Platinum Disulphate, Pt(SO₄)₂

Platinum sponge dissolves in concentrated sulphuric acid at about 350° C., and the solution assumes a yellow colour in consequence of the solution of platinum disulphate, Pt(SO₄)₂.

The tetrahydrate, Pt(SO₄)₂.4H₂O, is deposited from solution when an alternating current is passed through platinum electrodes immersed in sulphuric acid of density 1.840. On repeated recrystallisation from water in vacuo, the salt is obtained in the form of large, orange leaflets. These are very stable so long as they contain a trace of free sulphuric acid; but when quite pure they lose water upon prolonged exposure in a desiccator, yielding the anhydrous salt in the form of pretty crystals, exhibiting a green metallic reflex.

Both salts are soluble in water, the hydrated one yielding a yellow solution which deposits a brown basic salt on warming.

Hydrated platinum dioxide dissolves slowly in 50 per cent, sulphuric acid, and from the solution a basic sulphate, PtO₂.SO₃.4H₂O or Pt(OH)₄.H₂SO₄.H₂O, separates out in orange- yellow microscopic needles on addition of excess of acid in the cold. The salt is hydrolysed by water. Heated to 100° C. three molecules of water are lost, the residue having the composition PtO₂.H₂SO₄ or H₂PtO₂.SO₄.

When dissolved in sulphuric acid, platinic sulphate is reduced by oxalic acid, yielding a complex sulphate of trivalent platinum, namely, Pt₂(OH)₆.(SO₃)₄.(OH)₂.8.5H₂O. This yields well-defined triclinic prisms which gradually lose water when dried over sulphuric acid under reduced pressure, yielding a stable complex, Pt₂O₃.(SO₃)₃.H₂SO₄.4H₂O. This acid is dibasic, and yields crystalline potassium, sodium, and barium salts.