Alloys of Rhodium

Rhodium does not alloy with silver. When added to molten silver it floats on the surface and is recovered, on cooling, in the amorphous condition.

It alloys with gold, an alloy, which probably contains a real compound, being obtained with 1 per cent, of rhodium, and which is entirely soluble in aqua regia. A 10 per cent, mixture yields free rhodium, on cooling, in the form of feathery crystals, whilst if still more rhodium is present, it separates as the amorphous metal.

Added to molten bismuth, a crystalline tetrabismuthide is formed, of composition corresponding to the formula RhBi₄.

With tin, the compound RhSn₃ is produced, which may be isolated from the melt by treating with diluted hydrochloric acid.

Rhodium dissolves in molten zinc at dull redness, and on treating the product with concentrated hydrochloric acid, the excess of zinc dissolves, leaving a crystalline powder of composition corresponding to the formula RhZn₂. The formation of this insoluble alloy affords a convenient method of purifying rhodium.

Rhodium readily alloys with platinum, stiffening it and yielding mixtures that are useful for a variety of laboratory purposes. Rhodium reduces the loss in weight of platinum by volatilisation at all temperatures above 900° C., and it has therefore been suggested that a useful alloy for best quality crucibles "would be platinum containing 3 to 5 per cent, of rhodium, practically free from iron and iridium, and containing no other detectable impurities."

Alloys containing less than 5 per cent, of rhodium are soluble in aqua regia. An alloy containing 30 per cent, of rhodium is insoluble in aqua regia, and is more readily fused than rhodium itself.

No compounds of platinum and rhodium appear to exist between the limits of 0 and 55 per cent, of rhodium.

Triple alloys, containing rhodium, iridium, and platinum are found in nature.