Phosphorus Thiophosphates

The general methods of preparation recall those used in the preparation of phosphates, sulphides being used in place of oxides, with protection of the product from oxidation or hydrolysis:—

1. The heating of metallic sulphides with phosphorus penta-sulphide gives the most highly thionised thiophosphates. The poly-sulphides which are formed at the same time may be removed by alcohol. From the solutions dithiophosphates may be isolated.
   
   3M₂S + P₂S₅ = 2M₃PS₄
2. The heating of metallic chlorides with phosphorus penta-sulphides gives thiophosphates together with PSCl₃.
3. By the action of alkalies on thiohalides. PSCl₃ is the chloride of monothiophosphoric acid, and with alkalies it gives monothio-phosphates.

By fusing crystalline sodium sulphide with phosphorus penta- sulphide and dissolving the product in a little water, crystals of tri-sodium tetrathio-orthophosphate were obtained.

3(Na₂S.9H₂O) + P₂S₅ =2(Na₃PS₄.8H₂O) + 11H₂O

The crystals were needle-shaped or tabular, in the monoclinic system. The corresponding potassium salt, K₃PS₄, was obtained as a yellow crystalline mass by fusing KCl with P₂S₅.

These thiophosphates are hydrolysed in dilute solution with evolution of H₂S. They can be dissolved unchanged in alkalies. With salts of the heavy metals they give precipitates which are decomposed on warming, giving H₂S. They are easily oxidised by nitric acid, potassium dichromate, etc. with deposition of sulphur. When treated with solutions of sulphides, e.g. BaS, the trithiophosphates were produced, as represented by the equation

2Na₃PS₄ + 3BaS + 2H₂O = Ba₃(PS₃O)₂ + 3Na₂S + 2H₂S

Dithiophosphates, M₃(PS₂O₂), were also produced by this reaction.

Other tetrathiophosphates which have been prepared by the foregoing methods are Cu₃PS₄ (CuCl and P₂S₅), Ag₃PS₄ (AgCl and P₂S₅), Hg₃(PS₄)₂ (HgS and P₂S₅), Pb₃(PS₄)₂ (PbCl₂ and P₂S₅), Fe₃(PS₄)₂ (FeS and P₂S₅), Ni₃(PS₄)₂ (NiCl₂ and P₂S₅).

The preparation of barium trithiophosphate has been described above. The magnesium salt, Mg₃(PS₃O)₂.20H₂O, was prepared by the action of the tetrathiosodium salt on magnesium hydrosulphide. The ammonium salt, (NH₄)₃(PS₃O).H₂O, was prepared by the action of water on ammonium imidothiophosphate. From a solution of this compound by interaction with salts of various metals, e.g. CuSO₄, their trithiophosphates have been prepared.

Trisodium dithiophosphate, Na₃(PS₂O₂).11H₂O, has been prepared from P₂S₅ and a rather concentrated solution of sodium hydroxide. The solution was heated to 50° to 55° C. until the trithio-salt was decomposed. The salt was precipitated by alcohol, and when recrystallised from water appeared as colourless six-sided prisms. The ammonium salt, (NH₄)₃(PS₂O₂).2H₂O, was prepared similarly from aqueous ammonia and P₂S₅. From these soluble thiophosphates those of the heavy metals may be obtained by double decomposition.

Trisodium monothiophosphate was obtained from the solution of mixed thiophosphates by heating to 90° C. in order to decompose dithiophosphate. On cooling, the crystalline salt Na₃(PSO₃).12H₂O separated in six-sided tables, melting at 60° C. This salt was also prepared from PSCl₃ as follows:—

PSCl₃ + 6NaOH = Na₃(PSO₃) + 3NaCl + 3H₂O.

Ammonium dihydromonothiophosphate was prepared by the hydrolysis of imidotrithiophosphoric acid, thus

H₃[P(NH)S₃] + 3H₂O = (NH₄)H₂(PSO₃) +2H₂S

The salt was repeatedly precipitated by alcohol and dissolved in water. A similar or identical salt having the constitution SP(OH)₂(ONH₄) was prepared by the action of phosphorus pentasulphide on acetoxime in carbon disulphide solution. The part insoluble in CS₂ was extracted with alcohol, boiled and crystallised from cold water in monoclinic prisms.

From the alkali monothiophosphates those of other bases have been obtained by precipitation.

A series of pyrothiophosphates, M₂P₂S₇ (in which M is a divalent metal), has been prepared by the methods already described. The free acids decompose immediately on liberation, but a mixture of them has been prepared as a yellow oil having the composition H₄P₂O₂S₅ by the action of liquid hydrogen chloride on ammonium trithiophosphate at low temperatures.

Esters of the thiophosphoric acids have been prepared, e.g. ethyl tetrathiophosphate, (C₂H₅)₃PS₄.

Many of the reactions already described, such as the production of H₂S and phosphoric acid when these salts are treated with acids, will serve to detect the thiophosphates. The alkali and ammonium salts are soluble, the others mostly insoluble. Calcium, barium and strontium monothiophosphates, barium and strontium dithiophosphates and barium trithiophosphate are insoluble, the other alkaline earth salts soluble. Thiophosphate solutions mixed with alkali sulphides give a green colour with ferric chloride. Monothiophosphates give a blue precipitate with cobalt sulphate soluble in excess of the cobalt salt, dithiophosphates a green precipitate soluble in excess of dithiophosphate, and trithiophosphates a brown solution. Several other tests have been described.