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Ruthenochlorides, M2RuCl5

Chlor-ruthenites or Ruthenochlorides, M2RuCl5 are Double salts of ruthenium trichloride and the chlorides of the alkali metals have been prepared and are known as chlor-ruthenites or ruthenochlorides, their general formula being RuCl3.2MCl or M2RuCl5. The first of these to be discovered was:

Potassium Chlor-ruthenite, K2RuCl5

This salt may be prepared by the reduction of ruthenium nitrosotrihydroxide, Ru(NO)(OH)3, in alkaline solution by boiling with formaldehyde, dissolving in hydrochloric acid, and separating out the salt by addition of potassium chloride. Obtained in this way the crystals are brown in colour.

The salt may be prepared in an impure form by dissolving ruthenium in fused potassium hydroxide, adding small quantities of potassium nitrate the while, until all the ruthenium has passed into solution. On cooling the green mass becomes orange, and treatment with concentrated hydrochloric leaves a residue of potassium chlor-ruthenite.

A convenient method of preparing a fairly pure specimen of potassium chlor-ruthenite consists in adding freshly distilled ruthenium tetroxide to concentrated hydrochloric acid and digesting on the water- bath until evolution of chlorine ceases. This requires about two days. To the resulting strongly acid solution of ruthenium trichloride, potassium chloride is added in small quantities at a time, whereby a precipitate of crystals of potassium chlor-ruthenite is obtained. These are washed free from acid with alcohol, and dried by exposure over concentrated sulphuric acid.

Potassium chlor-ruthenite rapidly hydrolyses in aqueous solution, the liquid, originally red, gradually becoming black. The velocity of the hydrolysis admits of determination by electric conductivity measurements in consequence of the hydrochloric acid set free.

The equilibrium of the hydrolysed solution is not altered by dilution, by addition of acid, or by change of temperature. It thus appears that the reaction is irreversible, and that the final state does not represent a true equilibrium.

The reaction appears to take place according to the equation : K2RuCl5 + 2H2O = 2KCl + 2HCl + Ru(OH)2Cl.

Addition of alkali hydroxide to the hydrolysed solution yields an imediate precipitate of hydrated ruthenium sesquioxide, Ru2O3.3H2O.

Caesium Chlor-ruthenite, Cs2RuCl5

H2O.Of particular interest is Caesium Chlor-ruthenite, Cs2RuCl5.H2O, which Howe obtained by the action of hydrochloric acid on ruthenium tetroxide and subsequent addition of caesium chloride to the solution. The salt is precipitated as a dark brown powder, fairly soluble in water and hydrochloric acid, exhibiting the chemical reactions of a trivalent ruthenium salt. Howe also describes an isomeride of this salt, which he termed, in accordance with Werner's nomenclature, Caesium Aquo-chlor-ruthenate (vide infra).

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