallic acid is precipitated from the solution by the addition of hydrochloric acid. About 60 grams each of gallamide and gallic acid are obtained.
Technical Observations. Large scale preparations use tannin solutions which are obtained by countercurrent extraction of the tannin-containing material with
hot water. The extracts are concentrated in vacuum to 30° Be. The hydrolysis of the tannin is carried out in huge concrete vats, adaptable to work under either reduced or increased pressure; Crystallization of the gallamide requires 10 to 14 days, that of the gallate even longer. Tannin solutions have a strong tendency to ferment, and must be worked up rapidly, especially during the summer. The purity of the gallamide is determined by distilling a weighed sample with sodium hydroxide, collecting the ammonia in 1 N hydrochloric acid, and back-titrating. A good grade of gallamide is 92 per cent pure.
Gallamide and gallic acid are used in large’ quantities for the preparation of oxazines (see gallamine blue).
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