Platinum – facts and properties

Position in the periodic table

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Occurrence and extraction

Platinum is among the scarce metals. It occurs native, usually as small grains, and bound in minerals such as sperrylite, a compound of platinum and arsenic. The largest deposits lie in South Africa, where one huge rock formation, the Bushveld Complex, supplies most of the world's output. Other important producers are Russia and Zimbabwe. Platinum is recovered together with palladium, rhodium and other platinum metals and is also a by-product of nickel and copper production. A large share of the supply now comes from recycling, above all from old catalytic converters.

Long before the Europeans arrived, people in South America worked alloys of gold and platinum. The Spanish first met the metal in Colombia, where it turned up while gold was being panned. They took it for a nuisance contaminating the gold, because it could not be melted, and called it platina, from plata, the Spanish word for silver. In 1735 the Spanish naval officer and scientist Antonio de Ulloa came across the metal on an expedition in South America and later described it for Europe. Only in the nineteenth century did chemists learn to separate platinum cleanly from gold and the other platinum metals and to work it.

Properties and reactions

Platinum is in period 6 and group 10, directly below palladium. Its electron configuration, [Xe] 4f¹⁴ 5d⁹ 6s¹, has a small oddity: one electron sits in the 5d rather than the 6s orbital. Its electronegativity comes to 2.28. Platinum has a density of 21.46 g/cm³, a melting point of 1768.4 °C and a boiling point of 3825 °C. The metal is tough and can be drawn into fine wire.

Like gold, platinum is a precious metal. It does not oxidise in air even when heated, and most acids leave it alone. It dissolves in aqua regia, a mixture of hydrochloric and nitric acid, forming hexachloroplatinic acid, H₂[PtCl₆].

Its catalytic action is especially important. On the surface of platinum, gas molecules such as hydrogen, oxygen and carbon monoxide attach themselves, their bonds are loosened, and reactions run at a much lower temperature. As early as 1823 Johann Wolfgang Döbereiner built a lighter on this principle, in which platinum sponge ignited hydrogen in air.

Uses in everyday life and technology

One of the biggest customers is the vehicle catalytic converter. In the three-way converter of a petrol car, platinum, palladium and rhodium work together: platinum and palladium oxidise carbon monoxide and unburnt hydrocarbons to carbon dioxide and water, while rhodium converts nitrogen oxides to nitrogen. In fuel cells, fine platinum particles on carbon speed up the reaction of hydrogen and oxygen to water.

In the chemical industry platinum is the catalyst of the Ostwald process. Ammonia is oxidised with air on hot gauzes of platinum and rhodium to nitrogen monoxide, from which nitric acid is later made. In the laboratory platinum electrodes are used for measurements and electrolysis because they do not react themselves, and platinum resistance thermometers measure temperature precisely. Platinum jewellery usually has a fineness of 950 parts per thousand.

Platinum has even led to a medicine. In the mid-1960s the biophysicist Barnett Rosenberg noticed that bacteria near platinum electrodes stopped dividing. A platinum complex that had formed from the metal was responsible: cisplatin, cis-[Pt(NH₃)₂Cl₂]. It binds to the genetic material of cells and keeps them from dividing, which is why it acts against tumours, for example testicular cancer. It also hits healthy cells, which explains its side effects. Only the cis isomer is active, and it is used solely in medical cancer treatment.

Quick question

What effect does a catalyst have on activation energy?

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