platinum

platinum. A dense, malleable, and highly unreactive transition metal with a distinctive silvery-white appearance. It is a member of the platinum group metals, which also includes ruthenium, rhodium, palladium, osmium, and iridium. Renowned for its exceptional resistance to corrosion and high melting point, it is a precious metal of significant industrial and economic importance, widely used in catalytic converters, jewelry, and laboratory equipment.

Properties

Platinum possesses several notable physical and chemical characteristics that define its utility. It is one of the densest elemental metals, with a value exceeding that of lead and gold, and has a remarkably high melting point, surpassed among pure metals only by a few like tungsten and rhenium. Chemically, it is extremely inert, resisting attack by most reagents including hydrochloric acid, nitric acid, and even aqua regia under standard conditions, though it will dissolve in hot aqua regia to form chloroplatinic acid. This inertness stems from its strong covalent bonding and high reduction potential. The metal exhibits excellent catalytic properties, particularly in reactions involving hydrogen, such as in the Ostwald process for producing nitric acid and in fuel cell electrodes. Its crystal structure is face-centered cubic, contributing to its ductility and malleability.

Occurrence and production

Native platinum is rarely found in pure form and typically occurs as an alloy with other platinum group metals, often within nickel and copper ores. Major geological deposits are associated with mafic and ultramafic igneous rocks. The primary sources are located in the Bushveld Igneous Complex in South Africa, which dominates global supply, along with significant deposits in the Norilsk region of Siberia and the Sudbury Basin in Canada. Extraction is complex and costly, often involving froth flotation, smelting in electric furnaces, and a series of chemical processes including dissolution in aqua regia and precipitation. Refining is frequently carried out by specialized companies like Anglo American Platinum and Impala Platinum. Annual production is measured in only a few hundred tons, making it far rarer than gold.

Applications

The use of platinum spans diverse fields due to its unique properties. Its most significant application is in catalytic converters for automobiles, where it facilitates the conversion of harmful exhaust gases like carbon monoxide and nitrogen oxides into less toxic substances. The chemical industry employs it as a catalyst in critical processes, including the production of silicones and in petroleum refining for catalytic reforming. In electronics, it is used in hard disk drive coatings, thermocouples, and electrodes. The jewelry sector values it for its lustrous, hypoallergenic, and tarnish-resistant qualities, often alloyed with iridium or cobalt for strength. Other important uses include laboratory crucibles, pacemaker electrodes, and in anticancer drugs such as cisplatin and carboplatin.

History

Archaeological evidence suggests pre-Columbian South American cultures used platinum alloys. The first European description is credited to Antonio de Ulloa following his expedition with the French Academy of Sciences to Peru. Initially, Spanish metallurgists in the 18th century considered it a nuisance in silver mining, referring to it as "little silver" or *platina*. Its scientific investigation was advanced by William Hyde Wollaston, who developed a method for its purification and isolated palladium and rhodium in the process. The metal gained prominence in the 19th century, with standards like the International Prototype Metre and the International Prototype Kilogram being crafted from a platinum-iridium alloy. The establishment of the Johnson Matthey company was pivotal in its commercial refinement and distribution.

Biological role and precautions

Platinum has no known native biological role in any organism. However, certain coordination complexes, notably cisplatin, are critically important in chemotherapy, where they inhibit the division of cancer cells by binding to DNA. Despite this therapeutic use, soluble platinum salts, such as hexachloroplatinic acid, can pose health hazards, potentially causing allergic reactions, respiratory issues like platinosis, and damage to kidneys and the nervous system. Occupational safety is managed by agencies like the Occupational Safety and Health Administration, which sets exposure limits for dust and soluble salts. Environmental release primarily occurs from automotive catalytic converters, though the metal is generally considered inert in the environment.

Category:Chemical elements Category:Transition metals Category:Precious metals