§ 165. QUERCITANNIC ACID. 161 



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

 soluble in alcohol. 



Caffea-tannk^ acid yields sugar and crystalline cafFeic acid. 

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

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

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

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

 yields pyrocatechin by dry distillation. The former is also coloured 

 green by ferric chloride. Its ammoniacal solution turns green 

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

 experiments, it cannot be quantitatively determined by precijiita- 

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

 of potassium might possibly yield approximate results. 



The folloAA'ing tannins are provisionally considered by many 

 chemists to be glucosides (see note) ; they yield amorphous de- 

 composition-i^roducts resembling phlobajihenes. (Cf. § 160.) 



Quercitannic acid, Avhich is probably identical Avith the tannic 

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

 is, therefore, extremely difficult to piu'ify and to estimate (^ 161). 

 The copper and lead salts seem specially liable to be decomposed 

 by the combined action of air and Avater, Avhilst 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 Avith perfectly fresh infusions. 

 But the mucilaginous and other substances that are simultane- 

 ously dissolved l^y Avater from the oak-bark, also act upon the 

 reagents and render the estimation inaccurate. By extracting 

 Avith alcohol such foreign substances are excluded ; but the estima- 

 tion cannot be made in alcoholic solution, and distillation can 

 scarcely be efFected Avithout partial decomj)osition of the tannin. 



^ See Hlasiwetz, Annal. d. Chem. und Pharm, cxlii. 220, 1867 (Amer. Joiirn. 

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



- Compare E. Johanson, 'Beitr. zur Chem. d. Eichen-, Weiden-, und Ulmen- 

 rinde,' Diss. Dorpat, 1875 ; Grabowski, Annal. d. Chem. und Pharm, cxlv. 1, 

 1868. For oak-red see also Böttinger, ibid. ccii. 269, 1880. Loewe has re- 

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

 f, anal, Chem. xx. 208, 1881 (Journ. 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. Böttinger (Ber. d. d. Chem. Ges. xiv. 2390 ; Journ. 

 Chem. Soc. xl. 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 the 

 body isolated by Loewe. 



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