Platinum

Platinum: the essentials

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

Platinum is a beautiful silvery-white metal, when pure, and is malleable and ductile. It has a coefficient of expansion almost equal to that of soda-lime-silica glass, and is therefore used to make sealed electrodes in glass systems.

The metal does not oxidise in air. It is insoluble in hydrochloric and nitric acid, but dissolves when they are mixed as aqua regia, forming chloroplatinic acid (H₂PtCl₆), an important compound. It is corroded by halogens, cyanides, sulphur and alkalis. Hydrogen and oxygen gas mixtures explode in the presence of platinum wire.

The above shows a platinum crucible.

Isolation

Isolation: it would not normally be necessary to make a sample of platinum in the laboratory as the metal is available commercially. The industrial extraction of platinum is complex as the metal occurs in ores mixed with other metals such as palladium and gold. 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 platinum 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 palladium as well as H₂PtCl₆. The gold is removed from this solution as a precipitate by treatment with iron chloride (FeCl₂). The platinum is precipitated out as impure (NH₄)₂PtCl₆ on treatment with NH₄Cl, leaving H₂PdCl₄ in solution. The (NH₄)₂PtCl₆ is burned to leave an impure platinum sponge. This can be purified by redissolving in aqua regia, removal of rhodium and iridium impurities by treatment of the solution with sdoium bromate, and precipitation of pure (NH₄)₂PtCl₆ by treatment with ammonium hydroxide, NH₄OH. This yields platinum metal by burning.

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