4. Phosphotungstic Acid—Scheibler‘s reagent.—This acts in a manner very similar to phosphomolybdic acid.
Methods for detecting
Vegetable Alkaloids
There are several methods recommended for the isolation and detection of the vegetable alkaloids, and their separation from the contents of the stomach or from the membranes and tissues of the body. The process, however, most generally pursued is that of Stas, which may be briefly described as follows:
(a) The substance to be examined is mixed with twice its weight of absolute alcohol, to which from ten to thirty grains of tartaric or oxalic acid—preferably the former—have been added, and the mixture subjected to gentle heat in a flask, 70° to 75° C., or 158° to 167° F.
(b) If the membranes or organs have to be examined, they are finely divided, treated with absolute alcohol, squeezed, and again treated with fresh alcohol as in (a).
In either case, the mixture, when quite cold, is filtered, and the alcoholic solution is concentrated by evaporation, either in vacuo or in a current of air not exceeding 95° F. or 35° C.
The liquid residue is now passed through a moistened filter, which separates the fat and other insoluble matters. The filtrate is evaporated to dryness over sulphuric acid or in vacuo, and the acid residue of this evaporation dissolved in the smallest possible quantity of distilled water. The acid liquid is then gradually neutralised with the bicarbonate of potash or soda until effervescence ceases, and afterwards shaken in a flask with four or five times its bulk of pure ether, and allowed to settle. When the ether has become quite clear, a small portion of it is decanted into a small glass capsule, and allowed to spontaneously evaporate in a dry place. If during evaporation streaks of liquid appear on the side of the capsule, running together at the bottom, a liquid volatile alkaloid is probably present. If none of these manifestations occur, the alkaloid is in all probability solid and non-volatile.
| The Alkaloid is Volatile. | The Alkaloid is Non-Volatile. |
|---|---|
| To the original mixture in a flask | To the original mixture in a |
| add a moderate quantity of a strong | flask add strong caustic potash or |
| solution of caustic potash or soda, | soda solution, and agitate with |
| mixed with ether; agitate, and allow | successive portions of pure ether |
| the mixture to settle. Pour off the | allowing it to completely settle |
| ethereal solution, and re-shake | each time. The ethereal solutions, |
| residue with a fresh quantity of | being mixed, are evaporated, leaving |
| ether; decant, and mix both solutions. | the alkaloid in an impure state. |
| The ethereal solution is now shaken | To purify it, the solid residue left |
| with a mixture of four parts of water | on evaporation is treated with a |
| and one of sulphuric acid, which | small quantity of dilute sulphuric |
| withdraws the alkaloid from its | acid, which dissolves the alkaloid, |
| solution, leaving any fatty matter | leaving any fatty impurities behind. |
| dissolved in the ether. The acid | The acid liquid is evaporated to |
| solution is now mixed with strong | three-quarters of its bulk over |
| potash or soda solution in excess,[20] | strong sulphuric acid, and then a |
| agitated with ether, the ether poured | saturated solution of carbonate of |
| off, and then evaporated at as low | potash or soda added. The absolute |
| a temperature as possible,[21] | alcohol will then dissolve out the |
| leaving the pure alkaloid with all its | pure alkaloid, giving it, on in the |
| characteristic chemical and | crystalline form, and in evaporation, |
| physical properties. | a state to show its characteristic |
| reactions. |
If morphine has to be sought for, the liquid should be shaken with ether immediately after being neutralised with carbonate of sodium, and the ether poured off as quickly as possible; for if the alkaloid have time to separate in the crystalline form, scarcely any of it is dissolved by the ether (Otto).
The method of Stas is based upon the fact that the salts of the alkaloids, as a class, are soluble in water and alcohol, but are insoluble in ether; and that these salts when in solution are readily decomposed by the mineral alkalies with the elimination of the alkaloids, which, in their free and uncombined state, are more or less readily soluble in ether.