Estimation of Osmium

Osmium may be estimated with considerable accuracy by decomposing an alkali osmate with a slight excess of dilute sulphuric acid, in the presence of a little alcohol to prevent reoxidation. After ten to twelve hours a bluish black deposit containing the whole of the osmium is obtained. This is filtered and reduced in hydrogen. Fremy's method of precipitating osmium as sulphide does not yield accurate results.

When present in various compounds or alloys, osmium is conveniently estimated by ignition in a current of oxygen, collection in alcoholic alkali of the volatile tetroxide, and reduction with formaldehyde. The reduction product first forms a colloid, but is gradually precipitated and converted in metallic osmium by reduction in hydrogen at 230° to 250° C. Or the alcoholic solution may be treated with ammonium chloride and the precipitated osmyl diammine chloride reduced to metal in hydrogen.

Volumetric methods have been suggested, notably that of Klobbie, which consists in treating a solution of osmium tetroxide in dilute sulphuric acid with potassium iodide and titrating the liberated iodine with sodium thiosulphate. One molecule of osmium tetroxide liberates four atoms of iodine. The solution becomes dark green, but no further reduction takes place even after several days.

Separation of Osmium from ruthenium, rhodium, iridium and platinum may be effected by addition of zinc or magnesium to the solution containing these metals, whereby they are all precipitated as a black deposit.

Addition of hydrogen peroxide effects the solution of the osmium, the other metals remaining unaffected. The osmium may then be estimated by any of the foregoing methods.

The metals are precipitated together from solution as sulphides by passage of hydrogen sulphide through the acidified solution. The precipitate is dried and ignited in a platinum-boat in a current of oxygen; the ruthenium remains behind as the dioxide, RuO₂, in which condition it is weighed, or, if desired, it may be reduced to metal by ignition in hydrogen, cooled in carbon dioxide, and weighed as metal. The osmium volatilises as tetroxide, OsO₄, and is collected in 12 per cent, sodium hydroxide solution admixed with 2 per cent, of alcohol.

Addition of aluminium-strip effects the precipitation of metallic osmium, which is collected in an asbestos-packed tube and dried at dull red heat in hydrogen, cooled in carbon dioxide, and weighed.