Chemical Properties of Iridium

Iridium is not attacked by fluorine in the cold, but on warming to dull redness, vapours of the fluoride are evolved. Also, when employed as anode in the preparation of fluorine, it is rapidly attacked. Chlorine at red heat unites with iridium to form the trichloride; a mixture of chlorine and carbon monoxide at 240° C. has no action on the metal although platinum is attacked by it.

Finely divided iridium, when heated to redness in air, absorbs a certain amount of oxygen, forming an indefinite mixture of oxides. It is attacked by sulphur vapour at red heat, and under similar conditions phosphorus combines with it to form a fusible phosphide, which decomposes at white heat in a lime furnace, yielding the free metal.

Acids, even aqua regia, have no action on iridium, unless it is very finely divided, in which case it is very slowly attacked.

Fusion with a mixture of potassium hydroxide and nitrate results in the formation of iridates. Fused with potassium hydrogen sulphate it yields the sesquioxides. Alkalis alone have no action when fused with iridium.

Heated for some hours in a Bunsen burning with a smoky flame, an iridium crucible is unharmed, since the deposit of carbon can be entirely burnt away. Sulphur in the gas is likewise without effect. Prolonged ignition with copper renders the metal brittle, but the copper can be burnt off.

When strips of iridium are heated to high temperatures in carbon monoxide or in nitrogen, no loss in weight occurs. In carbon dioxide and in nitrogen containing oxygen appreciable losses in weight occur at 1500° C. upwards. This is attributed to the formation of oxide, which is volatile at these high temperatures.