The year 1803 represents a significant moment in chemistry, yielding the discovery of four new elements belonging to the Platinum Group Metals (PGMs). These elements—Rhodium (Rh), Palladium (Pd), Iridium (Ir), and Osmium (Os)—were all isolated from the same source of crude platinum ore. Their concurrent identification highlighted the complex nature of the ore and established the existence of this rare and valuable family of metals, prized for their resistance to chemical attack.
The Context of the 1803 Discoveries
The simultaneous discovery of these four elements resulted from a commercial venture aimed at purifying crude platinum. English chemist and physicist William Hyde Wollaston and his partner, Smithson Tennant, worked with platinum ore sourced primarily from South America. Purification involved dissolving the crude metal in aqua regia, a powerful mixture of nitric and hydrochloric acids.
This chemical treatment dissolved the bulk of the platinum and some impurities, but consistently left behind an insoluble, fine black powder residue. Wollaston focused on the liquid solution, successfully isolating two new elements. He announced the discovery of Palladium in 1802 and subsequently isolated Rhodium in 1803, naming it for the rose-red color of its compounds in solution, derived from the Greek word rhodon.
Tennant dedicated his attention to the undissolved black residue left after the aqua regia treatment. Through a series of chemical reactions, he separated this residue into two distinct metallic substances. He named one Iridium, after Iris, the Greek goddess of the rainbow, due to the striking variety of colors its compounds displayed in solution. The second element he isolated was Osmium, named from the Greek word osme, meaning “smell,” because of the distinct, chlorine-like odor of its volatile tetraoxide compound.
Defining Physical and Chemical Properties
Rhodium, Palladium, Iridium, and Osmium are classified as Platinum Group Metals (PGMs) based on their similar physical and chemical characteristics. They are dense, silvery-white transition metals with high melting points and exceptional resistance to corrosion, even at elevated temperatures. This shared chemical inertness makes them highly valued as “noble metals” that resist tarnishing and reaction with most common acids.
Among the group, certain elements possess unique properties. Osmium is the densest naturally occurring element on Earth, with a density of 22.61 grams per cubic centimeter. Iridium is similarly dense and holds the distinction of being the most corrosion-resistant metal known, making it extremely durable.
Rhodium is characterized by its high reflectivity and hardness, making it a popular choice for specialized optical applications and protective coatings. Palladium is unique for its relatively lower density and melting point compared to the other PGMs. It also has an extraordinary ability to absorb up to 900 times its own volume of hydrogen gas.
Current Commercial and Industrial Uses
The unique properties of the 1803-discovered elements have translated into diverse modern applications. The largest consumer of Rhodium and Palladium today is the automotive industry, where they are used as catalysts in three-way catalytic converters. These devices rely on the metals’ catalytic activity to convert harmful exhaust emissions, such as nitrogen oxides and carbon monoxide, into less harmful substances.
Palladium is also utilized in the electronics sector, particularly in multilayer ceramic capacitors and as an alloy in electrical contacts due to its stable electrical properties. Rhodium, due to its brightness and durability, is often electroplated onto jewelry (rhodium flashing) to provide a highly reflective, tarnish-resistant finish. It is also alloyed with platinum to create thermocouples capable of accurately measuring temperatures up to 1,800°C.
Iridium’s extreme resistance to heat and corrosion makes it indispensable for specialized, high-performance applications. It is used in the manufacture of high-temperature crucibles for growing single crystals, and its alloys are utilized in medical devices like pacemakers and stents due to their biocompatibility. Osmium’s hardness makes it suitable for use in applications requiring extreme wear resistance, such as in instrument pivots and fountain pen tips, often alloyed with Iridium.