Palladium (Pd, atomic number 46), a distinct member of the Platinum Group Metals (PGMs), was formally discovered by the English chemist and physicist William Hyde Wollaston in the early 19th century. Wollaston recorded the new noble metal in his laboratory notes in 1802, with the official public announcement occurring in 1803. The discovery was a byproduct of a commercial venture focused on refining platinum.
Context The Search for Platinum Refinement
In the early 1800s, William Hyde Wollaston focused on developing a profitable technique to process crude platinum ore, often called platina. This raw material, imported largely from South America, was difficult to work with due to its high melting point and numerous impurities. Wollaston sought to produce a pure, malleable form of platinum for sale to manufacturers.
The crude ore was an unrefined mixture containing platinum and other chemically similar, unknown elements. To achieve commercial purity, Wollaston developed a sophisticated chemical process to separate platinum from these metallic contaminants.
The substantial financial incentive led Wollaston to partner with fellow chemist Smithson Tennant to advance their understanding of the ore. This pursuit of a commercially viable platinum refining method provided the necessary framework for the discovery of palladium and other new elements.
The Chemical Separation Method
Wollaston’s method began by dissolving the crude platinum ore in aqua regia, a potent mixture of nitric and hydrochloric acids. This solvent broke down the resistant platinum and other metals into a soluble mixture of metal salts.
Next, the acidic solution was neutralized, typically using a base like sodium hydroxide. Wollaston then added ammonium chloride, which selectively reacted with the dissolved platinum to form a solid salt called ammonium chloroplatinate. This precipitation allowed for the physical separation of most of the platinum from the remaining liquid solution, or mother liquor.
The mother liquor, largely free of platinum, still contained dissolved salts of palladium and other metals. To isolate palladium, Wollaston introduced mercuric cyanide to the solution. This reagent caused the palladium to precipitate out as palladium(II) cyanide.
Finally, the isolated palladium cyanide compound was collected and subjected to intense heat. This thermal decomposition separated the cyanide component, leaving behind the pure, elemental palladium metal. This multi-step procedure demonstrated that palladium was a distinct element, not merely an alloy or an impurity.
Public Announcement and the Element’s Name
Wollaston chose an unconventional way to announce the discovery of palladium, keeping his refining process a trade secret for commercial gain. In April 1803, he anonymously offered the new “noble metal” for sale in a London mineral dealer’s shop. A flyer was circulated, describing the metal’s properties and challenging other chemists to identify or synthesize it.
This anonymous challenge sparked controversy, particularly from the Irish chemist Richard Chenevix, who claimed the substance was merely an alloy of platinum and mercury. Chenevix received an award for his work, believing he had debunked a fraud. Wollaston stood by his discovery, anonymously offering a reward for the successful synthesis of the metal to prove his claim.
Wollaston did not publicly reveal himself as the discoverer until 1805, confirming the element’s distinct nature and his role in its isolation. The name “palladium” was chosen as a nod to astronomy, referencing the asteroid Pallas, which had been discovered in 1802.