§ 105. QUERCITANNIG ACID. 161 



adding ammonia. It is pale yellow, amorphous, and easily 

 soluble in alcohol. 



Caffeortammic^ acid yields sugar an,d crystalline caffeie acid. 

 Caffeic acid is easily soluble in alcohol, sparingly in cold water, 

 and strikes a dark green colour with feme chloride, which is 

 turned red by soda. It reduces silver salts on warming, but not 

 alkaline copper solution. Like caffea-tannic acid, caffeic acid, 

 yields pyrocateohin by dry distillation. The former is also coloured 

 green by ferric chloride. Its a' moniacal solution turns green 

 when exposed to the air (viridic acid). According to Giinther's 

 experiments, it cannot be quantitatively determined by precipita- 

 tion with copper, lead, or gelatine. Titration with permanganate 

 of potassium might possibly yield approximate results. 



Tlie following tannins, are provisionally considered by many 

 chemists Lo be glucosides (see note) ; they yield amoi^hous de- 

 comjjosition-products resembling phlobaphenes. (Cf. § 160.) 



Qmrcitarmic acid, wbich is probably identical with the tannic 

 acids of willow- and elm,bark,^ is one of the less stable tannins, and 

 is, thereforCj extremely difficult to purify and to estimate (§ 161), 

 The copper and lead salts seem specially liable to be decomposed 

 by tbe combined action of air and water, whilst even the tannic 

 acid itself in aqueous solution rapidly undergoes change. To 

 have any value, therefore, estimations by gelatine or perman- 

 ganate of potassium must be made with perfectly fresh infusions. 

 But the mucUaginons and other substances that are simultane- 

 ously dissolved by water from the oak-bark, also act upon the 

 reagent-s and render the estimation inaccurate. By extracting 

 with alcohol such foieign substances are excluded'; but the estima- 

 tion cannot be made in alcoholic solution, and distillation can 

 scarcely be effected without partial decomposition of the tannin. 



* See HIasiwetz, Anijal. d.Chem. und Ph^nu. cxIiL 220, 1867 (Amer, Jaurn. 

 xxxviii. 504) ; alao Mulder und Olaanderen, Jahresb. f. Chem. 261, 1858. 



^ Compare E. Johanson, ' Bejtr. zur Chem. d. Eichen-, WeidenT, nndXJlnieB- 

 rinde,' Diss. Dorpat, 1875 ; Grabowsld, Annal. d. Chem. undPhann. cxiv. 1, 

 1868. For oak-red see also Bdttinger, ibid. ccii. 269, 188^. Loewe has re- 

 cently contested the glucosidal nature of quercitannic acid. Compare Zeitschr. 

 f. anaL Chem. xx. 208, 1881 (Joum. Chem. Soc. xL 901). Loewe believes 

 oak-red to be a kind of anhydride produced from the tannic acid by loss of 4 or 

 3 molecules of water. Bbttinger (Eer. d. d. Chem. 6es. xiv. 2390 ; Joum. 

 Chem, Soc. zl, 1041} considers quercitannic acid itself to be a glucoside, and 

 points out the difference between that substance and another constituent of 

 oak-bark also soluble in water and capable of tanning. The latter was tho 

 body isolated by Loewe. 



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