Art. LXI.—On the Preparation of Spontaneously Inflammable Phosphine.
[Read before the Philosophical Institute of Canterbury, 13th October, 1881.]
Plate XXXVII., fig. 1.
Phosphuretted hydrogen, now generally known as phosphine, being one of the gases commonly made for experimental purposes by the practical student in the chemical laboratory, it is essential that the apparatus for its production should be so contrived as to avoid all risk of an explosion.
In the preparation of spontaneously inflammable phosphine it is usual to employ a glass flask, fitted with delivery tube, to contain the alkaline solution and phosphorus, from which the gas is generated. Before applying heat to the flask a current of coal-gas is passed through, in order to displace the contained air; and the chief fault of the apparatus rests in the fact that, unless this is continued for some time, it is impossible to expel all the air from the vessel. Instead of using gas to displace the air, ether is sometimes poured into the flask; but this is open to the same objection, letting alone the expensiveness of the liquid employed.
Another and more satisfactory method of producing the gas is to generate it in a glass retort completely filled with the alkaline solution, the mouth, of course, being immersed in the water of the pneumatic trough. By this means we at once get rid of every particle of air, and all chance of an explosion is done away with. There is, however, one fault to be found with this form of generator. A considerable quantity of solution must be used, and as most of this has to be heated to a high temperature, some delay takes place before the required gas is given off.
To avoid all these difficulties I have designed a piece of apparatus which is both simple and effective. An ordinary glass flask is taken and fitted with cork and delivery tube; but in addition to this, also passing through the cork, is a deflagrating spoon, which should be so adjusted that the bowl is about half an inch above the surface of the liquid when in the flask (pl. XXXVII., fig. 1). The pieces of phosphorus and the solution of caustic potash (or other suitable alkaline hydrate) having been placed in the flask, a small piece of dried phosphorus is put into the deflagrating spoon and ignited. The flask is now corked and the oxygen, present as an objectionable gas, combines with the phosphorus to form acids, which in no way
interfere with the success of the experiment. Heat may be at once applied, and before the white fumes have quite disappeared the end of the delivery tube should be placed under water. The experiment may then be conducted as usual, care being taken when it is finished to withdraw the source of heat, and so allow the water to rush up the delivery-tube into the flask, which latter should not be opened till the water has ceased to flow. The phosphorus in the deflagrating spoon should be more than is required to combine with the oxygen in the flask, which if of moderate capacity will need a piece about the size of a pea.
This method of preparing phosphine will be found very useful, even in a well-furnished laboratory; but its utility will be more especially felt in places where coal-gas is unobtainable.