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| | Rhodium Chemical Properties |
| Melting point | 1966 °C (lit.) | | Boiling point | 3727 °C (lit.) | | density | 12.41 g/cm3 (lit.) | | storage temp. | Inert atmosphere,Room Temperature | | form | wire | | color | Red | | Specific Gravity | 12.41 | | resistivity | 4.33 μΩ-cm, 20°C | | Water Solubility | Insoluble | | Merck | 14,8186 | | Exposure limits | ACGIH: Ceiling 2 ppm
OSHA: Ceiling 5 ppm(7 mg/m3)
NIOSH: IDLH 50 ppm; Ceiling 5 ppm(7 mg/m3) | | CAS DataBase Reference | 7440-16-6(CAS DataBase Reference) | | NIST Chemistry Reference | Rhodium(7440-16-6) | | EPA Substance Registry System | Rhodium (7440-16-6) |
| | Rhodium Usage And Synthesis |
| 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 radioand 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.
| | 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.
| | 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.
| | 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.
| | Description | Rhodium is one of the platinum group elements, and is found
at very low concentrations in the Earths crust. Rhodium was
discovered by William Hyde Wollaston (England) in 1804. The
origin of the name comes from the Greek word rhodon,
meaning rose. The plated solid is very corrosion resistant and
exceptionally hard. While inert in air and acids, it can produce
a violent reaction to chlorine, bromine pentafluoride, bromine
trifluoride, and fluorine monoxide. | | Chemical Properties | Rhodium, together with platinum, palladium, iridium, ruthenium, and osmium, is one of the platinum-group metals in Group VIII of the Periodic Table. Rhodium metal is a white, hard, ductile, malleable solid with a bluish-gray luster. soluble in ether, alcohol, and water. The alloys of rhodium can also be used in high temperature conditions (i.e., thermocouples and crucibles). It also can be used in electroplating glass products due to its reflective properties.
| | Physical properties | Rhodium is a hard shiny-white metal that resists corrosion from oxygen, moisture, andacids at room temperatures. As a member of group 8 (VIII), 45Rh shares many chemical andphysical properties with cobalt (27Co) just above it and iridium (77Ir) below it in the verticalgroup. Therefore, it is considered one of the elements that are transitory between metals andnonmetals. It is rare and only found in combination with platinum ores.
Rhodium’s melting point is 1,966°C, its boiling point is 3,727°C, and its density is 12.41g/cm3. | | Isotopes | There are 52 isotopes of rhodium, ranging from Rh-89 to Rh-122. All are producedartificially with relatively short half-lives except one stable isotope, Rh-103, whichconstitutes 100% of the element’s existence in the Earth’s crust. | | Origin of Name | Named after the Greek word rhodon, which means “rose,” because of
the reddish color of its salt compounds. | | Occurrence | Rhodium is rare, but not as rare as ruthenium. It makes up only 1 part in 20 million of theelements found in the Earth’s crust. Even so, it is considered the 79th most abundant elementand is found mixed with platinum ore, and to a lesser extent, it is found with copper andnickel ores. It is found in Siberia, South Africa, and Ontario, Canada.
Rhodium is recovered from platinum and other ores by refining and purification processesthat start by dissolving the other platinum group metals and related impurities with strongacids that do not affect the rhodium itself. Any remaining platinum group elements areremoved by oxidation and bathing the mixture in chlorine and ammonia.
Rhodium is usually produced as a powder and can be formed by either casting or powdermetallurgy. | | Characteristics | Rhodium is one of the six platinum transition elements that include Ru, Rh, Pd, Os, Ir, andPt. Of these metals, rhodium has the highest electrical and thermal conductivity. Although arelatively scarce metal, rhodium makes an excellent electroplated surface that is hard, wearswell, and is permanently bright—ideal for plating the reflectors in automobile headlights. | | History | Wollaston discovered rhodium in 1803-4 in crude
platinum ore he presumably obtained from South America.
Rhodium occurs native with other platinum metals in river
sands of the Urals and in North and South America. It is also
found with other platinum metals in the copper-nickel sulfide
ores of the Sudbury, Ontario region. Although the quantity
occurring here is very small, the large tonnages of nickel processed
make the recovery commercially feasible. The annual
world production of rhodium in 1999 was only about 9000
kg. The metal is silvery white and at red heat slowly changes
in air to the sesquioxide. At higher temperatures it converts
back to the element. Rhodium has a higher melting point
and lower density than platinum. Its major use is as an alloying
agent to harden platinum and palladium. Such alloys are
used for furnace windings, thermocouple elements, bushings
for glass fiber production, electrodes for aircraft spark plugs,
and laboratory crucibles. It is useful as an electrical contact
material as it has a low electrical resistance, a low and stable
contact resistance, and is highly resistant to corrosion. Plated
rhodium, produced by electroplating or evaporation, is exceptionally
hard and is used for optical instruments. It has a high
reflectance and is hard and durable. Rhodium is also used for
jewelry, for decoration, and as a catalyst. Fifty-two and isomers are now known. Rhodium metal (powder) costs
about $180/g (99.9%).isotopes | | Uses | Rhodium is a transition metal catalyst used in a multitude of inorganic synthesis. | | 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. | | Uses | Rhodium is commercially used as an alloy metal with other metals to form durable hightemperatureelectrical equipment, thermocouples, electrical contacts and switches, and laboratorycrucibles.
Because of its high reflectivity, it is used to electroplate jewelry, silverware, optical instruments,mirrors, and reflectors in lighting devices.
When rhodium is combined with platinum and palladium, the elements together form theinternal metals of automobile catalytic converters, which convert hot unburned hydrocarbonexhaust gases to less harmful CO2 and H2O. Similar alloys are used to manufacture hightemperatureproducts such as electric coils for metal refining furnaces and high-temperaturespark plugs. | | Definition | Metallic element having
atomic number 45, group VIII of the periodic table,
aw 102.9055, no isotopes, valence = 3.
| | Definition | A rare silvery hard
transition metal. It is difficult to work and
highly resistant to corrosion. Rhodium occurs
native but most is obtained from copper
and nickel ores. It is used in protective
finishes, alloys, and as a catalyst.
Symbol: Rh; m.p. 1966°C; b.p. 3730°C;
r.d. 12.41 (20°C); p.n. 45; r.a.m.
102.90550. | | Definition | rhodium: Symbol Rh. A silvery whitemetallic transition element;a.n. 45; r.a.m. 102.9; r.d. 12.4; m.p.1966°C; b.p. 3727°C. It occurs withplatinum and is used in certain platinumalloys (e.g. for thermocouples)and in plating jewellery and opticalreflectors. Chemically, it is not attackedby acids (dissolves only slowlyin aqua regia) and reacts with nonmetals(e.g. oxygen and chlorine) atred heat. Its main oxidation state is+3 although it also forms complexesin the +4 state. The element was discoveredin 1803 by William Wollaston(1766–1828). | | Production Methods | Pure rhodium is prepared by the reduction of its ammonium
salt (dichloropentaaminorhodium). | | General Description | This product has been enhanced for energy efficiency. | | Hazard | The powder and dust of rhodium metal are flammable in air. Some of the compounds maycause skin irritations. It is best to use approved laboratory procedures when handling any ofthe six elements in the platinum family of metals. | | Hazard | Flammable in powder form. Upper respira-
tory tract irritant. Questionable carcinogen.
| | Health Hazard | There are no data demonstrating
acute or chronic rhodium-related diseases;
irritation and sensitization have occasionally
been reported in humans from exposure to the
salts of rhodium. Solutions of insoluble salts
splashed in the eye may cause mild irritation. | | Flammability and Explosibility | Notclassified | | Industrial uses | Metallic rhodium is the whitest of the platinum metals and does not tarnish under atmospheric conditions. It is insoluble in most acids, including aqua regia, but is attacked by chlorine at elevated temperatures and by hot fuming sulfuric acid. Liquid rhodium dissolves oxygen, and ingots are made by argon-arc melting. At temperatures above 1200 C, rhodium reacts with oxygen to form rhodium oxide, Rh2O3. Rhodium is used to make the nibs of writing pens, to make resistance windings in high-temperature furnaces, for high-temperature thermocouples, as a catalyst, and for laboratory dishes. It is the hardest of the platinum-group metals; the annealed metal has a Brinell hardness of 135. Rhodium is also valued for electroplating jewelry, electric contacts, hospital and surgical instruments, and especially reflectors.
The most important alloys of rhodium are rhodium platinum. They form solid solutions in any proportion, but alloys of more than 40% rhodium are rare. Rhodium is not a potent hardener of platinum but increases its high-temperature strength. It is easily workable and does not tarnish or oxidize at high temperatures. These alloys are used for thermocouples and in the glass industry. | | Safety Profile | Handle carefully. It may be a sensitizer but not to the same extent as platinum. Most rholum compounds have only moderate toxicity by ingestion. Flammable when exposed to heat or flame. Violent reaction with chlorine, bromine pentafluoride, bromine trifluoride, and OF2. A catalytic metal | | Potential Exposure | Rhodium has few applications by
itself, as in rhodium plating of white gold jewelry or plat-
ing of electrical parts, such as commutator slip rings, but,
mainly, rhodium is used as a component of platinum alloys.
Rhodium-containing catalysts have been proposed for use
in automotive catalytic converters for exhaust gas cleanup. | | Carcinogenicity | Chick embryos exposed to rhodium on the
eighth day of incubation were stunted; mild
reduction of limb size and feather growth inhibition
were also observed. A number of
rhodium compounds have tested positive in
bacterial assays for genetic altering capability. | | Environmental Fate | The most common route by which rhodium enters the environment
is as a component of automobile exhaust resulting
from use of catalytic converters. Rhodium is insoluble in water
and all acids, with the exception that very finely separated
material may be dissolved in concentrated sulfuric acid and
aqua regia.
Being largely inert, rhodium can undergo long-range
transport, and particulate phase matter generally leaves the
atmosphere by wet or dry deposition. In an aqueous environment,
rhodium can form complexes with halide and
nitrogen donor ligands, which may be water soluble, but
reactions can be dictated by pH, redox potential, and what
material is available for creating ligands. Reactions in soil can
depend on these same factors, as well as chloride concentrations, and rhodium is seen to be mobile only in
highly acidic soils.
Rhodium has been seen to bioaccumulate in both fresh and
salt water species, and has the potential to biomagnify. | | Shipping | Flammable powder, Hazard Class: 4.1; Labels:
4.1-Flammable solid. | | Toxicity evaluation | Enzymes that have essential sulfhydryl groups in or near the
activation sites are completely inhibited. It is thought that
rhodium compounds bond to DNA, RNA, or the corresponding
purines and inhibit their synthesis. | | Incompatibilities | Flammable as a dust, fume, or powder
may form explosive mixture with air. Incompatible with
strong oxidizers (chlorates, nitrates, peroxides, permanga-
nates, perchlorates, chlorine, bromine, fluorine, etc.); con-
tact may cause fires or explosions. Keep away from
alkaline materials, strong bases, strong acids, oxoacids,
epoxides, bromine pentafluoride, and bromine trifluoride;
chlorine trifluoride; oxygen difluoride. | | Waste Disposal | Recovery in view of the high
economic value. Recovery techniques for recycling of
rhodium in plating wastes and spent catalysts have been
described in the literature. |
| | Rhodium Preparation Products And Raw materials |
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