Products of the Decomposition of Tannins by Heat.—Pyrogallol, pyrogallic acid, C6H6O3, has a bitter, but not sour taste, and feebly reddens litmus, but the addition of the smallest trace of alkali gives it an alkaline reaction. It is poisonous, 2 gr. having killed a dog. It is soluble in less than 3 parts of cold water, and still more freely in hot. It is also soluble in alcohol, ether, acetone, ethyl acetate, and glycerin, but not in absolute chloroform, or petroleum spirit. It fuses at 268° F. (131° C.) (Etti), and sublimes at about 410° F. (210° C.).
With pure ferrous sulphate it gives a white precipitate, which redissolves to a fine blue liquid in presence of the least trace of ferric salt. Mineral acids change this to red, and the blue tint is restored by cautious neutralisation with ammonia, and is not destroyed, but sometimes rendered greenish by excess of acetic, and other organic acids. Any excess of ammonia produces an amethyst-red, and acetic acid restores the blue. Its solution is turned brown by traces of nitrous acid. With lime-water it produces a beautiful but evanescent purple, rapidly turning brown. In presence of alkalies it absorbs oxygen from the air with great avidity, turning orange, brown, and black. Pyrogallol does not precipitate gelatin. Its solution rapidly reduces permanganate, Fehling's solution, and salts of gold, silver,[F] mercury, and platinum. It precipitates copper and lead acetates, and with ammoniacal cupric sulphate it gives an intense purple-brown coloration. Gum arabic, saliva, and various other organic matters cause solutions of pyrogallol exposed to the air to absorb oxygen, by which purpurogallin is formed and separates in small yellow capillary crystals. If 0·2 per cent. of pyrogallol be added to a 1 per cent. solution of gum arabic, it becomes yellow in a few hours, and purpurogallin separates in hairlike crystals, which continue to increase for some months. It these crystals are freed from pyrogallol by washing with water, and a trace of alkali is added, they dissolve with an intense blue colour. Purpurogallol is also formed by oxidation with silver nitrate, potash permanganate, and many other reagents. Pyrogallol forms compounds with aldehydes; with formaldehyde, a body which reacts like a tannin and precipitates gelatin is produced. If pyrogallol be heated with hydrochloric acid, and aldehyde, chloral, or acetone, a red substance is produced. The less volatile portions of crude beech-tar creasote contains ethers of pyrogallol, methyl-pyrogallol, and propyl-pyrogallol, from which these bodies may be obtained by the action of hydrochloric acid under pressure. Methyl- and propyl-pyrogallols differ from ordinary pyrogallol in having an atom of hydrogen replaced by the groups CH3 or C3H7 respectively; and it is very probable that some tannins are derivatives of such modified pyrogallols.
[F] Hence its use as a "developer" in photography.
If pyrogallol be heated rapidly to 482° F. (250° C.) it parts with the elements of water, and is converted into metagallic acid, C6H4O2, a black amorphous body, insoluble in water, soluble in alkalies. When pyrogallol is made in the ordinary way by heating gallic or tannic acids to 410° F. (210° C.), much of this body is formed, even if the process be conducted in a stream of carbonic acid, and the yield of pyrogallol usually amounts to only about 5 per cent. of the gallic acid employed (see [p. 65]).
Catechol.—Pyrocatechol, pyrocatechin, oxyphenic acid, C6H4(OH)2.
Sources.—Beside that of the decomposition of certain tannins by heat (see [p. 63]), catechol is produced by the dry distillation of catechin and some allied bodies which frequently accompany the tannins. It is also formed together with pyrogallol and its homologues (see above) by the dry distillation of wood, wood tar creasote consisting largely of ethers of pyrocatechol and its homologues, methyl and pyrocatechol, &c., and hence it is also found in crude pyroligneous acid. It has also been produced by heating carbohydrates with water under pressure, and is found ready formed in Virginia-creeper (Ampelopsis hæderacea), and probably in other plants. It has also been formed synthetically.
Reactions.—Catechol melts at 232° F. (111° C.) and sublimes at about the same temperature, condensing in brilliant laminæ like benzoic acid. It is readily soluble in water, alcohol, and ether, and is extracted from its aqueous solution when shaken with the latter. Its aqueous solution precipitates lead acetate but not gelatin or alkaloids. With lime-water or caustic soda solution it becomes reddish, but remains clear for some time. It does not colour ferrous salts, but gives a dark green with ferric (avoiding excess); and after some time a black precipitate. The green is changed to a fine violet-red by alkalies and hydric sodic carbonate, and restored by acids. To fir-wood moistened with hydrochloric acid it gives, like phloroglucol, a violet coloration by combination with the trace of vanillin which this wood contains. This reaction does not seem to be given by pyrogallol or by common phenol. Catechol gives a red coloration with citric acid, and after standing, ceases to react with iron.
Decomposition of Tannins by Dilute Acids.—It has been stated that tannins when heated with dilute sulphuric or hydrochloric acids are decomposed, yielding frequently glucose, and either gallic or ellagic acids, or red anhydrides. To determine whether glucose is produced, the tannin must first be carefully purified from glucose, gums, or other bodies likely to interfere, by the methods mentioned on [p. 58]. Either the tannin itself or its washed lead-salt may be used, and must be heated to 212° F. (100° C.) for some hours in a sealed tube, or tightly closed bottle with dilute hydrochloric acid. After cooling, the mixture must be allowed to stand for some time to separate any sparingly soluble products, which must be filtered off. The filtrate must be shaken with ether and acetic ether to remove gallic acid ([p. 59]), the aqueous solution must be boiled, neutralised with soda, precipitated with basic lead acetate to remove any traces of tannin or colouring matters, the liquid again filtered, and excess of lead removed with dilute sulphuric acid, the mixture again neutralised with soda, and heated to boiling with Fehling's copper-solution, when a yellow or red precipitate of cuprous oxide will prove the formation of glucose. The precipitate produced by cooling may consist (of lead chloride, if the lead salt has been used,) of ellagic acid, or of red anhydrides or phlobaphenes of the tannin. The lead chloride may be removed by washing with boiling water. If the remaining precipitate has a pale yellow or fawn colour it probably consists of ellagic acid (see [p. 71]), soluble in ammonia and hot alcohol and dissolving freely in strong nitric acid, forming an intense crimson liquid.
The ethereous layer will contain the gallic acid, if any has been formed, and must be evaporated to dryness, and the residue taken up with cold water, and filtered. Addition of a few drops of solution of potassium cyanide will produce a fine red coloration if gallic acid be present, which rapidly fades, but is restored by shaking. A solution of picric acid, to which excess of ammonia has been added, gives a red coloration rapidly changing to a fine green, even in very dilute solutions of gallic acid.
It is not, however, generally necessary to resort to so elaborate a process merely to distinguish the class to which tannins belong. The tannin, or its infusion, may be simply boiled with dilute hydrochloric acid for some time, replacing the acid lost by evaporation. The solution is diluted to 50 c.c. and allowed to cool. Ellagic acid and phlobaphenes may separate, and must be filtered off. If the precipitate is pale, it is probably ellagic acid, and maybe recognised by the nitric acid test. If red, it probably consists of phlobaphenes, and may be treated with cold alcohol, in which phlobaphenes are freely soluble, but ellagic acid very little. The ellagic acid will therefore be left on the filter if present in any quantity, while the alcoholic solution may be precipitated by the addition of water, and the phlobaphenes further examined by treatment with potash.