Art. XLVI.—The Action of Phosphorus on Solutions of Copper Sulphate and certain other Metallic Salts.

By H. Rands, M.A.
Communicated by Dr. W. P. Evans.

the phosphide plays an important part in the reduction, which is explained by the action between metallic copper and phosphorus when placed in water in a vessel open to the air. In this case the water round the phosphorus slowly becomes black and opaque, and then dark-reddish, owing to the growth in it of exceedingly delicate feathery crystals of copper. Straub gives the following steps in the reaction:—

He concludes that in the well-known action of phosphorus upon a solution of copper sulphate the same series of changes takes place, except that oxidation by the air is replaced by more rapid oxidation by the copper sulphate. The present paper is a summary of a series of experiments showing the inaccuracy of some of Straub's conclusions, and also extending the investigation to other metallic salts.

Phosphorus our Copper Sulphate.

By sealing up phosphorus and copper sulphate solution with the complete exclusion of air it can be shown that atmospheric oxygen is unnecessary for the progress of the reaction.

If the sulphate solution is very dilute, the phosphide formed is not at first attached to the phosphorus, but remains suspended as a black cloud in the solution. After a time, however, some of the phosphide may form a film on the phosphorus, and metallic copper will then soon appear. Thus, while the phosphorus and the solution are directly in contact, phosphide alone is formed. Further, the copper phosphide cloud and the sulphate solution may remain indefinitely in contact without change, showing that the bodies do not react in the manner stated by Straub.

If the phosphorus is suspended in the sulphate solution by means of a copper wire, metallic copper is deposited directly upon the wire as well as upon the phosphorus. This deposition of copper, which may take place several inches away from the phosphorus, is evidently not due to the intermediate formation of phosphide, and is not accounted for in Straub's explanation Under suitable condi ions, long strands of bright metallic crystals of copper grow from the phosphorus down into the solution, clearing it of its blue colour as they develop. Each crystal of copper acts as a nucleus for the deposition of more metal, and beautiful fern-like growths several inches in length may be obtained. Here again we have direct deposition of copper independent of the phosphide. It can be shown to be due to the presence of reducing acid in the solution. Thus, after the copper strands have grown to some length, the decolorized liquid surrounding them will readily reduce potassium permanganate, bromine water, mercuric chloride, and gold chloride; warmed with zinc and sulphuric acid, phosphine is formed (by the nascent hydrogen), which turns a crystal of silver nitrate yellow (forming PAg₃ 3AgNO₃); with silver nitrate it gives a white precipitate which gradually turns brown, but which if quickly washed and dissolved in dilute sulphuric acid decolorizes potassium permanganate solution. These facts prove that the liquid contains a lower acid of phosphorus. Although the available methods of analysis did not enable a definite distinction to be made between phosphorous and hypophosphorous

acids in these solutions, the behaviour when titrating with permanganate in acid solution indicated the presence of two different reducing agents.

The Reaction between Phosphorus and Metallic Copper in Water.

This reaction, which is used by Straub to explain the reducing action, of phosphorus on all metallic salts, proceeds in the following manner: A black cloud, consisting of extremely fine particles of copper phosphide, appears round the phosphorus. After a few days some of the phosphide forms a film on the phosphorus, and then metallic copper soon appears. As Straub suggests, the copper and the phosphorus are both oxidized by the air, and by diffusion soluble acid copper phosphate is formed. The case is then exactly analogous to that of phosphorus acting on very dilute copper sulphate, and it will be noted that the effects observed are the same. The presence of air is necessary, but the only function of the metallic copper and the atmospheric oxygen is to keep up the supply of copper phosphate. The fact that copper in the presence of water and acids—even very feeble acids like carbonic—readily absorbs oxygen from the air, and forms salts, is well known (vide Mendeléeeff's Principles, ii, 425). The formation of metallic copper round the phosphorus in this case is seen to be really more complex than in the copper-sulphate case, and Straub is not justified in explaining the one by the other in the manner he does.

The black phosphide mentioned above was separated from the rest of the system, and found to consist of very fine particles, which pass through all ordinary filter papers and even through special baryta papers.

If left in the liquid and exposed to the air, the phosphide is slowly oxidized to the white phosphate; readily on boiling. It decolorizes acidified permanganate, being itself oxidized to phosphate and dissolved. It is slowly soluble in dilute mineral acids, most readily in nitric. With stronger acids it is dissolved readily with evolution of a gas. The phosphide does not react with copper sulphate alone.

Phosphorus on other Metallic Salts.

With copper nitrate reduction takes place with the formation of phosphide and copper, as in the case of the sulphate. On cupric chloride the action is somewhat different, the reduction being chiefly to white cuprous chloride. After several days a very little dark-red copper may appear here and there on the phosphorus. The reason a metallic film is not formed is that if metal is produced it is immediately acted on by the excess of cupric chloride with the formation of the cuprous salt. That the phosphide formed is not merely a secondary product due to combination of the reduced copper and phosphorus is shown by the fact that the black film appears long before any metallic copper, and also before the cuprous chloride.

In the action of phosphorus on copper acetate, besides the phosphide and metallic copper, a flocculent white precipitate appears round the phosphorus. This is copper orthophosphate (a mixture of the normal and the acid salts), which after a time decreases in amount, and ultimately disappears, its copper being deposited as metal.

Phosphorus readily reduces acid copper phosphate to phosphide and copper. Straub's explanation would require modification in this case, for we would have copper phosphate oxidizing copper phosphide to phosphate and itself being reduced to copper.

Phosphorus acts on silver salts with the formation of silver phosphide and metallic silver. The reduction proceeds much more quickly than with