Rhodium

Typical wastewaters

• road runoff and stormwater (automobile catalytic converters) Rh(0) particles on road dust and runoff solids, settling in infiltration basins and wetlands 1.5 to 17.2 ng/g in Perth runoff basin sediment; no dissolved value read

1 · Identity

Symbol, number

Rh, 45

Oxidation states in water

0 as converter particles; +3 as chloro and hydroxo complexes if dissolved

Note

The element entry covers the metal, its inertness and Rh₂O₃. No dissolved speciation measurement in natural water was read.

2 · Occurrence in water

Natural sources

Negligible; native and sulfide ores (element entry).

Anthropogenic sources

Automobile three-way catalysts (the NOx reduction metal); nitric acid plant gauzes and chemical catalysts (element entry). Toronto inhalable road dust geomean 21 µg/kg Rh (Pd 152, Pt 55); Canadian road dust average 7.1 ng/g Rh.

3 · Speciation

Particulate metal from converters; the abstracts read give no dissolved rhodium speciation, and the Pt/Pd ratio shift seen in drainage was attributed to palladium solubilisation, not rhodium.

Solubility

Very low.

Hydrolysis

Not read.

Complexation

Not read.

Precipitates

Not relevant.

4 · Role in treatment

Not relevant or not given for this element.

5 · Removal and control

6 · Analytics

Sampling pitfalls

Interference correction dominates the rhodium result (Cu, Sr, Rb, Pb monitored in the isopod study); at natural levels the number is mostly blank and correction.

7 · Regulatory limits

Limits change, and many are set locally. Treat these as the published values to start from, not as your compliance target: check the standard in force at your site and the numbers written into your own permit.

8 · Health and environmental effects

Toxicity

No human health guideline anywhere read. Rhodium in inhalable road dust was 14 percent bioaccessible in simulated lung fluid.

Bioaccumulation

The least accumulated converter metal: Asellus aquaticus from an urban river held 17.9 plus or minus 12.2 ng/g Rh (Pd 155.4, Pt 38.0) and the 24 h bioaccumulation factor was 7 (Pd 150, Pt 85); Chaoborus assimilated only 1.6 percent of dietary Rh.

Ecotoxicity

Not read.

Flags

• No dissolved rhodium concentration in water was read as a number.
• Sediment and dust figures are the only quantitative environmental data and are quoted as such.

Gaps

• No dissolved Rh concentration for any water was read.
• No speciation, solubility or ecotoxicity data were read.
• No removal efficiency in wastewater treatment was read.
• No GCC standard was read.

Sources

Identity

Name and symbol

Rhodium, Rh

Atomic number

45 protons

Position

group 9 · period 5 · d-block · transition metal

CAS number

7440-16-6

Atomic structure

Atomic mass

102.905 u

Electron configuration

1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s¹ 4d⁸
[Kr] 5s¹⁴d⁸

Electrons per shell

2, 8, 18, 16, 1

Valence electrons

9 ns and (n-1)d

Physical properties

State at room temperature

Solid

Melting point

2,237 K (1,963.85 °C)

Boiling point

3,968 K (3,694.85 °C)

Density

12.4 g/cm3

Appearance

Silvery white metallic

Thermal conductivity

150 W/(m·K)

Electrical resistivity

43.3 nΩ·m at 0 °C

Electrical conductivity

23.09 MS/m

Crystal structure

face-centered cubic

Molar heat capacity

24.98 J/(mol·K)

Chemical properties

Oxidation states

+3

Electronegativity

2.28 (Pauling Scale)

Ionisation energy

7.459 eV
1st 719.7, 2nd 1,740, 3rd 2,997 kJ/mol

Electron affinity

1.137 eV

Atomic radius

empirical 142, covalent 142, van der Waals 195 pm

Ionic radius

Rh³⁺ 67; Rh⁴⁺ 60; Rh⁵⁺ 55 pm

Reactivity

One of the least reactive metals: a platinum-group metal that does not tarnish, resists nitric acid completely, dissolves only reluctantly in aqua regia, and forms its compounds mainly in the +1 and +3 states.

with water

Does not react with water.

with oxygen, air

Stable in air; at red heat it slowly forms the sesquioxide Rh2O3, which decomposes back to the metal at higher temperature: $\ce{4 Rh + 3 O2 -> 2 Rh2O3}$.

with acids

Completely insoluble in nitric acid and only slightly soluble in aqua regia, a difference once used to separate it from platinum; hot concentrated sulfuric acid attacks it slowly.

with halogens

Combines with chlorine on heating to the trichloride: $\ce{2 Rh + 3 Cl2 -> 2 RhCl3}$, and with fluorine to RhF3 and higher fluorides.

Typical compounds

• Rh₂O₃ rhodium sesquioxide black paramagnetic oxide from red heat
• RhCl₃.3H₂O rhodium trichloride hydrate starting material for rhodium complexes
• Na₃RhCl₆ sodium hexachlororhodate rose-coloured salt of Wollaston's discovery
• RhCl(P(C₆H₅)₃)₃ Wilkinson's catalyst homogeneous hydrogenation catalyst
• Rh₂(O₂CCH₃)₄ rhodium(II) acetate cyclopropanation catalyst

Occurrence, production and use

Crustal abundance

1×10-3 milligrams per kilogram

Oceanic abundance

Not Applicable

Occurrence and sources

Rhodium occurs natively 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 area of the Sudbury, Ontario region. Although the quantity occurring there is very small, the large tonnages of nickel processed make the recovery commercially feasible. The annual world production of rhodium is only 7 or 8 tons.

• native, alloyed with other platinum metals placer river sands of North and South America
• in copper-nickel sulfide ores Sudbury, Ontario; Bushveld Complex, South Africa (largest reserves)
• crustal abundance 0.000037 ppm (BGS group figure for all PGMs via RSC); 0.001 mg/kg crust (PubChem)

Extraction, production

By-product of copper and nickel refining

world production about 30 t per year (RSC, undated); no separation chemistry stated

Uses

Rhodium is used to make electrical contacts, as jewelry and in catalytic converters, but is most frequently used as an alloying agent in other materials, such as platinum and palladium. These alloys are used to make such things as furnace coils, electrodes for aircraft spark plugs and laboratory crucibles.

Rhodium's primary use is as an alloying agent to harden platinum and palladium. Such alloys are used for furnace windings, thermocoupling 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. Rhodium is also used for jewelry, for decoration, and as a catalyst.

• Automotive catalysts: three-way catalytic converters, where rhodium reduces nitrogen oxides about 80 percent of rhodium use (rsc-element-45, undated)
• Chemicals: platinum-rhodium gauze catalyst for ammonia oxidation in nitric acid manufacture; catalysts for acetic acid and hydrogenation reactions
• Electronics and optics: electrical contacts; coatings on optical fibres, mirrors and headlight reflectors; crucibles and thermocouple elements
• Mining: co-product of PGM and nickel-copper sulfide mining in South Africa, Russia, Zimbabwe, Canada, United States

Safety, toxicity

Exposure to rhodium (metal fume and dust, as Rh) should not exceed 1 mg/m^3 (8-hour time-weighted average, 40-hour week).

GHS classification, signal word Danger

• H228 Flammable solid Flammable solids
• H334 May cause allergy or asthma symptoms or breathing difficulties if inhaled Sensitization, respiratory

Discovery and name

Discovered by

William Hyde Wollaston

Discovered

1804

First isolated

not in sources

Named by

not in sources

Origin of the name

from Greek ῥόδον, 'rose', for the color of one of its chlorine compounds

The metal is silvery white and at red heat slowly changes in air to the resquioxide. At higher temperatures it converts back to the element. Rhodium has a higher melting point and lower density than platinum. It is highly reflective, hard, and durable.