Gallic Acid.—Dioxysalicylic acid, C6H2(OH)3CO.OH, exists ready formed in some plants, and is a product of the fermentation of gallotannic acid under the influence of the nitrogenous ferment, pectase, or of its decomposition by boiling with acids or alkalies. It crystallises in white, or yellowish white needles, containing 1 mol. (9·5 per cent.) of water, which it loses at 212° F. (100° C.). It is soluble in 100 parts of cold or 3 of boiling water, in alcohol or glycerin, and slightly so in ether, by agitation with which it may however be removed from its aqueous solution. Gallic acid fuses at a temperature of about 449° F. (232° C.) (Etti, Chem. Soc. Jour., xxxvi. 160), but at about 410° F. (210° C.) begins to lose carbonic dioxide, and yields a crystalline sublimate of pyrogallol (see [p. 66]). If the heat be raised suddenly to 482° F. (250° C.) a considerable quantity of black shining metagallic acid is formed.
Aqueous solution of gallic acid gives the following reactions:—Solution of ferric chloride gives a deep blue coloration which is destroyed by boiling. Ferrous sulphate, if free from ferric salt, gives no reaction in dilute solutions, but a white precipitate in strong ones. The mixture rapidly darkens by oxidation. In alkaline solution gallic acid absorbs oxygen from the air and darkens from the formation of tannomelanic acid. Lime-water produces a white precipitate which rapidly becomes blue from oxidation. The same reaction is produced by baryta-water, or by the chlorides of barium or calcium on addition of ammonia (distinction from pyrogallol). It is distinguished from gallotannic acid by the following:—It does not precipitate gelatin, except in the presence of gum. It does not precipitate tartar emetic in presence of ammonic chloride, though both tannin and gallic acid are precipitated by tartar emetic alone. It precipitates lead acetate but not lead nitrate, while tannin precipitates both. A dilute solution of potassium cyanide gives a red coloration which disappears on standing, but is restored by shaking with air. If to even a very dilute solution of gallic acid, sodic arsenate, or some other faintly alkaline salts be added, the mixture absorbs oxygen and becomes a deep green. Aqueous solution of picric acid to which excess of ammonia has previously been added gives a red coloration, changing to green. Tannic and pyrogallic acid produce no reaction with cyanide, and with ammonic picrate a reddish coloration only. Gallic acid reduces silver nitrate and gold chloride rapidly when hot, but not Fehling's solution, and decolorises acidified potassic permanganate. If tannin and other oxidisable bodies be removed from its solution it may be estimated quantitatively by titration with permanganate in presence of indigo (see [p. 118]). It may be separated from tannin by gelatin or hide raspings (see pp. [121], [124]). Gallotannic and quercitannic acids may also be removed by precipitation with ammoniacal solution of cupric sulphate, or by cupric acetate, in presence of excess of ammonic carbonate (see also [p. 125]). Many other tannins, however, give precipitates with cupric salts which are soluble in ammonia and ammonic carbonate. In absence of such tannins it may be estimated gravimetrically by precipitation with cupric acetate. The precipitate is rapidly washed with water and digested with a solution of ammonic carbonate, in which it dissolves; any insoluble cupric tannate is filtered off, the solution is evaporated to dryness and the residue moistened with nitric acid and ignited. The weight of the remaining cupric oxide multiplied by 0·9 gives the weight of the gallic acid plus a little tannin dissolved by the ammonic solution.
Gallic acid may also be separated from tannin by lead acetate strongly acidified with acetic acid, by which tannic acid is precipitated, while lead gallate is dissolved.
Ellagic acid C14H8O9, when pure, is a sulphur-yellow crystalline body almost insoluble even in boiling water, and only slightly so in alcohol and ether, though by agitation with the latter, small quantities may be completely removed from aqueous solution. In hot alcohol it dissolves with a yellow colour, and crystallises on cooling. Solid ellagic acid gives with ferric chloride at first a greenish, and then a black coloration. In strong nitric acid it is soluble with a deep crimson coloration: that from divi-divi gives a crimson liquid on dilution with water, but from other sources it is rather orange.
Ellagic acid may be obtained in considerable quantity by pouring a concentrated alcoholic extract of divi-divi into water, when it separates and may be filtered off and recrystallised from hot alcohol. It may also be obtained by boiling the aqueous extracts of divi, myrabolans, pomegranate rind, &c., with dilute hydrochloric acid, and purified by the same means. It may be prepared from gallic acid by heating the latter with dry arsenic acid to 320° F. (160° C.), but is difficult to purify from traces of arsenic. Ellagic acid has not been reconverted into gallic acid. Its constitutional formula is, according to Schiff,
| C6H2 |  | CO.OH | ||
| OH | ||||
| O─── |  | |||
| O |  | |||
| C6H2 | | CO | ||
| O─── | ||||
| OH | ||||
| OH | ||||
differing from gallotannic acid only by the loss of two atoms of hydrogen.
Air-dried ellagic acid, C14H8O9 + OH2, contains 1 mol. of water, which it loses at 212° F. (100° C.) but reabsorbs in moist air. When heated to 392°-410° F. (200°-210° C.) it forms an anhydride, C14H6O8, losing another molecule of water, which it does not recover from moist air, but is slowly reconverted to ellagic acid by boiling with water.
The phlobaphenes or reds are chemically the anhydrides of the different tannic acids from which they are derived, or in other words they are formed from the tannins by the loss of one or more molecules of water. It is in this way that they are produced by the action of acids, and similarly they are often formed when alcoholic or highly concentrated aqueous extracts are poured into cold water, under which circumstances a part of the tannin seems unable to take up water again, and separates as a red precipitate. They exist ready formed in most tanning materials capable of producing them. They are soluble in alcohol, by which they may be extracted from tanning materials or dried residues containing them. They are also dissolved by dilute alkalies and alkaline carbonates, and by borax, which is said to be used in the preparation of some extracts, and was suggested by Sadlon as a means of making phlobaphenes available for tanning. Many of them are scarcely soluble in water even at a boiling temperature, though they become more so in presence of sugar, tannic acid, and some other substances. Their solubility in water depends on their degree of hydration, many tannins giving a series of anhydrides of which those containing only one molecule of water less than the original tannin are quite soluble in water, while the higher members of the series become less and less soluble as they lose water. Those which are soluble form the colouring matters of tanning materials, and generally are practically tannins, precipitating gelatin and combining with hide to form leather. Hemlock bark yields a series of such bodies, of which the lower members are deep red soluble tannins, while the higher form the red sediment, so well known to extract-tanners. Thus it is chemically impossible to decolorise hemlock extract without at the same time greatly lessening its tanning power, though by careful manufacturing and concentration at low temperature, the proportion of the higher anhydrides formed may be kept at a minimum. In many cases it is known, as in gambier, and in others it is probable that the tannin itself is merely the first anhydride of the series, and derived from a catechin which itself is a white crystalline body destitute of tanning properties (see [p. 79]).
Decomposition of the Phlobaphenes by Fusion with Caustic Alkalies.—It has been mentioned ([p. 64]) that the reds of different tannins yielded, in addition to protocatechuic acid, either phloroglucol, or acetic acid, or some other member of the fatty acid series. Some tannins, as those of alder and hop, give both phloroglucol and acetic acid, but it is very possible that this arises from the presence of two distinct tannins in these materials. It is stated that all those tannins which yield acetic acid on fusion with potash, also yield considerable quantities of glucose to dilute acids, while the phloroglucide tannins do not do so. Gallic acid fused with caustic soda has been found by Barth and Schroeder to produce a small quantity of phloroglucol, and it is similarly formed by resorcinol and common phenol (Chem. Soc. Jour., xliv. 60). It is therefore possible that in some cases where phloroglucol is detected, it may have been formed by the action of the alkali, and not have been originally a constituent of the tannin.