Rhodium is a silvery-white transition metal that holds the distinction of being the world's most expensive precious metal. It has an atomic number of 45 and is about as nonreactive as gold. The only way to dissolve rhodium is with sulfuric acid. Part of this metal's appeal comes from its high reflectance, almost unique among the metals. It is sometimes used as an expensive and flashy alternative to silver in jewelry, on which it is sometimes plated. Some of the most expensive consumer items in the world are made from rhodium.
This element was discovered in 1803 by William Hyde Wollaston, who isolated it from platinum. He named it after the Greek word rhodon, meaning rose, because of the color of the solution of salts. Wallaston was also the discoverer of palladium.
The price of rhodium is about six times more than gold by weight. It is never found in mineral form, only being found in trace amounts within platinum or nickel ores. 80% of the world's supply comes from South Africa, and world production of the metal is only about 20 tons per year.
Rhodium is a fission product in the decay of the isotope uranium-235, but only about 1% by weight, and most nuclear fuels only contain about 3% uranium-235, meaning only 1% of 3% of nuclear waste consists of this metal. Despite rhodium's great value, the cost of separating it out is generally greater.
There are 25 known isotopes of this metal in all, most with a half-life of less than an hour. This quantity of isotopes is typical among elements with atomic numbers in this range. Rhodium compounds exist, but not outside of the laboratory. They are highly poisonous.
Rhodium was made famous in 1979 when the Guinness Book of World Records awarded Paul McCartney a rhodium-plated disc to celebrate his status as history's all-time best-selling songwriter and recording artist. This metal is used when other precious metals such as silver, gold, or platinum are considered not enough.
What Is Rhodium Plated?
As previously mentioned, rhodium plating is often used on jewelry. You'll mostly find it on silver and white gold pieces to improve their durability and give them a little extra shine. Also known as rhodium dipping, this method uses electricity to apply rhodium to the existing metal.
The Right Tools for the Job
Rhodium electroplating uses several items: the rhodium itself plus activator and electrocleaner. THe process also requires gold or platinum wire, along with a platinized titanium anode and a stainless steel anode. Eye protection and ventilation are both essential to avoid injury/illness from chemicals and fumes. Before being rhodium-plated, the original jewelry piece to be impeccably clean. Even a small spot of dirt prevents rhodium plating from sticking to the metal.
The Rhodium Plating Process
The process starts with distilled water, electrocleaner and the stainless steel anode placed in one beaker. Activator powder and distilled water are poured into another beaker. A third beaker contains the platinized titanium anode and the rhodium bath solution. Both beakers are placed onto an electric burner and heated.
Next, a current is applied by an electroplating machine. The machine's positive lead connects to the stainless steel anode in the beaker with the electrocleaner. The negative lead clips onto a hooked gold or platinum wire that will hold the piece of jewelry. This item is suspended from a hook and dipped inside the electrocleaner solution. This removes any remaining debris on the piece.
Before dipping the jewelry in rhodium, it is rinsed in distilled water and then placed into the room temperature activator solution for a few minutes. After rinsing the piece again, the positive lead is connected to the anode in the rhodium bath. The jewelry is then dipped into the rhodium for a short time. Rinsing the piece off for a third time completes the process.
Black Rhodium Plating
Rhodium dipping uses either standard or black rhodium. The latter is a plating solution that also contains three additional metal alloys in trace amounts: tellurium oxide, arsenic trioxide and tin sulfate. Mixed with the rhodium bath, they create a gunmetal gray finish on jewelry instead of the silver-white finish with typical rhodium plating.
Does Rhodium Tarnish?
The Science Behind Tarnishing
Simply put, tarnish results from corrosion. It's a chemical reaction that occurs when metals like copper, aluminum or brass make contact with certain gasses or chemical compounds. While most tarnished metal surfaces exhibit a dull gray or black film, copper is one notable exception: It turns green when exposed to oxygen. Tarnish can protect the metal under the surface, but it can look unattractive depending on the type of metal.
Some precious metals can develop tarnished surfaces. Most gold jewelry isn't made from 24K gold but contains metal alloys for added durability. Pure silver on its own does not usually tarnish, but it's also too soft to use. That's why most commercially sold silver jewelry contains 92.5% silver and 7.5% other metals, usually copper.
Why Rhodium Does Not Tarnish
Rhodium is a unique metal. It's one of the hardest in existence, rated 6 on the Mohs hardness scale. That's the same as titanium and just below tungsten, which ranks at 7 on the scale. For reference, diamonds have a Mohs rating of 10.
Just because a metal is hard doesn't mean that it's a good candidate for jewelry in its pure form. Rhodium is one example. It's brittle in its native state, able to shatter almost like glass. However, it's an excellent plating for other metals like silver and gold. Besides its brilliance, it also offers more scratch resistance.
Rhodium is part of the platinum group of metals. Clustered together on the periodic table, these elements share some important characteristics: They tend to be chemically stable and have higher melting points. They also don't corrode or tarnish
Is Rhodium Hypoallergenic?
If you've ever had an allergic reaction to a piece of jewelry, chances are good that it contained nickel. Around 10% to 20% of the human population is allergic to nickel. That's because their immune systems see nickel as a foreign invader and respond, often producing red irritated skin and rashes.
Nickel is commonly combined with other metals, especially in costume jewelry. This is done to make the jewelry more durable and less likely to bend or break. For a piece to be hypoallergenic, it must either contain no nickel at all or extremely small amounts. Hypoallergenic metals and alloys include 24K gold, sterling silver, copper, titanium and rhodium.