Rhodium
Also: Rh
Rhodium (Rh) is a silvery-white platinum group precious metal used primarily as a catalyst in vehicle exhaust systems and for surface finishing, and is among the rarest and most expensive metals on Earth.
Rhodium (chemical symbol Rh, atomic number 45) belongs to the group of platinum group metals and was discovered in 1803 by the British chemist William Hyde Wollaston. The name derives from the Greek word rhodon (rose) — named after the rose-coloured solution that rhodium salts form in hydrochloric acid. The metal is silvery-white, extraordinarily corrosion-resistant, and is barely attacked even by aqua regia.
Occurrence and Extraction
Rhodium is among the rarest stable elements in the Earth's crust. It occurs almost exclusively as a by-product in the mining of platinum and palladium. More than 80% of the world's annual production comes from the Bushveld Complex mines in South Africa; further quantities are supplied by Russian nickel ore deposits and Zimbabwean platinum mines. Global annual output rarely exceeds 30 tonnes — for comparison: gold is produced at around 3,500 tonnes per year.
Extraction is complex: raw platinum concentrate undergoes several hydrometallurgical separation steps in a refinery before pure rhodium with a fineness of 99.9% is isolated.
Industrial Significance
By far the most important field of application is the three-way catalytic converter in the automotive sector. Rhodium reduces nitrogen oxides (NOₓ) to nitrogen and oxygen, and in this role cannot be fully replaced by any other metal. Around 80–85% of world rhodium demand is accounted for by this sector.
| Application area | Share of consumption (approx.) |
|---|---|
| Vehicle catalytic converters (NOₓ reduction) | 80–85% |
| Chemical industry (acetic acid synthesis) | 5–8% |
| Glass production (spinnerets) | 3–5% |
| Jewellery / surface finishing (rhodium plating) | 2–3% |
| Other (electronics, laboratory) | < 2% |
In the jewellery industry, rhodium is electroplated as an ultra-thin protective layer onto white and yellow gold as well as silver (rhodium plating). The layer gives the jewellery a radiantly white lustre and protects against tarnishing.
Pricing and Volatility
The rhodium price is not traded on a regulated exchange such as the COMEX, but is set by a small number of large traders and refineries in an OTC market (over the counter). This circumstance — combined with the extremely limited supply — leads to a volatility that significantly exceeds that of other precious metals.
The historical price trend illustrates this vividly:
- Around 2008, rhodium briefly reached over USD 10,000 per troy ounce, then fell to below USD 1,000.
- In spring 2021, rhodium quoted at just under USD 30,000 per troy ounce, at an all-time high, before correcting back to under USD 5,000 by 2023.
Current historical price trends as well as the daily GBP/USD exchange rates are essential for valuing rhodium positions, as rhodium is quoted internationally in US dollars.
Rhodium as an Investment Asset
Rhodium is not a classic investment precious metal like gold or platinum. There are no exchange-traded standard products (ETFs, ETCs) and hardly any standardised bars. The market is extremely illiquid: small shifts in automotive demand or production outages in South Africa can multiply or halve the price within a few weeks.
Aspects relevant to private investors in the United Kingdom (not a substitute for tax or investment advice):
- Rhodium is subject to VAT at 20% on physical purchase — it does not qualify as investment gold and is not VAT-exempt.
- Gains on the disposal of rhodium may be subject to Capital Gains Tax above the annual exempt amount; there is no German-style one-year speculation period in the UK. The gain equals disposal proceeds minus acquisition cost. Professional advice is required for individual tax and investment decisions.
Physical Properties at a Glance
Density: 12.41 g/cm³ (platinum: 21.45 | gold: 19.32)
Melting point: 1,964 °C (higher than platinum and palladium)
Reflection: ~80% (silver: ~95%, platinum: ~70%)
Mohs hardness: 6.0 (gold: 2.5 — significantly harder)
The high hardness makes rhodium more resistant than all other platinum group metals and is a main reason for its use as a protective coating in rhodium plating.
In Brief
Rhodium is the rarest and most price-volatile platinum group precious metal, whose key industrial role in the vehicle catalytic converter almost entirely determines its demand — and thus its price. Anyone monitoring current platinum prices or palladium prices should always keep an eye on rhodium as a closely linked PGM.