Physical Properties of Iridium

Native iridium has been found crystallised as cubes, and is stated to occur less frequently as rhombohedra. In an examination of a pure preparation belonging to Stas, Printz could find no evidence of dimorphism or of hexagonal (rhombohedral) crystals.

Iridium is a hard, brittle metal, which can be filed and which takes a polish. In appearance it lies between silver and tin; it is not ductile, however, even at red heat. Its specific heat is 0.0323, and its coefficient of linear expansion with rise of temperature (0.80° C.) is 0.000,007. The density of the native metal is 22.6 to 22.8. For the pure cast metal the value 22.42 has been found. It melts at 2290° C. and distils in the electric furnace, its boiling-point being approximately 2550° C. Its vapour, on cooling, is deposited as small crystals. Liquid iridium dissolves carbon, but liberates it, on cooling, in the form of graphite.

Iridium foil, when subjected to prolonged cathodic pulverisation in vacuo, becomes capable of absorbing some 800 times its volume of hydrogen at ordinary temperature. The metal becomes considerably altered in appearance, turning dull grey and brittle. Iridium which has been treated in this way yields an amalgam with mercury which is completely soluble in aqua regia; furthermore the iridium becomes capable of exploding electrolytic gas.

The most intense lines in the spectrum are as follow:

Arc: 2924.94, 2943.30, 3100.50, 3220.91, 3266.59, 3368.64, 3449.13, 3513.82, 3516.11, 3522.21, 3573.89, 3638.84, 3800.25.

Spark: 2833.32, 3513.85, 3573.90, 3606.01, 3731.49, 3800.25, 3895.72, 3976.49, 4020.20, 4070.07, 4399.72.

Iridium Black consists of an indefinite mixture of finely divided iridium and its oxides, and is obtained by reducing iridium compounds. For example, the sesquioxide dissolved in alkaline solutions yields a deposit of iridium black on boiling with alcohol, the latter being oxidised to formic acid and aldehyde.

Iridium black is soluble in aqua regia, and like its platinum analogue possesses considerable catalytic activity. For example, it converts ozone into oxygen, hypochlorites into chlorides, and free oxygen and chlorine water into hydrochloric acid and oxygen.

Colloidal Iridium is readily obtained by reducing an aqueous solution of the chloride with hydrazine hydrate or sodium amalgam, in the presence of a protective colloid such as gum acacia, or sodium protalbat (or lysalbate). Hydrogen, sodium formate, or formaldehyde may also be used, instead of the hydrazine, as reducing agents.

When evaporated to dryness over concentrated sulphuric acid in vacuo the solid hydrosol is obtained. This dissolves in warm water, yielding the colloidal solution again. The solution, when shaken with barium sulphate or animal charcoal, is decomposed, the metal being coagulated and thrown out as a precipitate.

Colloidal iridium may also be prepared by Bredig's method, which consists in sparking between iridium electrodes immersed in ice-cooled water. A current of 20 to 25 amperes at 220 volts gives satisfactory results. The colour of the hydrosol ranges from red to black, according to the method of preparation. A small current favours the formation of the black solution.

Colloidal iridium readily decomposes hydrogen peroxide solution, the reaction being, as with platinum, mono-molecular; the rate of decomposition is approximately proportional to the concentration of catalyst. Alkali does not affect the reaction velocity, but dilute acids accelerate it. Hydrogen sulphide, mercuric chloride, and certain other substances act as poisons. Colloidal iridium is less active, however, than platinum.

Carbon monoxide combines with oxygen, yielding the dioxide at ordinary temperatures when shaken with the hydrosol of iridium.

Explosive Iridium is obtained by dissolving an alloy of the metal with excess of zinc in hydrochloric acid, the zinc passing into solution, leaving metallic iridium in a finely divided condition.

The explosive property, first discovered by accident by Bunsen in 1868, appears to be due to the union of occluded oxygen and hydrogen, since the metal is not explosive if prepared in the entire absence of air.. Furthermore the explosive metal on being kept at 100° to 200° C. for several days ceases to be explosive.