Platinum Trioxide, PtO₃

Platinum Trioxide, PtO₃, has been obtained by electrolysing a well-cooled solution of hydrated platinum dioxide in 2-Normal potassium hydroxide, using platinum electrodes. The anode becomes covered with an amorphous, golden-coloured film of potassium platinate, having the composition K₂O.PtO₃ or K₂PtO₄. This, by treatment with ice-cold, dilute acetic acid, yields the free trioxide as a reddish brown substance, which gradually loses oxygen, but never becomes the pure dioxide on simply keeping in air, probably because a solid solution of trioxide in the dioxide is formed.

Platinum trioxide is not acted upon by dilute sulphuric, nitric, or acetic acid. Dilute hydrochloric acid dissolves it with evolution of chlorine, whilst sulphurous acid converts it into complex sulphites.

Concentrated hydrochloric acid rapidly dissolves the trioxide with violent chlorine evolution, whilst concentrated sulphuric and nitric acids slowly convert it into the dioxide in the warm.

When gently warmed oxygen is evolved, a residue of platinum dioxide being obtained.

A remarkable feature of the trioxide is the fact that it is not reduced by hydrogen peroxide, and Wohler therefore regards it, not as a peroxide, but as the anhydride of platinic acid, in which the platinum is regarded as hexavalent, thus:



It is suggested that the anodic passivity of platinum is attributable to the formation of a surface film of this oxide.