Pl. III.

General Methods of Examination of Tannins.

Decomposition by Heat.—The ordinary method is to distil the tannin or dried extract in a small retort, and examine the distillate for catechol and pyrogallol. Unless the heat be very carefully regulated, much loss is caused by the destruction of the catechol and pyrogallol with formation of metagallic acid, &c., and their detection is greatly complicated by the presence of secondary products. This difficulty is somewhat lessened by passing a stream of carbon dioxide through the retort, which carries the products quickly out of the heated portion. A better method is to heat the tannin in glycerin (Thorpe, Chem. Soc. Abstr., 1881, 663; Allen, 'Commercial Organic Analysis,' 2nd ed.). About 1 grm. of the sample is heated with 5 c.c. of pure glycerin to 392°-410° F. (200°-210° C.) for 20 minutes. After cooling, about 20 c.c. of water is added, and the liquid is shaken with an equal volume of ether, without previous filtration. The ethereous layer, which contains the pyrogallol and catechol, is separated from the aqueous portion, evaporated to dryness, and dissolved in 50 c.c. of water. The filtered solution is divided into several portions and tested with lime-water, ferric chloride, and ferric acetate (see pp. 66-7); by these means it is easy to distinguish between catechol and pyrogallol; and either may be detected in presence of a small portion of the other; but if in nearly equal quantities, their recognition is difficult. Catechol may be derived from catechin, &c., and pyrogallol from gallic acid, and it is therefore necessary in some cases to remove these bodies from the tannin before treatment. As a general rule, however, catechins and catechol derivatives are only present in any quantity with catechol-tannins, and the same is true of gallic acid with regard to pyrogallol. (For methods of separation see pp. 69, 71, 80). Catechol has been formed by long continued heating of cellulose, starch, and other carbohydrates with water under pressure (see p. 67).

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.

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.

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,

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.

The following is the best method in which to proceed to investigate the products of the action of potash. 20 grm. of the red, or of the tannin from which it is derived, or its lead salt, is boiled with 150 c.c. of solution of caustic potash of 1·20 sp. gr. for 3 hours, and the liquid is then concentrated with constant stirring till it becomes pasty. It is then cooled and treated with a volume of dilute sulphuric acid slightly more than enough to neutralise the alkali employed. After cooling it is filtered from potassium sulphate and other solid matters, and the filtrate treated with sodium bicarbonate till its wine-red reaction with litmus shows that the sulphuric acid is neutralised. The liquid is then shaken with an equal measure of ether, the ethereous layer drawn off and the treatment repeated several times. On distilling off the ether, phloroglucol is left and may be purified by solution in water, when protocatechuic acid and other products may be precipitated by neutral lead acetate, and filtered off, and the phloroglucol again extracted with ether, and recognised by its reaction with ferric chloride and bromine water, and by its sweet taste.

Phloroglucol.—Phloroglucin C₆H₆O₃ is a phenol isomeric with pyrogallol. It crystallises with 2 molecules of water, which it loses at 212° F. (100° C.). It melts at about 428° F. (220° C.), sublimes without odour, and solidifies again on cooling. It is soluble in water, alcohol, and ether, and by agitation with the latter it may be removed from its aqueous solution. It is not precipitated by any metallic salt but basic lead acetate. It is coloured deep violet red by ferric chloride. If bromine be added to its concentrated solution in water, it absorbs 3 atoms, forming tribromophloroglucol, C₆H₆Br₃O₃, which separates in crystalline needles, with evolution of heat and a very irritating odour. If a deal shaving be moistened with solution of phloroglucol, and then with strong hydrochloric acid, it soon takes a deep violet colour, from the formation of phloroglucol-vanillin with the trace of vanillin contained in all coniferous woods. Pyrocatechol gives a similar reaction, and it is stated by Etti (Chem. Soc. Jour., xliv. 60) that pyrogallol forms a similar compound; it does not, however, give the colour reaction on deal. It is extremely probable that this reaction may be used to detect phloroglucide tannins without the troublesome fusion with potash, in cases where pyrocatechol is absent. The reaction is strongly given by gambier, which is known to contain phloroglucol, but not by oak bark and valonia, though these contain protocatechuic acid. If to a dilute solution of phloroglucol a solution of aniline or toluidine nitrate be added, and then a trace of potassic nitrite, the liquid gradually becomes yellow or orange, then turbid, and finally deposits a cinnabar-red precipitate. This reaction is given by gambier, but is also produced by oak bark infusion, which is not supposed to contain phloroglucol; and gall tannin, pyrogallol, and other substances give similar but browner precipitates.

Protocatechuic Acid.—C₆H₃(OH)₂CO.OH, one of the six isomeric dihydroxybenzoic acids of this formula (see Miller, Chem. Soc. Jour., xli. 198), crystallises in needles and plates with 1 mol. water, which it loses at 212° F. (100° C.). It melts at 228° F. (109° C.) and on further heating is decomposed into pyrocatechol and carbonic acid. It is somewhat soluble in cold water, and readily so in hot water, alcohol, and ether. It is coloured bluish green by ferric chloride, which is changed to red by alkalies. Solutions of protocatechuates give a violet coloration with ferric salts. It is precipitated by lead acetate, and reduces silver ammonio-nitrate, but not Fehling's solution (see also p. 107).

Constitution of Tannins.

Having described the products of decomposition, something must be said of the way in which these constituents are combined to form the unaltered tannins. The only tannin of which we have as yet anything approaching complete knowledge is that obtained from galls, sumach, and myrabolans, and which is thence called gallotannic acid.

Gallotannic, or digallic acid exists as the principal tannic acid of the galls of oak, tamarisk, &c.; and, in mixture with more or less ellagitannic acid, in myrabolans, divi-divi, sumach, pomegranate rind, and many other plant-products. It has also been formed by Schiff from gallic acid, by mixing it, after drying at 230° F. (110° C.), with phosphorus oxychloride to a thin paste, and heating, first to 212° F. (100° C.) and then to 248° F. (120° C ). Much hydrochloric acid was evolved, and the gallic acid was converted into a yellow powder, which after purification by washing with ether, dissolving in water, allowing the gallic acid to crystallise out, saturating with salt, washing the precipitated tannin in salt solution, and redissolving in alcohol and ether, had all the reactions of purified gall tannin, but was perfectly reconverted into gallic acid on boiling with hydrochloric acid, without the formation of any trace of ellagic acid, or glucose. By analysis of the tannin and its acetyl compounds it was shown to be digallic acid, and its constitutional formula is almost certainly as follows:—

By boiling gallic acid with solution of arsenic acid, Schiff obtained a product which precipitated gelatin, and otherwise reacted like tannin, and he regards this as digallic acid, but other experimenters have failed to obtain digallic acid by this means, and have found that on complete removal of arsenic, the compound was reconverted to gallic acid. It therefore remains a moot point whether digallic acid is really formed, and reconverted into gallic acid by the prolonged action of hydric sulphide, which is necessary to remove the arsenic, or whether, as seems more probable, the supposed tannin is merely an arsenical compound of gallic acid.

Gallotannic acid as obtained from plants invariably yields traces of glucose, as well as of ellagic acid, when boiled with dilute acids. It is still an open question whether the glucose exists in the plant as a glucoside of tannic acid or is always the product of some impurity (as is shown by Etti to be the case with oak bark, where lÆvulin is always present). It seems most probable however that natural gallotannic acid is really a glucoside of digallic acid, or possibly, according to the theory of Hlasiwetz, a gummide, or compound of dextrin, which, by the action of acids, is easily converted into glucose. What has been said of gallotannic acid in this respect, applies to many other tannins, which like it give glucose by treatment with acids.

Gallotannic acid is met with, in commerce, in the form of light buff-coloured scales, with a faint peculiar odour and a powerfully astringent taste. It is soluble in 6 parts of cold water or glycerin, and very readily in hot. It is also very soluble in alcohol containing water, but much less so in absolute. It is moderately soluble in washed, but scarcely at all in anhydrous ether, chloroform, benzene, or petroleum spirit.

The commercial acid usually contains more or less of gallic acid, which may be detected by dissolving in water, shaking with ether, and after decanting and evaporating the ether, applying the tests described under gallic acid (p. 70). It may also be frequently distinguished in the simple aqueous solution of the tannic acid by the tests given. Its quantity may be determined (in the absence of other impurities) by the LÖwenthal method (p. 121), the gallic acid forming the "not-tannin," by comparison with a solution of pure gallic acid.

Commercial tannic acid is sometimes adulterated with starch, which is left undissolved on treating the sample with ordinary alcohol.

For the estimation of gallotannic acid see pp. 118 et seq. For its principal reactions, Table, p. 113.

Ellagitannic acid, C₁₄H₁₀O₁₀, is contained in divi-divi, myrabolans, and as a glucoside in pomegranate rind. When boiled with dilute acids, or treated with water at 230° F. (110° C.) in a sealed tube, it yields its anhydride, ellagic acid (see p. 71), C₁₄H₈O₉. In its reactions ellagitannic acid closely resembles gallotannic acid.

Quercitannic acid. Oak-bark tannin.—This tannin may be prepared from oak bark by agitating an alcoholic extract with ethyl acetate, and separating and evaporating the ethereous layer. It is still contaminated with a brownish-green terpene resin, and with some of the higher anhydrides of the tannin. The resin may be removed by treating the dried extract with ether or benzene, in which it is readily soluble; and the phlobaphenes, or higher anhydrides, by dissolving the tannin in ether-alcohol, or probably to a great extent, by simple solution in cold water in which the phlobaphenes are scarcely soluble.

It may also be prepared by evaporating the alcoholic extract, and extracting with water, which leaves the phlobaphenes, or higher anhydrides undissolved. The first anhydride, which is partially soluble, may be precipitated by the addition of salt, and the quercitannic acid extracted by shaking the filtered solution with acetic ether. In very dilute alcohol it yields a pure yellow precipitate with lead acetate. In aqueous solution the precipitate is light-brown. It gives a blue-black with ferric salts. When pure, quercitannic acid yields nothing to pure ether or to benzene.

If quercitannic acid be heated to 266°-284° F. (130°-140° C.) it loses water, and yields a red anhydride slightly soluble in water, which constitutes the red colouring matter of oak bark, and which has also been called "difficultly soluble tannin." It precipitates gelatin and is one of the class which Eitner well designates "tanning colouring matters." It gives a brownish red with lead acetate, and a blue-black with ferric salts; it is difficultly soluble in water and ether, but readily so in alcohol of all strengths. Together with other anhydrides, it exists naturally formed in the bark. At higher temperatures, or by boiling with acids, a series of higher anhydrides may be obtained which are quite insoluble in water, but are soluble in alcohol and caustic alkalies. No glucose is produced by treatment of pure quercitannic acid with acids, that formed by so treating oak-bark extract being due to the alteration of the lÆvulose present. If oak-reds are fused with potash they yield, according to Johansen (Chem. Soc. Jour., xxxii. 721), protocatechuic and butyric acids. If heated in sealed tubes with dilute hydrochloric acid, gallic acid only is formed, with evolution of methyl chloride. The constitutional formula of quercitannic acid is as yet very uncertain: it is probably a methyl derivative of digallic acid. Its formula is variously given as C₁₇H₁₆O₉ (Etti, Chem. Soc. Jour., xliv. 994), C₂₈H₂₆O₁₅ (Ibid. xl. 901, LÖwe), C₁₉H₁₆O₁₀ (BÖttinger, Berichte, xvi. 2710). For further details the original memoirs must be consulted.

For the reactions of oak-bark and valonia infusions the Table, p. 113, may be consulted.

Before describing the catechol-tannins it will be necessary to speak of a group of compounds of which it is probable that these tannins are decomposition products. These are the catechins. It is as yet by no means certain how far they should be considered a group, some chemists holding the opinion that there is only one catechin, of which the rest are merely impure preparations.

Catechin is a white crystalline substance, contained to the extent of some 30 per cent. in cube gambier, and in smaller proportion in block gambier and cutch, and very probably in all tanning materials yielding catechol-tannins. It melts at 422¹/₂° F. (217° C.) and yields a sublimate of catechol on further heating. It is readily soluble in alcohol and in boiling water, but requires 1133 parts of cold water for its solution. Hence it separates on cooling from a hot solution of gambier, and may be purified by redissolving in hot water and treatment with animal charcoal, and subsequent crystallisation. It may also be extracted from its aqueous solution by agitation with ether. It possesses no acid properties, though some writers have incorrectly called it catechuic acid. The aqueous solution gives precipitates with lead acetate and mercuric chloride, and reduces ammonia-nitrate of silver; but, unlike tannins, it does not precipitate gelatin, alkaloids, or tartar emetic. It is oxidised by permanganate in presence of free acid, and hence, when in solution, is estimated by the LÖwenthal method as "not tannin." It dissolves in concentrated sulphuric acid with a deep purple coloration. By fusion with caustic alkalies it yields protocatechuic acid and phloroglucol together with hydrogen. Its constitution is very uncertain, but it is probable that the phloroglucol stands in a somewhat similar relation to the protocatechuic acid that glycerin does to the fatty acids in natural fats; and that its decomposition by fused alkalies is a process much akin to saponification. This constitution is represented by the following formula:—

When acted on by heat and dilute acids the following anhydrides are produced—the formulÆ given must be taken as to some extent provisional.

The white deposit which occurs on pit sides and in the interior of leather where gambier is largely used, and which is sometimes called "the whites," consists of catechin. It may be decomposed by warm sulphuric acid. This deposit is favoured by the use of hot gambier liquors. It is probable that by exposure to the air and by boiling, catechin is gradually converted into catechutannic acid during the tanning process, and hence the practical tanning value of cube gambier is probably greater than analysis indicates. Hunt has, however, shown (Jour. Soc. Chem. Ind., iv. 266) that, as estimated by the LÖwenthal process, the tannin is to some extent lessened by boiling.

Kinoin, C₁₄H₁₂O₆ (Etti, Berl. Ber., xi. 1879), obtained from green or malabar kino, a product very similar to cutch, by boiling with dilute hydrochloric acid and extraction by agitation with ether, is very similar in its properties to catechin. It does not itself precipitate gelatin, but like catechin, yields a series of anhydrides or reds, which do so. On dry distillation it yields catechol and common phenol; and when heated with hydrochloric acid at 248°-266° F. (120°-130° C.) methyl chloride, catechol and gallic acid. Hence its constitution is probably that of methyl-catechol gallate.

Quebracho-catechin was found by P. N. Arata (Chem. Soc. Jour., xl. 1152) in the wood of quebracho colorado (p. 40), but in too small quantity for detailed investigation. It probably bears a similar relation to quebrachitannic acid that ordinary catechin does to catechutannic acid. It is insoluble in cold and only slightly soluble in hot water, but very soluble in alcohol and ether. Its solution is clouded by normal lead acetate, and gives rose-coloured precipitates with basic lead acetate and mercurous nitrate, and blackish with ferric acetate; it reduces silver-nitrate and gold chloride, and is coloured yellow by nitric acid, red by sulphuric acid, yellowish by sodium hypochlorite, and green by Fehling's solution. It does not precipitate gelatin, or alkaloids.

Catechutannic acid has been pretty fully described under catechin, of which it is the first anhydride (p. 80). It is possibly identical with mimo-tannic acid, the tannin of cutch and mimosa bark, which is chemically very similar, but greatly differs in its practical effect in tanning. For some further reactions of cutch and gambier infusions see p. 113. It gives a greyish-green precipitate with ferric salts, and (distinction from gallotannic acid) precipitates cupric sulphate but not tartar emetic.

Quebrachitannic acid is got from the wood of the quebracho colorado, Quebrachia lorentzii (formerly Loxopterygium) which must not be confounded with the bark of the Aspidospermum quebrachia, which is valuable, not for its tannin, but for an alkaloid, aspidospermin, which is used for medical purposes. It has been pretty thoroughly investigated by P. N. Arata (Chem. Soc. Jour., xxxiv. 986 and xl. 1152). It seems, however, a little doubtful to the writer whether the substance investigated by Arata was not the anhydride of the tannin, rather than the tannin itself, as it presents many points of analogy to catechu-red and was less soluble in water than quebracho tannin appears in practice to be.

According to Arata, quebrachitannic acid is a pale red amorphous mass, having an astringent taste and yielding a light cinnamon coloured powder. It is insoluble in carbon-bisulphide, turpentine oil, and benzene. Its aqueous solution gives a white precipitate with both normal and basic lead acetate, which when heated, acquires first a rose, and then a chocolate colour; with ferric chloride a green liquid is produced, which changes after a time to red and becomes black on addition of sodium acetate. It forms white precipitates with gelatin, albumen, and alkaloids. By dry distillation it yields catechol. By fusion with potash or the action of sulphuric acid, phloroglucol and protocatechuic acid, while nitric acid converts it into oxalic and picric acids. While it shows great similarity in its reactions to catechutannic acid, it differs materially in percentage composition, containing only 52·5 as compared with 62·0 per cent. of carbon.