48, 49, 50. DETECTION OF TANNIN. 30 



water is treated with water containing a little ammonia (1 in 50) 

 as long as anything is removed. The ammoniacal extract may be 

 evaporated with a slight excess of acetic acid, the residue rinsed 

 on to a filter with a little water, washed, dried, and weighed. 

 The brownish mass thus left on the filter is, as a rule, to be 

 regarded as phlobaphene ( 108; see also 160, 163), re- 

 sulting from the decomposition of tannin. The portion of the 

 aqueous extract insoluble in ammoniacal water may be again 

 dried over sulphuric acid, and then subjected to similar treat- 

 ment as the resin soluble in ether (cf. 39 to 45 ; 145, 146). If 

 there is reason to suspect the presence of an alkaloid soluble in 

 alcohol, but insoluble in ether, the residue, after treatment with 

 ammoniacal water, may be digested with water containing a little 

 sulphuric acid. (For alkaloids see 55, et seq.; 63, et seq. ; 171, et seg?.) 



EXAMINATION OF THE TANNIN. 



49. Detection. If the aqueous solution obtained from the 

 evaporation residue of the alcoholic extract is coloured blue-black 

 by a ferroso-ferric salt and precipitated by gelatine, solution of 

 acetate of lead is to be added in slight excess. The resulting pre- 

 cipitate is immediately collected on a tared filter, washed with 

 water (not too long, three or four times, with 3 to 5 cc.), dried, 

 and weighed ( 52, I.). It is then removed from the filter, which 

 is burnt in a porcelain crucible with a little nitrate of ammonia ; 

 the precipitate itself is next incinerated, and the whole finally 

 ignited in the blow-pipe flame until the weight is constant. This 

 is then deducted from the weight of the precipitate, and the re- 

 mainder noted as tannic acid, or bitter principle precipitated by 

 oxide of lead, or vegetable acids precipitated by lead ( 80). The 

 filtrate from the lead precipitate is treated according to 70. 



50. Detection continued. The same treatment is repeated with 

 a similar quantity of the watery extract obtained in 48, substi- 

 tuting acetate of copper for acetate of lead ( 52, II.). Here, 

 too, the amount of oxide of copper in the precipitate is to be 

 determined by following the same directions and deducted from 

 the weight of the precipitate. If the estimation with the copper 

 salt yields the same result as that with the lead, it is tolerably 

 certain that only tannic acid has been precipitated. But if lead 

 throws down more matter than copper we are generally justified 

 in assuming that the former precipitates substances other than 



