Platinum (Pt)

Platinum's Spanish name was an insult. Colonial prospectors panning the rivers of the Chocó in what is now Colombia kept finding heavy, pale, stubborn grains mixed in with their gold, and called the stuff platina — the diminutive of plata, silver. Little silver. Not-quite-silver. Worthless.

Worse than worthless, from the crown's point of view. Platinum's density is close enough to gold's that a counterfeiter could pad gold coin and bullion with it, and the authorities in New Granada responded by ordering seized platinum thrown back into the rivers under supervision. An element with a melting point most eighteenth-century furnaces could not reach spent a century being treated as contamination.

Metalsmiths had already solved it, a thousand years earlier

The dismissal is more embarrassing given what had happened on the same coast much earlier. Craftsmen of the La Tolita culture, in coastal Ecuador and southern Colombia between roughly 100 BCE and 400 CE, made nose ornaments and masks in which platinum and gold appear together in a deliberate white-and-yellow contrast. They could not melt platinum either. Instead they mixed platinum grains with gold dust and heated the mixture until the gold flowed and cemented the grains into a workable mass — sintering, arrived at empirically, two millennia before European metallurgy managed anything comparable.

The European route in was slow and contested. Antonio de Ulloa, a Spanish naval officer attached to the French Geodesic Mission to measure the shape of the Earth, encountered the metal in the 1730s and published a description in 1748. Charles Wood independently brought samples from Jamaica to Britain around 1741, and William Brownrigg presented them to the Royal Society in 1750. Which of these counts as the discovery depends on whether you credit the first published description or the first specimens placed in front of chemists.

Wollaston's secret process

Platinum became a commercial material because of one man's manufacturing trick, kept deliberately hidden for a quarter of a century.

William Hyde Wollaston worked out, around 1800, how to turn crude platinum ore into a dense, malleable, forgeable metal without ever melting it: dissolve, precipitate as ammonium chloroplatinate, decompose to a fine sponge, then compact and work the sponge hot. It was powder metallurgy long before the term existed, and it made him rich supplying platinum vessels to the sulfuric acid industry, which needed containers that acid would not eat.

He described the method publicly only in his Bakerian Lecture of 1828, dictated as he was dying. Working through the same ores he had also pulled out palladium and rhodium — the insoluble residue from the same batches went to his colleague Smithson Tennant and yielded osmium and iridium.

Russia found placer platinum in the Urals in the 1820s and did something no other country has done before or since: minted it into circulating coinage. Platinum roubles were struck from 1828 to 1845 and then withdrawn, because the metal's market value had climbed past the denomination stamped on the coins.

The instrument that defines the temperature scale

Platinum's electrical resistance changes with temperature in a way that is smooth, repeatable and almost perfectly reproducible between one carefully made sensor and another. That reliability is why the standard platinum resistance thermometer is not merely a good thermometer but the defining one: across most of the International Temperature Scale of 1990, from the triple point of hydrogen at 13.8033 K up to the freezing point of silver at 1234.93 K, temperature is realised by interpolating the resistance of a platinum sensor between fixed points. Above and below that band other instruments take over.

Platinum held a second metrological job for longer. The International Prototype Metre, adopted in 1889, was an X-sectioned bar of 90% platinum and 10% iridium, chosen for stability and stiffness, and it defined the metre until krypton light replaced it in 1960.

Most of it goes into exhaust pipes

The single largest use is catalysis in vehicle emissions control. A three-way catalyst oxidises carbon monoxide and unburnt hydrocarbons while reducing nitrogen oxides, and platinum is the workhorse for the oxidation side — dominant in diesel aftertreatment, where palladium performs less well. Autocatalysts consistently account for the largest slice of annual platinum demand, and the value locked in that thin washcoat is high enough to have created a global trade in stolen converters.

Beyond exhaust, platinum's industrial jobs cluster around chemistry that would destroy anything else:

• Platinum–rhodium gauze in the Ostwald process, where ammonia is oxidised on its way to nitric acid and thus to most of the world's fertiliser and explosives.
• Bushings of platinum–rhodium alloy through which molten glass is drawn into fibre, because the alloy holds its dimensions at temperature and does not contaminate the melt.
• Electrodes and crucibles in laboratories and in crystal growth.
• Proton-exchange-membrane fuel cells and electrolysers, where platinum catalyses the oxygen reaction. Loadings have fallen by roughly an order of magnitude over two decades, and they need to fall further before hydrogen at scale stops being a platinum problem.

A bacterium that stopped dividing

In 1965 Barnett Rosenberg at Michigan State was studying whether electric fields affect cell division, running current through a bacterial culture using platinum electrodes chosen because platinum is inert. The bacteria stopped dividing and grew into long filaments instead. The field had nothing to do with it: the "inert" electrodes were shedding traces of platinum, and those traces were forming a compound that interfered with DNA replication.

The compound was cis-diamminedichloroplatinum(II) — cisplatin — first synthesised in 1845 and sitting unremarked in the literature ever since. Approved in 1978, it turned metastatic testicular cancer from a disease that killed roughly nine patients in ten into one that cures better than nine in ten. With its successors carboplatin and oxaliplatin, platinum chemistry now appears in a substantial fraction of all chemotherapy regimens given anywhere.

One geological body, most of the world's supply

Around 70% of mined platinum comes from a single intrusion: the Bushveld Complex in South Africa, a layered body of igneous rock roughly two billion years old whose Merensky Reef, UG2 chromitite and Platreef horizons are only centimetres to a few metres thick and are mined kilometres down. Russia's Norilsk operations, where platinum-group metals come out alongside nickel, supply most of the remainder.

That concentration makes platinum supply hostage to South African electricity and labour relations in a way few commodities are. It also means secondary supply matters: recycled autocatalysts already meet a significant share of demand.

Of platinum's six naturally occurring isotopes, one is not stable. Platinum-190 alpha-decays to osmium-186 with a half-life around 650 billion years, slowly enough to be irrelevant to anyone handling the metal and useful to geochemists dating platinum-group ore deposits.