§ 165. GALLOTANNIC ACID. 159 



is partially precipitated from aqueous solution by chloride of 

 sodium, and by acid sulphite of potassium. The aqrueous or 

 alcoholic solution turns dark cherry-red when warmed with 

 hydrochloric or sulphuric acid, and deposits red flocks ; bichromate 

 of potassium produces a dark colouration and brown precipitate ; 

 with ferric chloride it strikes a green, or, if the solution is am- 

 moniacal, a violet colour ; it is not precipitated by tartar emetic. 

 No method is known by which it can be accurately estimated. 



Amongst the glucosidcd tannins those may be mentioned first 

 which, when boiled with dilute acids, yield, in addition to glucose, 

 crystal-line decomposition-products. The most important of them 

 is — 



Cfallotannic add, the decomposition-product of which, gallic 

 acid, has been already alluded to (§151). Its quantitative esti- 

 mation is comparatively easy, as fairly accurate results can be 

 obtained both volumetricaUy by titration with gelatine-solution 

 or with permanganate of potassium, and gravimetricaUy by pre- 

 cipitation as tin (stannous), copper, or lead-salt. A few sources 

 of error must, however, be here alluded to. In the first place, 

 if the tannic afiid has been extracted by water, mucilage atnd 

 gallic acid may also have passed into solution ; the latter is pre- 

 cipitable by gelatine in the presence of mucilage. That is avoided 

 by extracting with spirit. In the second place, gallic acid, as 

 already pointed out, acts ttpon permanganate of potassium. In 

 titrating with a solution of that substance, therefore, either the 

 gallic acid must be removed by shaking with ether, or, as sug- 

 gested by Lowenthal, and mentioned in §§ 52, 53, two estima- 

 tions must be made, the one before, the other after, precipitating 

 the tannic acid with gelatine, the calculations being made from 

 the difference. In precipitating with acetate of lead or copper 

 (but not with ammonio-chloride of tin), gallic acid is also partially 

 carried down, and should therefore be previously removed. From 

 a solution containing about 2 per cent of tannin the tin precipi- 

 tate wiU contain 1977 to 19-79 per cent, of stannous oxide, the 

 lead 50-00 per cent, of oxide of lead, and the copper .38-28 per 

 cent, of oxide of copper. 



Hammer's method (§ 52, xi.) may, as alieady stated, be best 

 applied to the estimation of gallo-tannic acid. 



The tannins of sumach,^ knoppem galls, valonia and algaro- 



' Compare Glinther, 'Beitr. zurKenntn. der im Sumach, etc, vork. Gerbs.,' 

 Diss. Dorpat, 1871 (Joum. Chem. Soo. zxir. 762) ; Loewe, Zeitschi-. f. anal. 



