Reaction:

Tannin or other tanning material.

Gallic acid.

Gallamide.

1 D. R.P. 65131 (1892).

2 It is dissolved in soda, the boiling solution piecipitated with hydrochloric acid, and the liquid filtered hot.

The most important raw material for gallamide and gallic acid are Gall-nuts and Sumach (Rhus coriaria). The tannin is either hydrolysed by caustic soda into sugar and gallic acid, or by the action of ammonium sulphite into sugar, gallic acid and gallamide, approximately equal parts of amide and acid being obtained.

200 Gms. tannin together with 200 c.cs. water, 400 gms. 20 % ammonia, and 100 gms. sodium bisulphite solution (25 % So2) are placed in a soda-water bottle fitted with a rubber stopper, which is then heated for 12 hours in a water bath at 500. The bottle must be shaken occasionally to ensure complete solution. The solution is then concentrated in a large glass flask to 400 c.cs. under reduced pressure.

After cooling, sufficient hydrochloric acid is added cautiously to render the liquid just acid to litmus. The gallamide is completely precipitated within 24 hours. (In the laboratory it is frequently necessary to cool down a small portion in a freezing mixture, and then to scratch the inside of the vessel in order to start the crystallization.)

The sparingly soluble gallamide is filtered off and well washed. The mother-liquor is treated with 100 gms. of 30 % caustic soda lye and the ammonia removed in vacuo. The liquid is then concentrated again to 300 c.cs. and acidified with sufficient concentrated hydrochloric acid to turn Congo paper just blue. The sodium salt of gallic acid separates out in the course of a few days in the form of a finely crystalline precipitate which is filtered off and pressed without washing. It is dissolved in 100 c.cs. water, and the gallic acid precipitated from the solution by means of hydrochloric acid. Yield of gallamide and gallic acid about 60 gms. each.

Notes on Works Technique and Practice. - On the large scale, solutions of tannin are used which are obtained by extracting the material containing the tannin with hot soft water on the counter-current principle, the solutions being afterwards evaporated in vacuo to 300 Be. The process is carried out in large concrete vats which may be used either for positive or negative pressures. The crystallization of the gallamide takes from 10-14 days,and that of the gallate still longer. Tannin solutions ferment readily, so that it is necessary to work quickly, particularly during the summer months. The purity of the gallamide is estimated by distilling off the ammonia from a weighed portion, by means of caustic soda, which is absorbed in normal hydrochloric acid, and the latter titrated back. Good gallamide should be 92 % pure.

Gallamide and gallic acid are used in large quantities for the preparation of Oxazines (see Gallamine Blue).

200 gms. Tannin. 200 gms. H2O.

400 gms. 20 % Nh3. 100 gms.

25% NaHso3.

100 gms.

30 % NaOH.