Rhodium


Rhodium was discovered, in 1804, by W. H. Wollaston, working in London at the time. Wollaston made a partnership with Smithson Tennant, in 1800, with the goal of developing and improving the technology of platinum refining. To one of the native platinum shipments from South America was applied an elaborated treatment. Mercury cyanide was added to the aqua regia platinum solution, the precipitate was removed and the resulting solution was evaporated. The residues were washed with alcohol, and the result was a dark-red material that was proved to be a sodium compound and a salt of a new metal. That salt would today be known as sodium chlorodite (Na3RhCl6.18H2O). This salt was heated up with hydrogen and was washed with water to remove the sodium chloride. The resulting residue was rhodium powder.

Wollaston chose the name rhodium (from the Greek rhodon, that means rose) due to the color of the chloride RhCl3 and of the respective aqueous solutions.

In 1885, Le Châtelier developed the first rhodium-platinum thermocouple, following a suggestion of Becquerel referring to the possibility of using the rhodium thermo-electric effect to measure temperature.