§ 529, Digitonin, a white amorphous body, has many of the characters of saponin. Like saponin, it is easily soluble in water, and the solution froths, and, like saponin again, it is precipitated by absolute alcohol, by baryta water, and by basic lead acetate. It may be readily distinguished from saponin by treating a watery solution with sulphuric or hydrochloric acid. On saponifying, it is split up into digitogenin, galactose, and dextrose. On heating, a beautiful red colour develops. It does not give the bromine reaction.
Digitogenin is insoluble in water and aqueous alkalies; it is somewhat soluble in alcohol, chloroform, and glacial acetic acid; it forms a crystalline compound with alcoholic potash, which is strongly alkaline, and not very soluble in alcohol.
§ 530. Digitalin, when perfectly pure, forms fine, white, glittering, hygroscopic needles, or groups of crystalline tufts; it is without smell, but possesses a bitter taste, which is at once of slow development and of long endurance. On warming, it becomes soft under 100°, and, above that temperature, is readily decomposed with evolution of white vapours. It is insoluble in water, in dilute soda solution, in ether, and in benzene. It is soluble in chloroform, especially in chloroform and alcohol, and dissolves easily in warm acetic acid; twelve parts of cold and six of boiling alcohol of 90 per cent. dissolve one of digitalin. Dilute hydrochloric or sulphuric acid decompose it into glucose and digitaletin (C44H30O18); if the action is prolonged, digitaleretin (C44H38O18), and finally dehydrated digitaleretin, are formed. Concentrated sulphuric acid dissolves it with the production of a green colour, which by bromine passes into violet-red, but on the addition of water becomes green again. Hydrochloric acid dissolves it with the production of a greyish-yellow colour, passing gradually into emerald green; water precipitates from this solution a resinous mass.
§ 531. Digitaletin.—A substance obtained by Walz on treating his digitalin by dilute acids. It is crystalline, and its watery solution tastes bitter. It melts at 175°, and decomposes, evolving an acid vapour at about 206°. It dissolves in 848 parts of cold, and 222 of boiling, water; in 3·5 parts of cold, and in from 2 to 4 of boiling, alcohol. It is with difficulty soluble in ether. It dissolves in concentrated sulphuric acid, developing a red-brown colour, which, on the addition of water, changes to olive-green. On boiling with dilute acids, it splits up into sugar and digitaleretin.
§ 532. Digitoxin always accompanies digitalin in the plant, and may by suitable treatment be obtained in glittering needles and tabular crystals. It is insoluble in water and in benzene. It dissolves with some difficulty in ether, and is readily dissolved by alcohol or by chloroform. On boiling with dilute acids, it is decomposed into an amorphous, readily soluble body,—Toxiresin. Digitoxin, according to Schmiedeberg, only exists in the leaves of the digitalis plant, and that in the proportion of 1 part in 10,000. Digitalin and digitoxin are par excellence the poisonous principles of the plant. Toxiresin is also intensely poisonous. It may be obtained in crystals by extracting the dry exhausted leaves with alcohol of 50 per cent., precipitating with lead acetate, and washing the precipitate first with a dilute solution of sodium carbonate (to remove colouring-matter), and then with ether, benzene, and carbon disulphide, in all of which it is insoluble; on decomposing the lead compound, digitoxin may be obtained in colourless scales or needle-shaped crystals.
§ 533. Digitaleretin, the origin of which has been already alluded to, is a yellowish-white, amorphous powder, possessing no bitter taste, melting at 60°, soluble in ether or in alcohol, but insoluble in water.
Paradigitaletin is very similar to the above, but it melts at 100°, and is insoluble in ether.
§ 534. Several other derivatives have been obtained and described, such as the inert digitin, digitalacrin, digitalein, and others, but their properties are, as yet, insufficiently studied. Digitalin, as well as digitoxin, may now be obtained pure from certain firms, but the ordinary digitalin of commerce is, for the most part, of two kinds, which may be distinguished as French and German digitalin. The French digitalin, or the digitalin of Homolle, is prepared by treating an aqueous extract of the digitalis plant with lead acetate, and freeing the filtrate from lead, lime, and magnesia, by successive additions of alkaline carbonate, oxalate, and phosphate, and then precipitating with tannin. The tannin precipitate is treated with litharge, and the digitalins boiled and extracted from the mass by means of alcohol, and lastly, purifying with animal charcoal. Crystals are in this way obtained, and by removing all substances soluble in ether by that solvent, digitalin may be separated. The German digitalin is prepared according to the process of Walz, and is extracted from the plant by treatment with alcohol of ·852. The alcohol is removed by evaporation, and the alcoholic extract taken up with water; the watery extract is treated with lead acetate and litharge, filtered, the filtrate freed from lead by hydric sulphate, and the excess of acid neutralised by ammonia, and then tannin added to complete precipitation. The precipitate is collected and rubbed with hydrated oxide of lead, and the raw digitalin extracted by hot alcohol. The alcohol, on evaporation, leaves a mixture of digitalin mixed with other principles and fatty matter. If sold in this state, it may contain from 2 to 3 per cent. of digitalein and digitonin. On treating the mixture with ether, digitalin with some digitaletin is left behind, being almost insoluble in ether. Since, however, digitaletin is very insoluble in cold water, by treating the mixture with eight parts of its weight of cold water, digitalin is dissolved out in nearly a pure state. It may be further purified by treating the solution with animal charcoal, recrystallisation from spirit, &c.
§ 535. Reactions of the Digitalins.—Digitonin is dissolved by dilute sulphuric acid (1 : 3) without colour, and the same remark applies to hydrochloric acid; on warming with either of these acids, a violet-red colour appears; this reaction thus serves to distinguish digitonin from the three other constituents, as well as from saponin.
Sulphuric and gallic acids colour the glucosides of digitalin, digitalein, and digitonin, red, but not digitoxin, which can be identified in this way.