Platinum Dioxide, PtO₂

Hydrated Platinum Dioxide, PtO₂.xH₂O, is obtained1 by boiling platinum tetrachloride with an excess of twice normal sodium hydroxide solution, and then neutralising with acetic acid. The precipitate obtained when the foregoing reactions are carried out in dilute solution, and when the acetic acid is added in the cold, consists of a white deposit of tetrahydrate, PtO₂.4H₂O. This hydrate is acidic in character, and is probably more correctly represented by the formula H₂Pt(OH)₆, and is described in further detail under the name of hexahydroxy-platinic acid. It readily loses water, becoming yellow on drying in consequence of partial dehydration. When boiled with water the precipitated tetrahydrate becomes ochre-coloured, being transformed into the trihydrate, PtO₂.3H₂O. Upon prolonged exposure to sulphuric acid in a desiccator further dehydration ensues, the yellowish brown dihydrate, PtO₂.2H₂O, being produced. When maintained for some time at 100° C. the mass becomes black and resembles coal in appearance, but yields upon pulverisation a dark brown powder. This is the monohydrate, PtO₂.Tl₂O, which clings tenaciously to its combined water. It is soluble with difficulty in hydrochloric acid, although, when a portion of the oxide has passed into solution, the remainder dissolves more rapidly on gentle shaking.

On saturating a solution of chlor-platinic acid with hexahydroxy-platinic acid and dialysing, a product is formed which coagulates on warming, and has a composition represented by the formula (PtO₂)₅.2HCl.9H₂O. Boiling water converts it into metaplatinic acid, (PtO₂)₅.5H₂O.

When aqueous solutions of organic acids are boiled with the dioxide, reduction takes place, metallic platinum being deposited, and the acids oxidised. Hydrogen peroxide solution slowly reduces the dioxide.

Platinum dioxide, upon ignition, decomposes, evolving oxygen, a residue of metallic platinum being obtained together with a solid solution of either the monoxide or the sesquioxide in the dioxide.