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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:

Brom-platinites, M2PtBr4

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:

PtBr4PtBr2 + 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.

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