Brom platinous Acid, H₂PtBr₄

Brom platinous Acid, H₂PtBr₄, has not been isolated in the pure state, but exists in solution when platinous bromide is dissolved in aqueous hydrogen bromide. It combines with bases to form:

Of these the most important salt is:

Potassium Tetrabrom-platinite, K₂PtBr₄

Potassium Tetrabrom-platinite, K₂PtBr₄.2H₂O, which may be obtained by reduction of the hexabrom-platinate, K₂PtBr₆, with potassium oxalate solution at 100° C., or by addition of sodium bromide to potassium tetrachlor-platinite.

On concentration, the brom-platinite crystallises out in large, black, rhombic crystals:

a: b: с = 0.60582: 1: 0.70499.

The crystals are readily soluble in water. Their heat of formation is as follows:

[Pt] + Br₂ + 2KBr.Aq. = K₂PtBr₄.Aq. + 21.83 Cals.

The heat of solution of the crystals is:

[K₂PtBr₄] + Aq. = K₂PtBr₄.Aq. - 10.6 Cals.

Platinum Tetrabromide, PtBr₄

Platinic Bromide, Platinum Tetrabromide, PtBr₄ is obtained by maintaining brom-platinic acid at 180° C. in air. The product is not pure, for even at this temperature it undergoes partial dissociation into the dibromide:

PtBr₄ ⇔ PtBr₂ + Br₂.

As obtained in this way, platinic bromide is a dark powder, slightly soluble in water to a reddish brown solution which contains dihydroxy-tetrabrom-platinic acid, H₂PtBr₄(OH)₂. It is fairly soluble in a mixture of alcohol and water, and very soluble both in alcohol and in ether. On warming, however, these latter solutions become reduced, yielding a deposit of platinum black. Its solution in aqueous hydrogen bromide contains brom-platinic acid.

The salt has not as yet been obtained in a perfectly pure condition. Its heat of formation is given by Pigeon:

[Pt] + 2Br₂ = [PtBr₄] + 42.4 Cals.

[Pt] + 2(Br₂) = [PtBr₄] + 56.8 Cals.