III.—Saponin—Saponin Substances.
§ 563. The term “saponin” of late years has been applied to a class of glucosides which possess the common property of being poisonous, and, when dissolved in water, forming solutions which froth on shaking like soap-suds.
The substances which have these properties are not all of the same series chemically, but those of the general formula, CnH2n-8O10, are most numerous, and the following is a list:—
Possibly also dulcamarin C22H34O10 and syringen C17H26O10 may belong to this series.
There are some 150 distinct plants which thus yield saponins; a few of these plants are as follows:—Saponaria officinalis, Gypsophila struthium, Agrostemma githago (corn cockle), Polygala senega, Monimia polystachia, the bark of Quillaja saponaria, and Chrysophyllum glycyphleum.
The saponin separated from Saponaria, and from the corn cockle will be here described.
§ 564. Properties.—Saponin is a white amorphous powder, very soluble in water, to which it gives the curious property of frothing just like soap solution. To obtain this effect there must be at least 1 mgrm. in 1 c.c. of liquid. Saponin is neutral in reaction, it has no odour, but causes sneezing if applied to the mucous membrane of the nose; the taste is at first sweet, and then sharp and acrid. It is almost entirely insoluble in absolute alcohol, but dissolves in hot alcohol of 83° to separate again nearly completely on cooling. It is precipitated by basic lead acetate, and also by baryta water, but in each case it is advisable to operate on concentrated solutions. Picric acid, mercuric chloride, and alkaloidal “group reagents” give no precipitate. When a little of the solid substance is treated with “Nessler” reagent, there is a greenish or yellow colour produced. A drop of strong sulphuric acid, mixed with a minute quantity of saponin, strikes slowly a bright red colour, which, on heating, deepens to maroon-brown. Nordhausen sulphuric acid shows this better and more rapidly. If saponin is boiled with dilute acid it breaks up into sapogenin and sugar, and therefore the liquid after neutralisation reduces “Fehling.” This reaction is probably after the following equation:—
2C17H26O10 + 2H2O = 2C8H11O2 + 3C6H12O6.
Sapogenin may be separated by evaporating the neutralised liquid to dryness, treating the dry residue with ether, which dissolves out the sapogenin, and finally recovering the substance from the ethereal solution, and crystallising it from hot alcohol. Crystals are readily obtained if the alcoholic solution is allowed to evaporate spontaneously. A solution of saponin exposed to the air gets turbid, and develops carbon dioxide; not unfrequently the solution becomes mouldy.