Osmium Dioxide, OsO₂

Osmium Dioxide, OsO₂, results when salts of tetravalent osmium are heated with sodium carbonate in a current of carbon dioxide, and by heating finely divided osmium in the vapour of osmium tetroxide.

Osmium dioxide may also be obtained in the anhydrous condition by ignition of potassium osmiamate, OsNO₃K, in vacuo at 350° C., or by heating the hydrate, OsO₂.2H₂O, to a little below 200° C. in an inert gas such as nitrogen or carbon dioxide. Obtained in this manner its density is 7.71. Further heating to 400° C. changes the colour progressively from bluish black, the colour of the dihydrate, to brown, and slightly increases the density. At about 460° C. the dioxide begins to decompose into osmium and its tetroxide according to the equation:

2OsO₂ = Os + OsO₄,

but by heating the dioxide in the vapour of tetroxide this decomposition is retarded until a temperature of about 650° C. is arrived at. The dioxide then has a density of 7.91, and consists of copper-coloured crystals of octahedral and hexahedral forms. They are insoluble in acids, but are readily reduced by hydrogen.

The oxide prepared by reduction of alkali osmates with alcohol or by hydrolysis of ammonium chlorosmate is not pure, but contains small quantities of organic substances or alkalies. When dry it is liable to ignite upon exposure to air, and even to detonate if warmed to 300° C. directly it meets the air.

Hydrated Osmium Dioxide exists in two stages of hydration. The dihydrate, OsO₂.2H₂O, is obtained by the reduction of alkali osmates with alcohol:

K₂OsO₄ + 2H₂O + C₂H₅OH = OsO₂.2H₂O + 2KOH + CH₃.CHO.

It also results upon hydrolysing an alkali chlorosmate, thus: K₂OsCl₆ + 4H₂O = OsO₂.2H₂O + 2KCl + 4HCl.

The oxide is thrown out in colloidal form, but addition of electrolytes to its neutral solution effects its precipitation. The method may be varied by heating potassium chlorosmate with pure sodium hydroxide in solution in the absence of air, on the water-bath.

When dried over sulphuric acid it has the composition OsO₂.2H₂O. It gradually oxidises in air, yielding the tetroxide. When heated to 120° С. in an inert gas it loses a molecule of water, being converted into the monohydrate, OsO₂.H₂O.

It has already been mentioned that osmium dioxide, obtained either by reduction of an alkali osmate with alcohol or by hydrolysis of an alkali chlorosmate, is precipitated in the colloidal form.

A more permanent colloidal solution is obtained in the presence of a protective colloid. It may be prepared, for example, by impregnation of lanolin with a solution of potassium osmate, and subsequent reduction with hydrazine hydrate. The lanolin serves as the protective colloid. The product is dissolved in light petroleum and may be precipitated by addition of alcohol. By heating the precipitate to 50° C. in hydrogen, reduction to colloidal metallic osmium takes place.