Osmium Tetroxide, OsO₄

Osmium Tetroxide, OsO₄, frequently but incorrectly known as osmic acid, is the highest oxide of osmium known, and is formed in a variety of ways. Finely divided metallic osmium slowly oxidises in air to the tetroxide, and more rapidly on heating in air or, better, in oxygen. At high temperatures the compact metal yields vapours of the volatile tetroxide, and this affords a useful means of quantitatively separating osmium from its iridium alloy.

The tetroxide is also formed by oxidation of the lower oxides with nitric acid. The metal itself, if it has not been previously ignited, may also be oxidised with nitric acid to the tetroxide. When purified by sublimation, osmium tetroxide is obtained as transparent needles, which soften at the temperature of the hand and melt at 45° C. They slowly dissolve in water to a colourless solution, which is used in microscopic work for staining purposes, the oxide being reduced to the metal. The solution possesses a burning taste, but does not redden litmus. It is, however, coloured yellow, brown, green, and, finally, indigo blue by sulphur dioxide. The crystals also dissolve in alcohol and in ether.

The tetroxide readily sublimes on heating. When fused it boils at 100° C., yielding a vapour of density 8.89 (air = 1) or 128 (H = 1), the theoretical requirement for the formula OsO₄ being 127.5 (H = 1). The vapour is very penetrating and exceedingly poisonous, producing temporary blindness and other alarming symptoms. If inhaled, the best antidote appears to be hydrogen sulphide, which neutralises the action of the tetroxide on the respiratory organs.

The vapour pressures of osmium tetroxide at various temperatures are as follow:

Temperature ° C	95	115	125	135
Pressure, mm.	275	182	640.4	779

Mild reducing agents such as alcohol convert the tetroxide into lower oxides of osmium. When thrown on to red-hot charcoal it deflagrates. Its vapour, when passed over glowing copper turnings, is reduced, metallic osmium being deposited on the copper. If a current of hydrogen gas is simultaneously passed through, the copper oxide is reduced as rapidly as formed, and the alteration in weight of the copper is due entirely to the deposit of metallic osmium. This affords a convenient method of quantitatively estimating osmium with very considerable accuracy.

Osmium tetroxide catalytically assists the oxidation of certain oxidisable substances. For example, a mixture of 15 grams of arsenic with 10 grams of potassium chlorate in 50 c.c. of water remains unaltered even after addition of a few drops of dilute sulphuric acid. Upon introducing a trace of osmium tetroxide (c. 0.015 gram) in solution, the temperature immediately rises, the arsenic being rapidly oxidised to arsenic acid. Hydrazine sulphate may be oxidised in a similar manner, the reaction proceeding quantitatively and admitting of exact measurement by collection of the liberated nitrogen. This activity is attributed to the formation of an additive compound between osmium tetroxide and the chlorate, the complex thus formed acting as the oxidiser. That it is caused by the formation of a higher oxide of osmium seems unlikely, since osmium tetroxide does not take up oxygen from solutions of chlorates. Furthermore, a purely catalytic decomposition of the chlorate seems improbable, since osmium tetroxide does not. appreciably influence the rate of decomposition of either the solid chlorate or its solutions.

On the other hand, osmium tetroxide may be used to catalytically assist the reduction or hydrogenation of unsaturated oils like linseed or cotton-seed oils. The oil is mixed with a small quantity of the tetroxide, hydrogen passed through, and the whole warmed, just as when nickel or its oxides is used as catalyst. The osmium can be recovered, by treatment with charcoal, in the form of its dioxide, a colloidal solution of which is frequently formed in the oil by the reduction of the tetroxide during the process.

Aqueous solutions of osmium tetroxide are readily reduced by the introduction of practically any metal except those known as the precious metals. Thus zinc, silver, mercury, etc., effect the precipitation of metallic osmium from acidulated solutions in a very pure form. In the last-named case an amalgam is produced from which the osmium is obtained by distilling off the mercury. Ferrous sulphate and stannous chloride also reduce the tetroxide solutions, but hydrogen, sulphur and selenium appear to have no action under ordinary conditions. Sulphur dioxide reduces the solution to osmium sulphite, whilst potassium iodide reduces it to dioxide with liberation of iodine - a reaction that may be utilised in the volumetric determination of osmium.

The acidic properties of osmium tetroxide are illustrated by the formation of definite compounds with the alkali hydroxides. These are crystalline, orange or brown in colour, and readily soluble in water, yielding strongly hydrolysed solutions. Of these the following derivatives have been obtained: OsO₄.2KOH, OsO₄.RbOH, OsO₄.CsOH, and 2OsO₄.CsOH.

Osmium tetroxide does not liberate iodine from neutral solutions of potassium iodide.