Hexahydroxy-platinic Acid, H₂Pt(OH)₆

Hexahydroxy-platinic Acid, H₂Pt(OH)₆, or PtO₂.4H₂O, is obtained as a white precipitate on neutralising with acetic acid in the cold a well-diluted solution of platinum tetrachloride which has been boiled with excess of caustic soda.

When freshly prepared it is readily soluble in dilute acids or alkalies. On drying it undergoes slight dehydration, becoming yellowish in colour.

On dissolving in dilute caustic alkalies it yields salts of the type M₂Pt(OH)₆. The sodium and potassium salts of this acid have been prepared, and that these salts correspond to the foregoing formula and are not simply hydrates of the type M₂PtOO₃.3H₂O, is established, not only by their being stable at 100° C., but by their precipitating from aqueous silver acetate solution the salt Ag₂Pt(OH)₆, which is also stable at 100° C. In a similar manner thallium hexahydroxy-platinate, Tl₂Pt(OH)₆, may be thrown out of solution.

Still further evidence in favour of the correctness of the foregoing formula is afforded by the interesting isomorphism existing between potassium platinate and the stannate and plumbate of the same metal. This is evident from the crystallographic measurements detailed below. The salts crystallise according to the rhombohedral system.

K₂Pt(OH)₆ a: с = 1: 1.9952 α = 69° 11' 4"
K₂Pb(OH)₆ a:c = 1: 1.9514 α= 70° 10' 26"
K₂Sn(OH)₆ а: с = 1: 1.9588 α = 70° 0' 44"

Electric conductivity measurements indicate that potassium platinate undergoes normal dissociation in dilute aqueous solution like other salts of dibasic acids. It is not hydrolysed by water at 25° C.

The sodium salt, Na₂Pt(OH)₆, after standing for several days becomes gelatinous owing to the separation of Na₂O.(PtO₂)₃.6H₂O, which, upon prolonged dialysis, yields Na₂O.(PtO₂)₃.9H₂O.

When mixed with excess of dilute hydrochloric acid, hexahydroxy-platinic acid dissolves, yielding dichlor-tetrahydroxy-platinic acid, H₂PtCl₂(OH)₄.