Process for preparing pure Gallic Acid. 75 



suit Is obtained by bringing oxide oHron in contact wllh a 

 solution of pure gallic acid. 



Iron treated with a solution of gallic acid soon commu- 

 nicates to it a black colour, which, however, can hardly 

 be precipitated from it. 



Tannin is soluble in gallic acid, and yields with it a 

 liquor resembling the infusion of galls, and which has the 

 property of instautlv precipitating acid solutions of iron 

 black, by a double affinity ; the taimin combining with the 

 acid, and disengaging oxide of iron, which unites wilh the 

 gallic acid. 



It results from these experiments : 



1. That gallic acid does not separate iron from sulphuric 

 and some other acids, excepting some change of combina- 

 tion takes place in the solutions : in this case the iron, 

 becoming more oxidated, requires a much greater quan- 

 tity of acid, and the redundant oxide of iron unites wilh 

 the g>illic acid. 



2. That when a solution of gallic acid immediately 

 forms a black precipitate in neutral solutions of iron, it is 

 not pure, and commonly contains tannin, which combines 

 v.'ith the sulphuric acid, and separates from it the oxide of 

 iron which is dissolved by the gallic acid. 



3. That during the prcqxuatiou of gallic acid, all contact 

 with iron must be avoided, even in the filtering paper, 

 which is rarely free from it, otherwise the acid is discolour- 

 ed by the iron. It may easily be discovered that the acid 

 contains iron, when, in evaporation, small violet spots are 

 formed in the places where the acid is about to crystallize. 



4. That as gallic acid dissolves very readily in alcohol, 

 and tannin scarcely at all, this reagent may be employed to 

 separate them. The alcohol, however, should be very 

 strong; for, if it contain ever so little water, some tannin 

 will be dissolved. 



5. Of all the ])rocesscs recommended for the preparation 

 of gallic acid, this furnishes it in the greatest abundance 

 and of the best rpiality. For, if we consider in particular 

 that recommcndc d some years ago, which consisted in em- 

 ploying the solution of muriate of tin, afterwards sul[ihu- 

 rized hvdrogcn cas, and lastly alcohol, wc may readily con- 

 ceive, parlicularlv if wc have practised it, how defective, 

 expensive, and disadvantageous it must be ; and at last it 

 amounts to noihinir more than rendering tannin insoluble 

 in alcohol ; and nothing is efleclcd excepting that this re- 

 agent dissolves gallic acid alone. As to the process which 

 recommends the use of g'uc, thai is quite as bad : you in- 



S deed 



