Tetrachlor-platinites, M₂PtCl₄

Salts of tetrachlor-platinous acid are known as tetrachlor-platinites, and have the general formula M₂PtCl₄.

They are both stable and well known. They may be prepared in a variety of ways, chief amongst which are:

1. Addition of chlor-platinous acid or a solution of platinous chloride to a metallic carbonate or chloride. For this purpose the chlor-platinous acid is best made by dissolving platinous chloride in hydrochloric acid. This ensures the freedom of the final product from the metallic chlor-platinate. A less troublesome method is to reduce chlor-platinic acid with sulphur dioxide if a pure product is not required, but this is not easy to carry out satisfactorily otherwise. If, for example, reduction of the chlor-platinic acid is not complete, the metallic platinite will be contaminated with platinate. If, on the other hand, excess of sulphur dioxide is present, the metallic chlor-platinite becomes contaminated with sulphite derivatives.
2. Probably the most convenient method of preparing metallic chlor-platinites consists in reducing the corresponding chlor-platinates. This may be effected with potassium oxalate in concentrated aqueous solution at the boiling-point, and containing a trace of iridium, the reaction proceeding according to the equation:
   
   M₂PtCl₆ + K₂C₂O₄ = M₂PtCl₄ + 2KCl + 2CO₂.
   
   Hydrogen sulphide, cuprous chloride, and the last named along with powdered zinc, have been used as reducing agents, but are open to several objections.
3. Double decomposition of metallic sulphates and barium chlor- platinite. Thus:
   
   M₂SO₄ + BaPtCl₄ = BaSO₄ + M₂PtCl₄.

The clear solution containing the soluble chlor-platinite is decanted or filtered free from the insoluble barium sulphate, and concentrated.

The chlor-platinites are mostly soluble salts yielding red solutions readily distinguished from solutions of the chlor-platinates, which are yellow. The silver, lead, mercury, and thallium salts are almost insoluble in water.

Chlor-platinites are oxidised by chlorine and by hypochlorites to chlor-platinates, whilst nascent hydrogen reduces them in aqueous solution with separation of metallic platinum.

When their solutions are warmed and a current of hydrogen sulphide is passed through, platinous sulphide is obtained as a black precipitate; whilst sulphur dioxide under like conditions effects their decolorisation, platinosulphites being produced. With ammonia, ammino derivatives are obtained.