2. The Production of Phosphine (PH3).—Any method which produces phosphine (phosphuretted hydrogen), enabling that gas to be passed through nitrate of silver solution, may be used for the detection of phosphorus. Thus, Sonnenschein states that he has found phosphorus in extraordinary small amount, mixed with various substances, by heating with potash in a flask, and passing the phosphine into silver nitrate, separating the excess of silver, and recognising the phosphoric acid by the addition of molybdate of ammonia.[317]
[317] Sonnenschein, Handbuch der gerichtlichen Chemie, Berlin, 1869.
The usual way is, however, to produce phosphine by means of the action on free phosphorus of nascent hydrogen evolved on dissolving metallic zinc in dilute sulphuric acid. Phosphine is formed by the action of nascent hydrogen on solid phosphorus, phosphorous acid, and hypophosphorous acid; but no phosphine can be formed in this way by the action of hydrogen on phosphoric acid.
Since it may happen that no free phosphorus is present, but yet the first product (phosphorous acid) of its oxidation, the production of phosphine becomes a necessary test to make on failure of Mitscherlich’s test; if no result follows the proper application of the two processes, the probability is that phosphorus has not been taken.
Blondlot and Dusart evolve hydrogen from zinc and dilute sulphuric acid, and pass the gas into silver nitrate; if the gas is pure, there is of course no reduction; the liquid to be tested is then added to the hydrogen-generating liquid, and if phosphorous or hypophosphorous acids be present, a black precipitate of phosphor-silver will be produced. To prove that this black precipitate is neither that produced by SH2, nor by antimony nor arsenic, the precipitate is collected and placed in the apparatus to be presently described, and the spectroscopic appearances of the phosphine flame observed.
3. Tests Dependent on the Combustion of Phosphine (PH3).—A hydrogen flame, containing only a minute trace of phosphorus, or of the lower products of its oxidation, acquires a beautiful green tint, and possesses a characteristic spectrum. In order to obtain the latter in its best form, the amount of phosphine must not be too large, or the flame will become whitish and livid, and the bands lose their defined character, rendering the spectrum continuous. Again, the orifice of the tube whence the gas escapes must not be too small; and the best result is obtained when the flame is cooled.
M. Salet has proposed two excellent methods for the observation of phosphine by the spectroscope:—
(1) He projects the phosphorus-flame on a plane vertical surface, maintained constantly cold by means of a thin layer of running water; the green colour is especially produced in the neighbourhood of the cool surface.