Rhodium (Rh) is a silver-white, rare precious metal in the platinum metal group. It is rarer than gold, silver, and platinum, which leads to a high price point. Rhodium is often found as a byproduct of copper and nickel mining but is sometimes present with other platinum metals such as palladium. South Africa is the leading provider of this scarce metal, but Russia and Canada are also significant suppliers. Industries such as automotive and jewelry manufacturers use most of the rhodium produced.
Due to the metal’s rarity and high price, investing in rhodium or rhodium bullion is not common. It is not considered an investment item, and most physical products you can buy are jewelry or plated coins, such as the 2022 Samoa 1 oz Silver 2 Tala Light of Christ. However, some private mints, like the Baird Mint in London, produce rhodium bars. Overall, it is a niche investment with a small market that does not make it a great store of wealth like other precious metals.
Rhodium’s atomic number is 45 and it has only a single naturally occurring isotope, but it also has 25 radioactive isotopes. William Hyde Wollaston discovered rhodium in 1803, shortly after he identified another previously unknown metal, palladium, in ore from South America.
Because of the refining process Wollaston used, which included Aqua Regia, rhodium’s first appearance was as rose red crystals, sodium rhodium chloride (Na3RhCl6). He eventually isolated and produced a sample of the metal from these crystals and named it rhodium from the Greek word for rose. The result was a hard, silver, and shiny metal with exciting potential for industrial use.
What is Rhodium Used for?
This silver-white transition metal is rare and corrosion-resistant. Because of its brilliance, it is often electroplated onto jewelry and other metals, like coins, although it is most likely to be used for commemorative or collector pieces.
The electroplated items are polished to provide an attractive and permanent surface.
Rhodium plating produces jewelry and other items that are more scratch resistant than most other metals. It is primarily alloyed with silver-colored metals like silver, white gold, and palladium.
While diamonds and other precious gems look beautiful on rhodium rings and jewelry, it requires occasional professional polishing.
Another application of rhodium’s brilliance and reflectivity is its use as a reflecting surface in optical equipment.
When rhodium is added in small quantities to platinum, the resultant alloy is even stronger and loses mass under high heat more slowly than pure platinum. This alloy is useful as a furnace crucible, a catalyst in high-heat environments like an engine, and as spark plug electrodes.
How is Rhodium Different from Platinum?
Rhodium is a member of the platinum group, and while they share similarities, rhodium differs from platinum in several ways. Rhodium is more robust in all ways except for density.
Melting Point of Rhodium
Rhodium can withstand greater heat and melts at a higher temperature than platinum. The melting point of rhodium is 3,567 degrees Fahrenheit. The melting point of platinum is 3,214.9 degrees Fahrenheit.
Thermal Conductivity of Rhodium
A material’s ability to conduct heat is measured in watts per meter-kelvin. This represents the amount of heat a metal can conduct with an area and thickness of one meter when there is a temperature difference of one kelvin between the two sides of the material.
The difference in thermal conductivity between platinum and rhodium is 78 W/mk. Rhodium can conduct 150 watts per meter-kelvin, and platinum’s conductivity is 72 W/mk.
Density of Rhodium
Platinum is almost double the density of rhodium. The measured density of platinum is 21.09 g/cm³, while the density of rhodium is 12.45 g/cm³.
For context, water has a density of 1 g/cm³, which means water has a mass of 1 gram for each cubic centimeter of volume. Therefore, platinum has a mass of 21.09 grams, and rhodium has a mass of 12.45 grams for each cubic centimeter of volume.
Hardness of Rhodium
Rhodium is harder than platinum. Rhodium has a Vickers hardness of 1246 megapascals (MPa), and platinum has a Vickers hardness of 550 MPa.
To appreciate the difference in hardness, it is helpful to understand how hardness is tested.
The Vickers hardness test provides a way to measure the hardness of a given material. It utilizes a diamond pyramid which is pressed into the surface of the testing material with a specific level of force.
The indentation that this pyramid leaves is measured for shape and size to determine the hardness. The unit of hardness used is the Vickers Pyramid Number (HV) or the Diamond Pyramid Hardness (DPH), which is converted into units of pascals (Pa), a unit of pressure.
What does it mean that there are 696 megapascals of difference between rhodium and platinum?
One megapascal is equivalent to around ten times sea-level atmospheric pressure. Another way of thinking about it is that 1 MPa is about 145 pounds of pressure per square inch, which is 4-5 times the pressure in a car tire.
Want to read more about the science of precious metals?
