| History, Occurrence, and Uses |
Rhodium was discovered by W. H. Wollaston in 1803-04 in the aqua regia platium hexachloride, (NH4)2PtCl6, from the aqua regia extract, the resulting filtrate contained two new metals, palladium and rhodium. The element was named rhodium, derived from the Greek word rhodon for the beautiful rose color of its chloro salt and its aqueous solution.
Rhodium occurs in nature in trace quantities, always associated with other platinum metals. It is found in native form. Its average abundance in the earth’s crust is estimated to be 1mg/kg. Rhodium is used as a precious metal for making jewelry and decorative. Other important applications of this metal or its compounds are in making glass for mirrors or filtering light; in catalytic reactions to synthesize a number of products; as an alloying element for platinum; as a hardening agent for platinum and palladium at high temperatures; in electrical contact plates in radio- and audio-frequency circuits. Rhodium alloyed with platinum is used in thermocouples. A 10% Rh-Pt alloy was introduced by LeChatelier in 1885 for use in thermocouples. Also, rhodium alloys are used in laboratory crucibles, electrodes, optical instruments, furnace linings, and making glass fibers. |
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| Physical Properties |
Grayish-white metal; face-centered cubic crystals; density 12.41 g/cm3; hardness, annealed 100-120 Vickers units; melts at 1,964°C; vaporizes at 3,695°C; electrical resistivity 4.33 microhm–cm at 0°C; tensile strength, annealed 50 tons/in2; Young’s modulus, annealed 2.3×104 tons/in2; magnetic susceptibility 0.99×10–6 cm3/g; thermal neutron absorption cross section 156 barns; insoluble in water; soluble in concentrated sulfuric or hydrochloric acid under boiling conditions; the metal in massive form is slightly soluble in aqua regia, but in small quantities or in thin plates it partially dissolves in aqua regia; forms solid solutions with platinum, palladium and iridium. |
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| Reactions |
At ordinary temperatures rhodium is stable in air. When heated above 600°C, it oxidizes to Rh2O3, forming a dark oxide coating on its surface. The gray crystalline sesquioxide has a corundom-like crystal structure. The sesquioxide, Rh2O3 , decomposes back to its elements when heated above 1,100°C. However, on further heating the metal starts to lose its weight similar to platinum, probably due to loss of its volatile oxide RhO2 dissolved in the metal. The molten metal readily absorbs gaseous oxygen.
The metal in powder form absorbs hydrogen when heated. When heated with carbon monoxide under pressure rhodium forms carbonyl, Rh4(CO)12. The metal combines with halogens at elevated temperatures. When heated with fluorine at 500 to 600°C, it forms a trifluoride, RhF3, a red rhombohedral crystalline powder insoluble in water, dilute acids, or alkalis. Also, a blue tetrafluoride, RhF4, is formed as a minor product. When heated with chlorine gas above 250°C, the brown-red trichloride, RhCl3, forms. It is hygroscopic, decomposing at 450°C.
Rhodium is attacked by fused caustic soda or caustic potash. Also, fused sodium or potassium cyanide and sodium bisulfate attack the metal. |
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| Recovery |
Wollaston’s earliest method involved recovery of rhodium from native platinum. Pt was digested with aqua regia. Rhodium in bulk form is slightly soluble in aqua regia. However, when present as a minor constituent in platinum alloys, the metal may be extracted with aqua regia. Platinum was precipitated from aqua regia extract as ammonium hexachloroplatinate, (NH4)2PtCl6. Addition of mercurous cyanide, Hg2(CN)2, to the filtrate separated palladium as yellow palladium cyanide, Pd(CN)2. Excess mercurous cyanide in the remaining solution was decomposed by evaporating the solution with hydrochloric acid. The residue was treated with ethanol. A dark red solid residue that remained after alcohol treatment was a double chloride, sodium chlororhodite, Na3RhCl6•18H2O. Heating this rhodium complex with hydrogen decomposed the double chloride forming sodium chloride, hydrogen chloride and rhodium metal:
2Na3RhCl6 + 3H2 → 6NaCl + 6HCl + 2Rh
Sodium chloride was removed by leaching with water. Rhodium powder was left as residue. |
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| Chemical Properties |
grey amorphous powder, soluble in ether, alcohol, and water. |
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| Uses |
Rhodium is a transition metal catalyst used in a multitude of inorganic synthesis. |
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| Uses |
As an alloy with platinum; as a corrosion-resistant electroplate for protecting silverware from tarnishing; for making high-reflectivity mirrors for cinema projectors, searchlights. Spongy or black rhodium is used as a catalyst in various organic hydrogenation and oxidation reactions. |
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