138 liHSmS, BITTER PRINCIPLES, ETC. 



Under the influence of alkalies gallic acid turns rapidly green, 

 red and reddish-brown. Like tannic acid, it yields inky mixtures 

 with ferrous and ferric salts, but is not precipitated by gelatine 

 from aqueous solution. It reduces nitrate of silver and alkaline- 

 copper solution. Gallic acid is precipitated by acetate of lead, 

 and is partially removed from aqueous solution Ijy digestion with 

 the hydrate of that metal. The precipitates are, however, neither 

 quite insoluble nor of constant composition, so that they cannot 

 be recommended as a means of estimating gallic acid except 

 under certain conditions. If a solution of hydrate of lead in 

 potash is boiled with very dilute solution of gallic acid, a rose or 

 violet colour is developed, which is persistent for some time, 

 especially in the presence of alcohol (Klunge). 



Catechin colours cone, sulphuric acid, on warming, purple, 

 changing to black. Solutions in aqueous potash, ammonia or 

 carbonated alkalies gradually absorb oxygen and become rose-red, 

 scarlet, passing to dark red and finally black. The alkaline solu- 

 tion produces at first no colouration with ferrous sulphate, but a 

 green tinge is subsequently developed ; acetate of soda is said to 

 turn the colourless mixture instantly violet-blue, and cause the 

 separation of a bluish-black precipitate. A very small quantity 

 of ferric chloride colours solutions of catechin green, but excess 

 causes decolourization and formation of a brown precipitate. 

 Like gallic acid, catechin does not precipitate gelatine, and acts as 

 a reducing agent. The lead salt obtained by precipitation is not 

 suited for the quantitative determination of catechin, as it easily 

 decomposes (turning red on exposure to the air). 



A better method for estimating both catechin and gallic acid 

 consists in agitating with ether or acetic ether and weighing the 

 evaporation-residue, or preferably, titrating it with perman- 

 ganate of potash. (Cf. § 52, VII. ; §§ 53 and 165.) 



Pyrocatechin is also easily soluble in alcohol, melts at 112°, and 

 can be sublimed. Exposed to the air in alkaline solution it turns 

 green and black ; ferroso-ferric salts colour it dark green. It 

 reduces gold and silver salts and alkaline-copper solution. With 

 acetate of lead a precipitate is formed, which is soluble in acetic 

 acid ; solution of gelatine is not precipitated. 



§ 152. Quercitrin, Querceiin, etc. — Quercitrin and quercetin, if 

 present in the material under examination, might be partially 

 extracted with ether (§ 3G), by which, however, they are not very 



