Palladium: the essentials

Palladium is a steel-white metal, does not tarnish in air, and is the least dense and lowest melting of the platinum group metals. When annealed, it is soft and ductile. Cold working increases its strength and hardness. It is used in some watch springs.

Ruthenium, rhodium, palladium, osmium, iridium, and platinum together make up a group of elements referred to as the platinum group metals (PGM).

At room temperatures the metal has the unusual property of absorbing up to 900 times its own volume of hydrogen. Hydrogen readily diffuses through heated palladium and this provides a means of purifying the gas.

Table: basic information about and classifications of palladium.

Palladium: historical information

Palladium was discovered by William Hyde Wollaston at 1803 in England. Origin of name: named after the asteroid "Pallas" which was discovered at about the same time and from the Greek name "Pallas", goddess of wisdom.

William Hyde Wollaston discovered palladium in 1803-4 in crude platinum ore from South America. He dissolved the ore in aqua regia (a mixture of hydrochloric and nitric acids), neutralised the acid with sodium hydroxide (NaOH), and precipitated the platinum by treatment with ammonium chloride, NH4Cl, as ammonium chloroplatinate. Palladium was then removed as palladium cyanide by treatment with mercuric cyanide. The metal was produced from this cyanide by heating.

Palladium: physical properties

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Palladium: orbital properties

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Isolation: it would not normally be necessary to make a sample of palladium in the laboratory as the metal is available commercially. The industrial extraction of palladium is complex as the metal occurs in ores mixed with other metals such as platinum. Sometimes extraction of the precious metals such as platinum and palladium is the main focus of a partiular industrial operation while in other cases it is a byproduct. The extraction is complex and only worthwhile since palladium is the basis of important catalysts in industry.

Preliminary treatment of the ore or base metal byproduct with aqua regia (a mixture of hydrochloric acid, HCl, and nitric acid, HNO3) gives a solution containing complexes of gold and platinum as well as H2PdCl4. The gold is removed from this solution as a precipitate by treatment with iron chloride (FeCl2). The platinum is precipitated out as (NH4)2PtCl6 on treatment with NH4Cl, leaving H2PdCl4 in solution. The palladium is precipitated out by treatment with ammonium hydroxide, NH4OH, and HCl as the complex PdCl2(NH3)2. This yields palladium metal by burning.

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palladium atomic number