# Platinum as a Metallic Material

## Platinum as a Metallic Material

Platinum is a noble metal and is currently second only to rhodium as the most expensive precious metal. It has the face centered cubic crystal structure. At room temperature, platinum is soft (HV 55) and very ductile. It can be cold worked to more than 90 % deformation by rolling and drawing. The undeformed metal has a Young’s modulus of 165 $GPa$.

Despite its high melting point, platinum recrystallizes at a temperature of 500-550 $°C$.

Fig.1
Microstructure of recrystallized platinum

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The mechanical strength of platinum is increased by additions of alloying elements such as rhodium or iridium (see Pt-Rh and Pt-Ir alloys). The strength at temperatures > 1,200 $°C$ is also increased by introducing small, finely divided oxide particles into the microstructure (oxide dispersion strengthening).

Platinum is resistant to most types of molten glass and is insoluble in acids. It only dissolves in hot hydrochloric acid containing an oxidizing agent, e.g. HNO3 (classical aqua regia) or elemental chlorine, to hexachloroplatinic(IV) acid, H2Cl6Pt·6 H2O:

$8HCl+ 2HNO3+ Pt → H2[PtCl6]+ 4H2O+ 2NOCl$

Platinum is attacked by molten alkalis, cyanides and many molten salts. Above 1,200 $°C$ it is the most oxidation resistant metal, but oxidizes slowly in air to the volatile oxides platinum monoxide, PtO, and platinum dioxide, PtO2. As a result of its resistance to chemical changes, the physical properties of platinum (mass, volume, electrical resistance, electromotive force) remain constant over very long periods of time.

Fig.2
Platinum sponge and bar

Platinum sponge from the refinery and a bar of pure platinum.

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