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:—
| C6H2 |  | CO.OH | ||
| OH | ||||
| OH | ||||
| O |  | |||
| C6H2 | | CO | ||
| OH | ||||
| OH | ||||
| OH | ||||
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.