Rhodium Rh

spore: profile
id: rhodium
spore_version: 0.2
family: precious-metals
name: Rhodium
abbreviation: Rh
role: reflective and high-temperature precious metal
category: metals
type: precious-metals
summary: |
  Rhodium is the pure metal Rh, a dense platinum-group metal (12.423 g/cm3 at 20 °C in Wikipedia) that melts at 1964 °C. Its largest use is automotive catalytic converters, which took about 81 % of 30,000 kg consumed in 2012, according to Wikipedia. It also appears in platinum-rhodium thermocouples, in high-reflectivity mirrors, and in first-mirror coatings for ITER optical diagnostics. A 1987 NASA Lewis test found that rhodium films survived an air plasma in an asher with negligible reflectance loss, but their low solar reflectance ruled them out for solar dynamic mirrors.
best_for: 'Corrosion-resistant reflective and high-temperature sensing surfaces: not attacked by most acids, and does not normally form an oxide even when heated (Wikipedia)'
weakest_at: 'Cost and supply concentration: USGS price 5,800 dollars per troy ounce in 2025e, and principal production in South Africa at 75 % (Wikipedia)'
identity:
  formula: Rh (element, atomic number 45); pure metal, CAS 7440-16-6
  cas: 7440-16-6
  crystal_system: face-centred cubic
  source: pubchem-rh
  where: Names and Identifiers, Other Identifiers, CAS 7440-16-6 (PubChem CID 23948); crystal structure from Chemical and Physical Properties, Color/Form ('face-centered cubic structure')
  designations: []
tags:
  environments:
    - low-earth-orbit
  stresses:
    - atomic-oxygen
  applications:
    - thermal-control
    - optics-and-cameras
    - electronics-and-chips
properties:
  - benchmark: density
    label: Density, rhodium (Wikipedia infobox)
    value: 12.423
    unit: g/cm3
    scope: material
    conditions:
      temperature: 20 °C
    method: Value printed in the infobox of the Wikipedia article on rhodium
    status: ground
    source: wikipedia-rh
    where: Infobox, Physical properties, density row
  - benchmark: density
    label: Density, rhodium (PubChem experimental property)
    value: 12.41
    unit: g/cm3
    scope: material
    conditions:
      temperature: 20 °C
    method: Experimental value listed in the PubChem record for rhodium (CID 23948)
    status: ground
    source: pubchem-rh
    where: Chemical and Physical Properties, Experimental Properties, Density, entry 12.41 g/cu cm @ 20 °C
  - benchmark: melting_point
    label: Melting point, rhodium (Wikipedia infobox)
    value: 1964
    unit: °C
    scope: material
    conditions:
      unit_in_source: 2237 K (1964 °C, 3567 °F)
      pressure: not stated
    method: Value printed in the infobox of the Wikipedia article on rhodium
    status: ground
    source: wikipedia-rh
    where: Infobox, Physical properties, melting point row
  - benchmark: melting_point
    label: Melting point, rhodium (PubChem experimental property)
    value: 1964
    unit: °C
    scope: material
    conditions:
      form: element, CAS 7440-16-6
      pressure: not stated
    method: Experimental value listed in the PubChem record for rhodium
    status: ground
    source: pubchem-rh
    where: Chemical and Physical Properties, Experimental Properties, Melting Point, entry 1964 °C
  - benchmark: boiling_point
    label: Boiling point, rhodium (Wikipedia infobox)
    value: 3695
    unit: °C
    scope: material
    conditions:
      unit_in_source: 3968 K (3695 °C, 6683 °F)
      pressure: not stated
    method: Value printed in the infobox of the Wikipedia article on rhodium
    status: ground
    source: wikipedia-rh
    where: Infobox, Physical properties, boiling point row
  - benchmark: thermal_conductivity
    label: Thermal conductivity, rhodium (Wikipedia infobox)
    value: 150
    unit: W/(m·K)
    scope: material
    conditions:
      temperature: not stated
    method: Value printed in the infobox of the Wikipedia article on rhodium
    status: ground
    source: wikipedia-rh
    where: Infobox, Thermal conductivity row
  - benchmark: electrical_resistivity
    label: Electrical resistivity, rhodium (Wikipedia infobox)
    value: 43.3
    unit: nΩ·m
    scope: material
    conditions:
      temperature: 0 °C
    method: Value printed in the infobox of the Wikipedia article on rhodium
    status: ground
    note: The PubChem value below differs from this one (4.51 µΩ·cm at 0 °C); both are kept as printed and the difference is not resolved.
    source: wikipedia-rh
    where: Infobox, Electrical resistivity row
  - benchmark: electrical_resistivity
    label: Electrical resistivity, rhodium (PubChem experimental property)
    value: 4.51
    unit: µΩ·cm
    scope: material
    conditions:
      temperature: 0 °C
    method: Experimental value listed in the PubChem record for rhodium, in the Other Experimental Properties text
    status: ground
    note: Differs from the Wikipedia value above; not converted and not resolved.
    source: pubchem-rh
    where: Chemical and Physical Properties, Experimental Properties, Other Experimental Properties, text beginning Electrical resistivity 4.51 microohm-cm
  - benchmark: cte
    value: 8.46
    unit: ppm/K
    scope: material
    conditions:
      temperature: 20 °C
      unit_in_source: 8.46×10−6/K (at 20 °C)
    method: Value printed in the infobox of the Wikipedia article on rhodium; written in ppm/K from the printed 8.46×10−6/K
    status: ground
    source: wikipedia-rh
    where: Infobox, Thermal expansion coefficient row
  - benchmark: specific_heat
    label: Specific heat capacity, rhodium
    value: 242.736
    unit: J/(kg·K)
    scope: material
    conditions:
      temperature: not stated
    method: Value printed in the infobox of the Wikipedia article on rhodium
    status: ground
    source: wikipedia-rh
    where: Infobox, Specific heat capacity row
  - benchmark: elastic_modulus
    label: Young's modulus, rhodium
    value: 380
    unit: GPa
    scope: material
    conditions:
      temperature: not stated
    method: Value printed in the infobox of the Wikipedia article on rhodium
    status: ground
    source: wikipedia-rh
    where: Infobox, Young's modulus row
  - benchmark: bulk_modulus
    label: Bulk modulus, rhodium
    value: 275
    unit: GPa
    scope: material
    conditions:
      temperature: not stated
    method: Value printed in the infobox of the Wikipedia article on rhodium
    status: ground
    source: wikipedia-rh
    where: Infobox, Bulk modulus row
  - benchmark: mohs_hardness
    label: Mohs hardness, rhodium
    value: 6
    unit: '-'
    scope: material
    conditions:
      temperature: not stated
    method: Value printed in the infobox of the Wikipedia article on rhodium, with no unit or condition
    status: ground
    note: mohs_hardness is not in the canonical property list.
    source: wikipedia-rh
    where: Infobox, Mohs hardness row
corrosion:
  - environment: Acids, nitric acid and aqua regia (Wikipedia)
    headline: Rhodium is not attacked by most acids and is insoluble in nitric acid; it dissolves slightly in aqua regia.
    effect: The Wikipedia characteristics section gives these statements. No rate or acid concentration is given.
    rating: low
    conditions:
      basis: encyclopaedia summary; concentrations not stated
    source: wikipedia-rh
    where: Characteristics, Chemical properties paragraph
  - environment: Acids and aqua regia, compact metal and fine powder (PubChem)
    headline: Compact rhodium is not attacked even by aqua regia, but finely divided rhodium reacts with it.
    effect: The PubChem experimental properties give this contrast between compact and finely divided metal. No rate is given.
    rating: low
    conditions:
      form: compact metal and finely divided metal
      basis: database compilation; no rate given
    source: pubchem-rh
    where: Chemical and Physical Properties, Experimental Properties, Other Experimental Properties
  - environment: Heating in air (conflicting sources)
    headline: Wikipedia says rhodium does not normally form an oxide even when heated. PubChem says it is slowly oxidised to the sesquioxide at red heat.
    effect: The two statements conflict and are both recorded. Wikipedia gives no rate; PubChem also says the metal absorbs oxygen when melted.
    rating: low
    conditions:
      temperature: red heat (PubChem); not stated (Wikipedia)
      basis: two summaries that disagree
    source: pubchem-rh
    where: Chemical and Physical Properties, Experimental Properties, Stability/Shelf Life
  - environment: Halogens, chlorine and bromine at red heat, fluorine
    headline: Chlorine or bromine converts rhodium to a trihalide at red heat; fluorine does not attack it.
    effect: PubChem gives both statements. The halogen reaction needs red heat; no rate is given.
    rating: moderate
    conditions:
      temperature: red heat (chlorine and bromine)
    source: pubchem-rh
    where: Chemical and Physical Properties, Experimental Properties, Stability/Shelf Life, second entry
  - environment: Air plasma in an asher (1987 NASA ground test, low Earth orbit concern)
    headline: Rhodium films showed negligible reflectance loss in an air plasma and need no protective coating in low Earth orbit.
    effect: NASA Lewis reports that platinum and rhodium films survived the plasma and showed negligible reflectance losses over the exposure. Their solar reflectance was low, which ruled them out for solar dynamic mirrors. The paper gives no exposure time in the text read here.
    rating: low
    conditions:
      specimen: ion-beam-sputtered rhodium films on a nickel substrate and other substrates
      test: exposure in an asher, RF-generated air plasma
      basis: ground test; not flight data
    source: gulino-1987
    where: Abstract; Corrosion-Resistant Metals section; Conclusions
risks:
  hazards:
    - text: PubChem reports that rhodium does not meet GHS hazard criteria for 81.3% (239 of 294) of company reports to the ECHA C&L Inventory. Reports that give codes list H228 flammable solid (15.3%) and H334 may cause allergy or asthma symptoms or breathing difficulties if inhaled, with signal word Danger.
      ghs:
        - H228
        - H334
      source: pubchem-rh
      where: Safety and Hazards, Hazards Identification, GHS Classification
    - text: The 2025 Implats safety data sheet for rhodium gives signal word Danger with H303 may be harmful if swallowed, H317 may cause an allergic skin reaction and H333 may be harmful if inhaled. It also notes long lasting harmful effects in aquatic life.
      ghs:
        - H303
        - H317
        - H333
      source: implats-sds-rh
      where: Section 2.1, Classification according to GHS, Health statements
    - text: The REACH lead registrant dossier for rhodium (EC 231-125-0) lists rhodium as not classified under CLP and GHS, with no signal word.
      ghs: []
      source: matthey-rh-reach
      where: Section 1, Classification and labelling table, Substance identity and Table 1.1
  failure_risks:
    - mode: Dust or powder ignition
      trigger: Very fine dust or powder in high concentration, exposed to high-energy heat or an ignition source
      consequence: The Implats SDS says very fine dust may explode in very high concentrations, but calls this highly unlikely in current form and classes the product as non-flammable. PubChem says rhodium is flammable in powder form and that its metal fume is flammable.
      severity: low
      source: implats-sds-rh
      where: Section 10.2, Flammability
    - mode: Violent reaction with halogens and interhalogens
      trigger: Contact with bromine pentafluoride, chlorine trifluoride, oxygen difluoride, or chlorine when heated
      consequence: PubChem reports violent reaction with BrF5, ClF3 and OF2, and reaction with chlorine when heated. The Chemical Dangers entry warns that this can cause fire or explosion.
      severity: moderate
      source: pubchem-rh
      where: Safety and Hazards, Reactivity Profile and Chemical Dangers
    - mode: Catalytic ignition of sorbed hydrogen
      trigger: Metal prepared by heating its compounds in hydrogen, then exposed to air while still hot
      consequence: The hydrogen sorbed in the metal can ignite catalytically on exposure to air, so the metal must be cooled in an inert atmosphere.
      severity: moderate
      source: pubchem-rh
      where: Safety and Hazards, Hazardous Reactivities and Incompatibilities
  side_effects:
    - text: 'PubChem summary: the platinum-group metals are generally nontoxic in the metallic state, but soluble halide salts are highly reactive. Occupational contact urticaria and hypersensitivity are reported.'
      affects: Workers handling metal powder, fume or dust
      source: pubchem-rh
      where: Safety and Hazards, Hazards Identification, Hazards Summary
    - text: 'USGS prices: 4,660.44 dollars per troy ounce in 2024 and an estimated 5,800 dollars in 2025. The 2025 price rose 24 % as demand grew in the hard disk and chemical catalyst industries.'
      affects: Cost of coatings, mirrors, thermocouples and catalysts
      source: usgs-pgm-2026
      where: Salient Statistics, Price, dollars per troy ounce, Rhodium row; Events, Trends, and Issues
  handling:
    - text: 'Keep dust controlled: the Implats SDS gives ACGIH TLV-TWA 1 mg/m3 for metal and insoluble compounds and 0.01 mg/m3 for soluble rhodium.'
      source: implats-sds-rh
      where: Section 8.1, Control parameters
    - text: PubChem gives the OSHA permissible exposure limit as 0.1 mg/m3 (as Rh, metal fume and insoluble compounds), with the same figure as the NIOSH recommended exposure limit.
      source: pubchem-rh
      where: Safety and Hazards, Safety and Hazard Properties, OSHA Standards and NIOSH Recommendations
    - text: Wear a Class P1 particulate respirator where large quantities create a dust inhalation hazard. Spilt bulk material calls for goggles and PVC or rubber gloves.
      source: implats-sds-rh
      where: Section 6.1, Personal precautions
    - text: Store in tightly sealed containers in a cool, dry, well-ventilated area, away from oxidising agents, acids and foodstuffs.
      source: implats-sds-rh
      where: Section 7.2, Conditions for safe storage
    - text: Keep powder away from halogens and interhalogens, which react violently with rhodium. Moisten spilt powder before sweeping to prevent dusting.
      source: pubchem-rh
      where: Safety and Hazards, Hazardous Reactivities and Incompatibilities; Accidental Release Measures, Spillage Disposal
  mitigations: []
uses:
  - application: thermal-control
    device: Platinum-rhodium thermocouples for high-temperature measurement
    example: Wikipedia says that by the early 1900s rhodium-containing thermocouples were used to measure temperatures up to 1800 °C.
    source: wikipedia-rh
    where: Uses section, thermocouple paragraph
  - application: optics-and-cameras
    device: High-reflectivity mirrors for cinema projectors and searchlights
    example: PubChem says rhodium is used for high-reflectivity mirrors for cinema projectors and searchlights, and as a corrosion-resistant electroplate.
    source: pubchem-rh
    where: Use and Manufacturing, Uses
  - application: optics-and-cameras
    device: Magnetron-sputtered rhodium first mirrors for ITER optical diagnostics
    example: Polyteknik, with F4E and ITER, applies magnetron-sputtered rhodium coatings to full-scale first mirror prototypes. The offer claims resistance to erosion, deposition and oxidation; it gives no numbers.
    source: polyteknik-f4e
    where: Technology offer, The technology and Advantages and Potential Applications sections
  - application: electronics-and-chips
    device: Computer hard disks and chemical catalysts (rhodium demand)
    example: USGS says the 2025 price increase reflected increased demand, particularly for rhodium in the hard disk and chemical catalyst industries.
    source: usgs-pgm-2026
    where: Events, Trends, and Issues
supply:
  - subject: Rhodium mine output
    producers: []
    total: Wikipedia says principal production is in South Africa, at 75 %, with much less production in Russia and Zimbabwe. Annual world production fluctuates around 30 tonnes. The year of these figures is not printed.
    source: wikipedia-rh
    where: Production and occurrence section
  - subject: Rhodium recycling
    producers: []
    recycling: The 2026 USGS platinum-group summary gives a recovery figure for palladium and platinum only. No rhodium recovery figure is given.
    source: usgs-pgm-2026
    where: Recycling paragraph
  - subject: United States rhodium imports for consumption
    producers: []
    total: 14,000 kg in 2025e, 14,500 kg in 2024 and 12,100 kg in 2023 (USGS, Salient Statistics, Imports for consumption, Rhodium row).
    source: usgs-pgm-2026
    where: Salient Statistics, Imports for consumption, Rhodium row
  - subject: Rhodium substitution
    producers: []
    substitutes:
      - 'USGS: for some industrial end uses one platinum-group metal can substitute for another, but with losses in efficiency.'
    source: usgs-pgm-2026
    where: Substitutes paragraph
  - subject: Rhodium critical-mineral status
    producers: []
    critical_lists:
      - list: USGS 2025 List of Critical Minerals
        listed: true
    source: usgs-crit-2025
    where: Final 2025 list entry, Rhodium, used in catalytic converters, electrical components, and as a chemical catalyst
documents:
  - title: 'Wikipedia: Rhodium'
    type: database
    url: https://en.wikipedia.org/wiki/Rhodium
    year: 2026
  - title: PubChem compound record for rhodium (CID 23948)
    type: database
    url: https://pubchem.ncbi.nlm.nih.gov/compound/23948
    year: 2026
  - title: 'USGS Mineral Commodity Summaries 2026: Platinum-Group Metals'
    type: report
    url: https://pubs.usgs.gov/periodicals/mcs2026/mcs2026-platinum-group.pdf
    year: 2026
  - title: USGS About the 2025 List of Critical Minerals
    type: report
    url: https://www.usgs.gov/programs/mineral-resources-program/science/about-2025-list-critical-minerals
    year: 2025
  - title: IUCLID dossier extract, rhodium, EC 231-125-0, classification and labelling of the lead registrant (generated 10/12/2020)
    type: report
    url: https://matthey.com/documents/161599/166998/Rh-rhodium.pdf/b2651f30-de8d-0e3d-4afe-2f15dfce8301
    year: 2020
  - title: Oxidation-Resistant Reflective Surfaces for Solar Dynamic Power Generation in Near Earth Orbit (NASA TM-88865)
    type: report
    url: https://ntrs.nasa.gov/citations/19870001527
    year: 1987
  - title: Rhodium-Coated Mirrors for harsh environments, Polyteknik technology offer (Fusion for Energy)
    type: report
    url: https://fusion-technology-transfer.europa.eu/wp-content/uploads/2026/05/Technology-offer_Polyteknik_final.pdf
    year: 2026
  - title: 'Implats safety data sheet: Rhodium (version 1, revision 1, date of issue not available)'
    type: sds
    url: https://implats.co.za/pdf/material-safety-data-sheets/2025/sds-rhodium.pdf
    year: 2025
gaps:
  - Country shares of rhodium mine output with a year are not in a fetched source. Wikipedia gives South Africa 75 % and no year, and the USGS 2026 summary has no rhodium mine figure, so the producers list is empty.
  - No designations (UNS, ASTM, EN, DIN, JIS) for pure rhodium were found in a fetched source, so none are listed. The Implats and REACH documents give no grade code.
  - Mechanical strength and hardness with temper (annealed or work hardened) were not found in a fetched source. A fetched summary of the Wikipedia infobox reported Vickers hardness as a range of 1100 to 8000 MPa, which is too wide to enter as one value and was not checked on the raw page, so it is left out.
  - The Johnson Matthey datasheet (metals.ulprospector.com listing) returned 403 and was not read, so no Johnson Matthey value is entered. The Dräger rhodium page also returned 403.
  - 'Melting point differs across listings: the Johnson Matthey listing gives about 1966 °C in a search summary, but the page was not read. Only the Wikipedia and PubChem values of 1964 °C are entered.'
  - Heat of fusion differs across sources (26.6, 20.5 and 21.5 kJ/mol in search summaries). None was read on a fetched page, so none is entered.
  - Hazard classification is not settled. The REACH dossier says not classified, PubChem reports H228 and H334, and the Implats SDS reports Danger with H303, H317 and H333. The flammability of the powder also differs (PubChem flammable, Implats non-flammable). Both are recorded and not resolved.
  - 'Oxidation in air is not settled: Wikipedia says no oxide forms even when heated, and PubChem says the metal is slowly oxidised to the sesquioxide at red heat.'
  - 'Electrical resistivity differs: Wikipedia gives 43.3 nΩ·m at 0 °C and PubChem gives 4.51 µΩ·cm at 0 °C. Both are kept as printed.'
  - No crustal abundance for rhodium was read. The European Commission critical raw materials list was not checked for rhodium, so no EU list entry is given.
  - No space flight, thruster, or catalyst life data for rhodium were found. The 1987 NASA test is a ground plasma test, and the paper states that low solar reflectance rules rhodium out for solar dynamic mirrors.
  - The refining process for rhodium was not read on a fetched page, so story.recipe gives composition and form only.
  - Automotive catalytic converters are the largest use (about 81 % of 30,000 kg in 2012, Wikipedia), but no application value in the enum fits catalysis. It appears in story.why and the uses text only.
  - Atomic oxygen erosion in flight, outgassing, radiation effects and cold welding were not searched. The environment tag low-earth-orbit rests only on the 1987 NASA ground test.
  - 'No media were added: no rhodium image was read or licensed from a fetched source.'
sources:
  wikipedia-rh:
    title: Rhodium (Wikipedia, English)
    url: https://en.wikipedia.org/wiki/Rhodium
    year: 2026
    kind: web
    license: CC BY-SA 4.0 (cited, not reproduced)
  pubchem-rh:
    title: 'PubChem compound record: Rhodium (CID 23948), experimental properties, GHS classification, reactivity'
    id: CAS 7440-16-6
    url: https://pubchem.ncbi.nlm.nih.gov/rest/pug_view/data/compound/23948/JSON
    year: 2026
    kind: database
    license: PubChem (NCBI), public domain; sections attributed to ECHA, NIOSH and HSDB
  usgs-pgm-2026:
    title: Mineral Commodity Summaries 2026, Platinum-Group Metals
    id: USGS MCS 2026, February 2026
    url: https://pubs.usgs.gov/periodicals/mcs2026/mcs2026-platinum-group.pdf
    year: 2026
    kind: report
    license: public domain (USGS)
  usgs-crit-2025:
    title: About the 2025 List of Critical Minerals (USGS, final list)
    id: Final 2025 List of Critical Minerals, November 2025
    url: https://www.usgs.gov/programs/mineral-resources-program/science/about-2025-list-critical-minerals
    year: 2025
    kind: web
    license: public domain (USGS)
  matthey-rh-reach:
    title: IUCLID dossier extract, rhodium (EC 231-125-0), classification and labelling of the lead registrant (Johnson Matthey)
    id: EC 231-125-0; CAS 7440-16-6; generated 10/12/2020
    url: https://matthey.com/documents/161599/166998/Rh-rhodium.pdf/b2651f30-de8d-0e3d-4afe-2f15dfce8301
    year: 2020
    kind: report
    license: REACH registration extract (cited, not reproduced)
  gulino-1987:
    title: Oxidation-Resistant Reflective Surfaces for Solar Dynamic Power Generation in Near Earth Orbit (Gulino, NASA Lewis, TM-88865)
    id: NASA NTRS 19870001527
    url: https://ntrs.nasa.gov/citations/19870001527
    year: 1987
    kind: report
    license: public domain (NASA)
  polyteknik-f4e:
    title: Rhodium-Coated Mirrors for harsh environments and high-performance optical systems (Polyteknik AS technology offer, Fusion for Energy)
    url: https://fusion-technology-transfer.europa.eu/wp-content/uploads/2026/05/Technology-offer_Polyteknik_final.pdf
    year: 2026
    kind: web
    license: Fusion for Energy technology offer (cited, not reproduced)
  implats-sds-rh:
    title: 'Safety Data Sheet: Rhodium (Impala Platinum / Implats, version 1, revision 1)'
    id: CAS 7440-16-6; date of issue not available
    url: https://implats.co.za/pdf/material-safety-data-sheets/2025/sds-rhodium.pdf
    year: 2025
    kind: datasheet
    license: supplier document, linked not reproduced
license: CC-BY-4.0
updated: '2026-10-26'
review:
  status: draft
  method: assistant
  checked_values: 13
  unreachable: 3
  caveats: 4
  note: 'Values were read by an AI assistant on 2026-10-26 from the pages named in each entry. Unreachable: the Johnson Matthey datasheet listing (403), the Dräger rhodium page (403) and the Johnson Matthey Platinum Metals Review article (bot check). A first PubChem CID tried (23949) is rubidium monohydride; the rhodium record is CID 23948. Caveats: electrical resistivity sources disagree (43.3 nΩ·m against 4.51 µΩ·cm at 0 °C); hazard classifications disagree; oxidation in air is described differently by Wikipedia and PubChem; the 1987 NASA test has no numeric values in the text read.'
story:
  recipe:
    line: Rhodium is a single-element metal (Rh, atomic number 45) with a face-centred cubic structure (PubChem, Color/Form). Platinum-rhodium alloys are the other common form, used for thermocouples, spinnerets, bushings and high-temperature furnace windings (PubChem, Uses). The refining route is not described in the sources read for this profile.
    parts:
      - name: Rhodium
        symbol: Rh
        atoms: 1
        atomic_weight: 102.9055
        role: The whole material. PubChem prints the computed molecular weight of the element as 102.9055.
  risk_groups:
    - title: For people
      level: 1
      verdict: Conflicting classifications
      lead: The REACH dossier, PubChem and a 2025 supplier safety data sheet do not agree on the hazard class of rhodium metal. Read the SDS for the product you buy.
      points:
        - label: Classifications differ
          line: The REACH lead registrant says rhodium is not classified. PubChem reports that 81.3 % of 294 company reports do not meet GHS criteria, and the rest give H228 (flammable solid, 15.3 %) or H334 (respiratory sensitisation). The Implats SDS gives signal word Danger with H303, H317 and H333.
        - label: Dust limits
          line: The Implats SDS gives ACGIH 1 mg/m3 for metal and insoluble compounds and 0.01 mg/m3 for soluble rhodium. PubChem gives the OSHA limit as 0.1 mg/m3 as Rh for metal fume and insoluble compounds.
        - label: Sensitisation
          line: The Implats SDS lists skin sensitisation (H317) and respiratory sensitisation (H333). PubChem's hazard summary says the metal is generally nontoxic but reports occupational contact urticaria.
    - title: If it fails
      lead: These are failure routes in the sources. None is a field incident.
      points:
        - label: Powder
          line: PubChem says rhodium is flammable in powder form. The Implats SDS calls the product non-flammable, but says very fine dust may explode in very high concentrations with a high-energy ignition source.
        - label: Halogens
          line: PubChem reports violent reaction with BrF5, ClF3 and OF2, and reaction with chlorine when heated.
        - label: Hot metal from hydrogen
          line: Metal made by heating its compounds in hydrogen must cool in an inert atmosphere, because sorbed hydrogen can ignite catalytically when the hot metal meets air.
        - label: Cost
          line: The USGS price was 4,660.44 dollars per troy ounce in 2024 and an estimated 5,800 in 2025, a rise of 24 %.
    - title: Handling and storage
      points:
        - label: Dust
          line: Avoid conditions that create dust or fumes. Use a P1 particulate respirator where large quantities create a dust hazard, and moisten spilt powder before sweeping.
        - label: Storage
          line: Store in tightly sealed containers in a cool, dry, well-ventilated area, away from oxidising agents, acids and foodstuffs.
        - label: Heating
          line: The Implats SDS notes that toxic metal oxides may evolve when the metal is heated.
  why: Rhodium (Rh) is a dense, silvery platinum-group metal with a melting point of 1964 °C, according to Wikipedia and PubChem. It is not attacked by most acids, and according to Wikipedia it does not normally form an oxide even when heated. Most of its use is in automotive catalytic converters, which took 81 % of 30,000 kg consumed in 2012, according to Wikipedia. Its uses that fit this collection are thermocouples, reflective mirrors and fusion-mirror coatings. A 1987 NASA Lewis test found that rhodium films showed negligible reflectance loss in an air plasma, but their low solar reflectance ruled them out for solar dynamic mirrors.
  uses:
    - where: Thermocouples
      line: Wikipedia says that by the early 1900s rhodium-containing thermocouples were used to measure temperatures up to 1800 °C. PubChem adds that rhodium-platinum alloys are used in high-temperature furnace windings and laboratory crucibles.
    - where: Mirrors
      line: PubChem says rhodium is used for high-reflectivity mirrors for cinema projectors and searchlights. Polyteknik, with Fusion for Energy and ITER, offers magnetron-sputtered rhodium coatings on full-scale first mirror prototypes for ITER optical diagnostics. The offer gives no numbers.
    - where: Electronics and catalysts
      line: USGS says the 2025 rise in rhodium prices reflected increased demand, particularly in the hard disk and chemical catalyst industries.
    - where: Automotive catalysts
      line: Wikipedia calls automotive catalytic converters the primary use, taking 81 % of the 30,000 kg consumed in 2012. This application has no matching value in the application list, so it is described here only.
    - where: Space
      line: A 1987 NASA Lewis technical memorandum tested rhodium films in an air plasma in an asher, in a study concerned with the low Earth orbit environment. The films showed negligible reflectance loss and need no protective coating there. Their low solar reflectance, compared with silver, ruled them out for solar dynamic mirrors. It is a ground test, not flight data.
  can_do:
    - label: Heat
      value: 1964 °C melting point
      benchmark: melting_point
      line: The melting point is 1964 °C in Wikipedia and PubChem, and the boiling point is 3695 °C in Wikipedia.
    - label: Weight
      value: 12.423 g/cm3
      benchmark: density
      line: Wikipedia gives a density of 12.423 g/cm3 at 20 °C, and PubChem gives 12.41 g/cm3 at 20 °C.
    - label: Conduct heat
      value: 150 W/(m·K)
      benchmark: thermal_conductivity
      line: Wikipedia gives a thermal conductivity of 150 W/(m·K), with no temperature stated.
    - label: Stiffness
      value: 380 GPa Young's modulus
      benchmark: elastic_modulus
      line: Wikipedia gives a Young's modulus of 380 GPa and a bulk modulus of 275 GPa, with no temperature stated.
    - label: Resist acids
      value: Not attacked by most acids
      line: Wikipedia says rhodium is not attacked by most acids and is insoluble in nitric acid. PubChem says compact rhodium is not attacked even by aqua regia.
  compare: Within this profile, the Wikipedia density of 12.423 g/cm3 and the PubChem density of 12.41 g/cm3 agree closely, while the two electrical resistivity figures (43.3 nΩ·m in Wikipedia and 4.51 µΩ·cm in PubChem, both at 0 °C) do not. The melting point of 1964 °C and the boiling point of 3695 °C come from Wikipedia, and PubChem agrees on the melting point. The thermal conductivity of 150 W/(m·K) is the only figure for that property in this profile.
  compare_rows:
    - label: Density
      benchmark: density
    - label: Melting point
      benchmark: melting_point
    - label: Thermal conductivity
      benchmark: thermal_conductivity
  handling:
    - Keep dust down. Use a particulate respirator where dust is generated in large quantities.
    - Store it sealed, dry and away from oxidising agents, strong acids and foodstuffs.
    - Keep powder away from halogens and interhalogens, which react violently with it.
    - Check the supplier SDS for the exact product, because the classifications differ between sources.
  good_for:
    - 'Thermocouples and high-temperature sensing: platinum-rhodium thermocouples were used to measure up to 1800 °C by the early 1900s (Wikipedia).'
    - 'Reflective optics: high-reflectivity mirrors for projectors and searchlights (PubChem), and first-mirror coatings for ITER diagnostics (Polyteknik).'
    - 'Corrosion-resistant surfaces: not attacked by most acids, and does not normally form an oxide when heated, according to Wikipedia.'
  watch_out:
    - 'Cost and price swings: the USGS price rose 24 % in 2025, to an estimated 5,800 dollars per troy ounce, from 4,660.44 in 2024.'
    - 'Hazard classifications disagree: the REACH dossier says not classified, PubChem reports H228 and H334, and the Implats SDS reports Danger.'
    - 'Low solar reflectance: the 1987 NASA test ruled rhodium out for solar dynamic mirrors, despite its corrosion resistance.'
    - The electrical resistivity values differ between Wikipedia (43.3 nΩ·m at 0 °C) and PubChem (4.51 µΩ·cm at 0 °C), and this is not resolved.
  verdict: Rhodium is a dense, corrosion-resistant precious metal with a melting point of 1964 °C. Its main use is automotive catalysis, but in the fields of this collection it is used for thermocouples, reflective mirrors and fusion-mirror coatings. It survived an air-plasma test in 1987, but its low solar reflectance ruled it out for solar dynamic mirrors, and no flight data were found. The hazard classification is not settled, and this profile has no grade designations, country shares or mechanical strength data.