Ruthenium Tetroxide, RuO₄

Ruthenium Tetroxide, also known as Ruthenium Peroxide, RuO₄, results in small quantity when ruthenium is heated to about 1000° C. in a current of oxygen. A more convenient method of preparing it is that first adopted by Claus, and consists in passing chlorine through a solution of alkali ruthenate and warming to 80° to 90° C. The tetroxide distils over and, on cooling, yields a golden yellow mass. It may be purified by repeated shaking with warm water to remove all traces of chlorine, separating as completely as possible from water, and finally subliming several times in vacuo, when it yields beautiful crystals melting at 25.5° C. to an orange liquid, and decomposing at 106° to 107° C., yielding the crystalline dioxide. Its vapour density at 100° C. as determined by Dumas' method corresponds to a molecular weight of 166.7. Theoretically the formula RuO₄ requires the value 165.7. The vapour vigorously attacks cork and rubber.

Ruthenium tetroxide dissolves to a slight extent in water. It is also soluble in caustic alkali, from which solutions a black precipitate of finely divided ruthenium is obtained on addition of alcohol. Both the aqueous solution and the pure substance itself possess an odour resembling that of ozone. Its vapour, however, is not poisonous like that of the corresponding tetroxide of osmium. In contact with alcohol the solid tetroxide is reduced with explosive violence. When covered with water, to which a concentrated solution of caesium chloride is subsequently added and a little hydrochloric acid, ruthenium tetroxide is gradually converted into the oxy-salt, Cs₂RuO₂Cl₄. The corresponding rubidium salt has likewise been prepared.

Ruthenium tetroxide is permanent when kept in sealed tubes in the dry state and protected from light. Exposed to light it assumes a brown colour, but. the brown product is soluble in alkali, yielding a ruthenate. Presumably the coloration is due to partial reduction. Owing to its ready reduction by organic substances whereby a black precipitate of finely divided ruthenium is obtained, potassium per-ruthenate has been found useful for histological microscopy.

Several other oxides have been described, namely, Ru₂O₅, Ru₂O₅.2H₂O, Ru₄O₉, RuO₃, and Ru₂O₇. Of these, the first three are probably indefinite mixtures rather than separate chemical entities. The last two oxides do not appear capable of a separate existence although their compounds are well known. These are termed: Ruthenates, M₂RuO₄, and Per-ruthenates, MRuO₄

Potassium Ruthenate, K₂RuO₄.H₂O, may be obtained by fusing ruthenium with a mixture of potassium hydroxide and nitrate. A green mass of potassium ruthenate is formed, which dissolves in water to an orange-coloured solution. It is also formed when 50 grams of ruthenium tetroxide are dissolved in 500 c.c. of water containing 70 grams of potassium hydroxide at 60° C. Crystals obtained by evaporation from its aqueous solution are rhombic in character, and possessed of a greenish metallic lustre. By transmitted light the crystals appear red. They absorb moisture from the air and darken owing to partial reduction. Heated to 200° C. they become anhydrous, but undergo no further change even at 440° C.

In solution potassium ruthenate is of a deep orange colour, but in dilute solution assumes a green colour, due to formation of per- ruthenate. Hydrochloric acid effects the precipitation of an oxide, chlorine being simultaneously evolved. The solution stains the skin black owing to separation of oxide.

The corresponding sodium derivative is readily prepared by fusing together ruthenium and sodium peroxide. The silver, barium, strontium, calcium, and magnesium salts are also known.

Potassium Peivruthenate, KRuO₄.H₂O results by the action of chlorine on an aqueous solution of the normal ruthenate, as also by the solution of 50 grams of ruthenium tetroxide, previously fused under water, in 250 c.c. of water at 60° C. containing 60 grams of potassium hydroxide. On crystallisation black tetragonal octahedra result, which, on heating in vacuo to 400° C. decompose to oxygen, ruthenium dioxide, and potassium ruthenate. The crystals are not isomorphous with those of potassium permanganate as was at one time believed to be the case.

Sodium Per-ruthenate, NaRuO₄.H₂O, is prepared in an analogous manner to the potassium salt as small, black, crystalline lamellae. It is considerably more soluble in water than the potassium analogue, and yields a dark green solution. Heated to 440° C. water and oxygen are evolved.