Most salts of the alkaloids of cinchona afford a beautiful purple tar when they are heated in a test tube, and the same is also produced with the powdered bark, provided alkaloids be present. No other bark, as far as we know, yields a similar product of the dry distillation. It is not observed even in using true Cinchona barks, which are devoid of alkaloids. This method for ascertaining the presence of alkaloids in Cinchona barks has been proposed in 1858 by Grahe of Kasan. Hesse has improved Grahe’s test in the following way: he extracts the powdered bark with slightly acidulated water and dries up the liquid with a little of the powder. Grahe’s test at once shows whether a given bark contains Cinchona alkaloids or not.
Acid principles of Cinchona Barks—Count Claude de la Garaye[1366] observed (1746) a crystalline salt deposited in extract of cinchona bark, which salt was known for some time in France as Sel essential de la Garaye. Hermbstädt at Berlin (1785) showed it to be a salt of calcium, the peculiarity of whose acid was pointed out in 1790 by C. A. Hoffmann,[1367] an apothecary of Leer in Hanover, who termed it Chinasäure. The composition of this substance, which is the Kinic Acid of English chemists, was ascertained by Liebig in 1830 to be C₇H₁₂O₆, or now C₆H₇(OH)₄COOH. The acid forms large monoclinic prisms, fusible at 162° C., of a strong and pure acid taste, soluble in two parts of water, also in spirit of wine, but hardly in ether. The solutions are levogyre. Kinic acid appears to be present in every species, and also to occur in barks of allied genera; and in fact to be of somewhat wide distribution in the vegetable kingdom. By heating it or a kinate, interesting derivatives are obtained; thus, by means of peroxide of manganese and sulphuric acid, we get yellow crystals of Kinone or Quinone, C₆H₄O₂,—a reaction which may be used for ascertaining the presence of kinic acid. Kinic acid is devoid of any noteworthy physiological action.
Cincho-tannic Acid—is precipitated from a decoction of bark by acetate of lead, after the decoction has been freed from cinchona-red by means of magnesia. Dr. de Vry informed us that the Indian barks are usually richer in cincho-tannic acid; their cold infusion becomes turbid on addition of hydrochloric acid, which forms an insoluble compound with the former.
The cincho-tannate of lead decomposed by sulphuretted hydrogen, and the solution cautiously evaporated in vacuo, yields the acid as an amorphous, hygroscopic substance, readily soluble in water, alcohol, or ether. The solutions, especially in presence of an alkali, are quickly decomposed, a red flocculent matter, Cinchona-red, being produced. Solutions of cincho-tannic acid assume a greenish colour on addition of a ferric salt. By destructive distillation, cincho-tannic acid affords pyrocatechin.
Quinovic (or Chinovic) Acid, C₂₄H₃₈O₄, crystallizes in hexagonal scales which are sparingly soluble in cold alcohol, more readily in boiling alcohol, but not dissolved by water, ether, or chloroform. It occurs in cinchona barks, and has been met with by Rembold (1868) in the rhizome of Potentilla Tormentilla Sibth.
Other Constituents of Cinchona Barks—Quinovic acid is accompanied by Quinovin (or Chinovin), C₃₀H₄₈O₈, an amorphous bitter substance, first obtained (1821) by Pelletier and Caventou under the name of Kinovic Acid, from China nova,[1368] in which it occurs combined with lime. Quinovin in alcoholic solution was shown in 1859 by Hlasiwetz to be resolved by means of hydrochloric gas into quinovic acid, C₂₄H₃₈O₄, and an uncrystallizable sugar, Mannitan, C₆H₁₂O₅, with subtraction of H₂O. The formation of quinovic acid takes place more easily, if quinovin is placed in contact with sodium amalgam and spirit of wine, when, after 12 hours, mannitan and quinovate of sodium are formed (Rochleder, 1867).
Quinovin, although an indifferent substance, may be removed from cinchona barks by weak caustic soda, from which it is precipitable by hydrochloric acid, together with quinovic acid and cinchona-red. Milk of lime then dissolves quinovin and quinovic acid, but not the red substance. Quinovic acid and quinovin again precipitated by an acid, may be separated by chloroform in which the latter only is soluble, or also by cold dilute alcohol sp. gr. about 0·926, quinovin being readily removed by this liquid.
Quinovin dissolves in boiling water; its solutions, as well as those of quinovic acid, are dextrogyre. Quinovin seems to be a constituent of almost every part of the cinchonas and the allied Cinchoneæ, although the amount of it in barks does not apparently exceed 2 per cent. It is accompanied by quinovic acid: both substances are stated to have tonic properties.
Cinchona-red, an amorphous substance to which the red hue of cinchona barks is due, is produced as shown by Rembold (1867), when cincho-tannic acid is boiled with dilute sulphuric acid, sugar being formed at the same time. By fusing cinchona-red with potash, protocatechuic acid, C₇H₆O₄, is produced. Cinchona-red is sparingly soluble in alcohol, abundantly in alkaline solutions, but neither in water nor in ether. Thick Red Bark in which it is abundant, affords it to the extent of over 10 per cent.
The Cinchona barks yield but a scanty percentage of ash, not exceeding 3 per cent., a fact well according with the small amount they contain of oxalate and kinate of calcium.