Rhodium
Also: Rh
Rhodium (Rh) is a silver-white platinum group precious metal used primarily as a catalyst in vehicle exhaust systems and for surface finishing; it is among the rarest and most expensive metals on Earth.
Rhodium (chemical symbol Rh, atomic number 45) belongs to the platinum group metals and was discovered in 1803 by British chemist William Hyde Wollaston. Its name derives from the Greek word rhodon (rose) — named after the rose-coloured solution that rhodium salts form in hydrochloric acid. The metal is silver-white, extraordinarily corrosion-resistant, and is barely attacked even by aqua regia.
Occurrence and Production
Rhodium is one of the rarest stable elements in the Earth's crust. It occurs almost exclusively as a byproduct of platinum and palladium mining. More than 80% of global annual production comes from the Bushveld Complex mines in South Africa; additional quantities are supplied by Russian nickel ore deposits and Zimbabwean platinum mines. Global annual production rarely exceeds 30 tonnes — by comparison, gold is produced at around 3,500 tonnes per year.
Production is complex: crude platinum concentrate undergoes several hydrometallurgical separation steps at a refinery before pure rhodium with a fineness of 99.9% is isolated.
Industrial Significance
By far the most important application is the three-way catalytic converter in the automotive sector. Rhodium reduces nitrogen oxides (NOₓ) to nitrogen and oxygen, and cannot be fully replaced by any other metal in this function. Around 80–85% of global rhodium demand comes from this sector.
| Application | Share of consumption (approx.) |
|---|---|
| Vehicle catalysts (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 gold, yellow gold, and silver (rhodium plating). The layer gives jewellery a brilliantly white shine and protects against tarnishing.
Price Formation and Volatility
The rhodium price is not traded on a regulated exchange like COMEX, but is set by a small number of major dealers and refineries in an OTC market (over the counter). This fact — combined with the extremely limited supply — leads to volatility that far exceeds that of other precious metals.
Historical price trends illustrate this strikingly:
- Around 2008, rhodium briefly reached over 10,000 USD per troy ounce, then fell below 1,000 USD.
- In spring 2021, rhodium was quoted at nearly 30,000 USD per troy ounce, an all-time high, before correcting back below 5,000 USD by 2023.
Current historical price charts and daily EUR/USD exchange rates are essential for evaluating rhodium positions, as rhodium is quoted internationally in US dollars.
Rhodium as an Investment
Rhodium is not a classic investment precious metal like gold or platinum. There are no exchange-traded standard products (ETFs, ETCs) and virtually no 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 matter of weeks.
Aspects relevant to private investors (not a substitute for tax or investment advice):
- In Germany, rhodium is subject to VAT on physical purchases — it does not qualify as investment gold within the meaning of § 25c UStG.
- The speculative holding period (§ 23 EStG) is one year; gains within this period are subject to income tax. 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)
Reflectivity: ~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 primary 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 industrial key role in vehicle catalysts almost entirely determines its demand — and therefore its price. Anyone monitoring current platinum prices or palladium prices should always keep rhodium in view as a closely linked PGM.