Brom-platinic Acid, H₂PtBr₆

Brom-platinic Acid, H₂PtBr₆, was first obtained by Balard in a more or less pure condition by acting on platinum with a mixture of nitric and hydrobromic acids.

A more satisfactory method consists in dissolving platinum sponge in hydrobromic acid saturated with bromine. This may be effected in a sealed glass tube at 180° C., but that is not necessary. If the platinum sponge is simply boiled for a short time with an excess of bromine and hydrobromic acid in a flask with a reflux condenser, the whole is dissolved, yielding a solution of brom-platinic acid.

Gutbier prepared pure brom-platinic acid by repeatedly evaporating the pure chloride with concentrated hydrobromic acid on the water-bath and then treating it similarly three or four times with hydrobromic acid containing bromine. The residue, on being taken up with dilute hydrobromic acid, yields a deep carmine-red solution.

Brom-platinic acid crystallises in red, monoclinic prisms, containing 9 molecules of water, thus: H₂PtBr₆.9H₂O. The crystals are deliquescent and readily soluble in water, alcohol, and ether. When heated in air to 180° C. they melt, water and hydrogen bromide being evolved, platinic bromide constituting the residue.

The heat of formation is as follows:

[Pt] + 2Br₂ + 2HBr. Aq. = H₂PtBr₆.Aq. + 60.7 Cals.

Brom-platinic acid possesses a strongly acid character, decomposing carbonates and uniting with bases generally, to form salts known as:

These salts are isomorphous with the chlor-platinates already described, and which they resemble in their general chemical reactions. The alkali salts are best known, and are readily prepared by addition of a solution of the alkali bromide to one of brom-platinic acid, and crystallising the precipitated complex from dilute hydrobromic acid.

The solubilities of the alkali brom-platinates in water and dilute hydrobromic acid decrease with increasing atomic weight of the alkali metal. The solutions obtained are red in colour, but turn yellow on dilution with water. Hydrazine hydrate decomposes them with ease, metallic platinum being deposited and nitrogen evolved.

Numerous brom-platinates of organic bases have been prepared and described, but to deal with these is beyond the scope of this work. The more important inorganic brom-platinates are as follow:

Ammonium Brom-platinate, (NH₄)₂PtBr₆

Ammonium Brom-platinate, (NH₄)₂PtBr₆, crystallises in cubic octahedra, which are dark brown in colour and isomorphous with the corresponding potassium salt.

When heated, ammonium brom-platinate decomposes, partly according to the equation

3(NH₄)₂PtBr₆ = 3Pt + 16HBr + 2NH₄Br + 2N₂,

and partly

(NH₄)₂PtBr₆ = Pt + 2NH₄Br + 2Br₂.

By thorough drying of the salt, however, the quantity of bromine liberated is reduced.

The density of the salt is 4.265. It dissolves in water to a deep orange-red solution, which, when saturated at 20° C., contains 0.59 grams of (NH₄)₂PtBr₆ per 100 grams of solution.

Caesium Brom-platinate, Cs₂PtBr₆

Caesium Brom-platinate, Cs₂PtBr₆, crystallises in reddish yellow octahedra.

Potassium Brom-platinate, K₂PtBr₆

Potassium Brom-platinate, K₂PtBr₆, results as dark brown octahedra, from which it is difficult to remove all traces of water. The crystals are not very soluble, a solution saturated at the ordinary temperature containing 2.02 per cent, of its weight of the salt. At 100° C. the salt dissolves in 10 times its weight of water. The density of the crystals is 4.658.

The heat of formation is:

[Pt] + 2(Br₂) + 2KBr.Aq. = K₂PtBr₆.Aq. + 59.3 Cals.

The heat of solution is:

[K₂PtBr₆] + Aq. = K₂PtBr₆. Aq. - 12.2 Cals.

Rubidium Brom-platinate, Rb₂PtBr₆

Rubidium Brom-platinate, Rb₂PtBr₆, yields reddish yellow octahedra. The salt retains traces of moisture most obstinately.

Sodium Brom-platinate, Na₂PtBr₆

Sodium Brom-platinate, Na₂PtBr₆.6H₂O, yields deep red, triclinic prisms, isomorphous with those of sodium chlor-platinate hexahydrate. The salt is readily soluble in water, stable in air, and has a density of 3.323.

Tetrabrom-dihydroxy-platinic Acid, H₂PtBr₄(OH)₂

Platinum tetrabromide is only slightly soluble in water, yielding a reddish brown solution containing the dihydroxy acid. With silver nitrate the acid solution yields a dark brown precipitate of the silver salt, Ag₂PtBr₄(OH)₂.

In an analogous manner salts of lead, PbPtBr₄(OH)₂.Pb(OH)₂; thallium, Tl₂PtBr₄(OH)₂; and mercury, HgPtBr₄(OH)₂, have been prepared, whilst electric conductivity measurements indicate that the sodium salt, Na₂PtBr₄(OH)₂, can exist in solution.