Rhodium volatilisation

The resistance of rhodium-platinum alloys to corrosion is about the same as or slightly better than that of pure platinum, but they are much more stable at high temperatures. They have excellent resistance to creep above 1 000°C, a factor which largely determines their extensive use in the glass industry, where continuous temperatures sometimes exceeding 1 500°C are encountered. Rhodium additions to platinum reduce appreciably the volatilisation of pure platinum at high temperatures. 

Rhodium-platinum alloys containing up to 40% Rh are used in the form of wire or ribbon in electrical resistance windings for furnaces to operate continuously at temperatures up to 1 750°C. Such windings are usually completely embedded in a layer of high-grade alumina cement or flame-sprayed alumina to prevent volatilisation losses from the metal due to the free circulation of air over its surface. Furnaces of this type are widely employed for steel analysis, ash fusions and other high-temperature analytical procedures. 

A still further purification of the rhodium is effected 3 by fusing the metal obtained in the preceding manner to dull redness with metallic zinc, whereby the compound RhZn2 is formed, with considerable heat evolution, a portion of the zinc being volatilised. On cooling, treatment of the mass with concentrated hydrochloric acid effects the solution of the excess of zinc, the insoluble crystalline alloy remaining unaffected. 

The last named is dissolved in aqua regia, excess of ammonia added, and the solution evaporated to dryness, whereby the monochlor-rhodium pentammine chloride is formed. This is recrystallised to remove traces of iridium, calcined with sulphur in a crucible, and finally heated in the oxyhydrogen flame to volatilise any still remaining osmium. 

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. 2... 

Detection of Ruthenium in Platinum Alloys.—In order to detect the presence of ruthenium in platinum alloys, a portion of the alloy is fused with lead. The melt is extracted with nitric acid and the residue ignited in contact with air in order to volatilise the osmium. The mass may now contain platinum, iridium, rhodium and ruthenium, and is fused with potassium nitrate and hydroxide. The whole is dissolved in water, treated with excess of nitric acid and allowed to stand in a flask covered with filter-paper. In a few hours (12-24) the filter-paper darkens if ruthenium is present, in consequence of the evolution of vapour of its tetroxide. To confirm the presence of ruthenium, the paper is ignited and the ash fused with potassium nitrate and hydroxide. On extraction with water the orange colour of potassium ruthenate is obtained.1... 

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