23. Properties of Precious Metals

Gold

(courtesy of the World Gold Council)

• Gold’s ability to conduct electricity makes it an indispensable component in electronics. Completely resistant to corrosion, it is the undisputed material of choice to guarantee reliability in a broad range of high-performance and safety-critical applications. Indeed, in recent years, the electronics sector alone has accounted for more than 300 tonnes of global annual demand, underlining its value in these applications.
• Exceptionally malleable and ductile, one ounce of gold can be beaten into a translucent sheet 0.000018 cm thick and covering 9 square metres, or pulled into a wire 80 km (50 miles) long. At 5 microns in diameter, the wire would be 20 times thinner than a human hair.
• Gold’s catalytic properties – it accelerates the rate of chemical reactions without being consumed – means that it is becoming an important component in many industrial processes. It is a proven material for use in catalytic converters, which reduce the toxicity of exhaust fumes, and plays a role in the production of a range of chemicals we all use on a day-to-day basis. The new gold catalysts that are currently being developed could reduce the impact of airborne and water-borne pollution, and the importance of gold in fuel cells puts the metal at the heart of technologies for a cleaner energy future.
• These unique qualities mean there are many industrial uses of gold, including for engineers in the aerospace and construction industries. NASA protects its astronauts and equipment from radiation and heat with a layer of gold, while a film of the metal coats all 14,000 windows of the Royal Bank Plaza building in Toronto, Canada.
• Because it does not corrode, gold has been used in dentistry for centuries, and gold alloys are still used today. The metal’s biocompatibility – it rarely has any negative effects when placed in contact with the human body and resists infection – makes it a natural choice for other sensitive implants, such as those in the inner ear or eyelids.
• The medical applications of gold are wider still. Gold-based drugs have been developed and used to treat illnesses such as rheumatoid arthritis. Research is currently ongoing into the role that gold can play in cancer treatment.
• As understanding of nanotechnology reveals gold’s unique qualities at the nano-scale, further biomedical uses are being found for the metal. It has been deployed in a range of cutting-edge techniques for diagnosing diseases.

Silver

(courtesy of the Silver Institute)

• Of all the elements, silver is the best conductor of electricity. On a scale of 0-100, silver ranks at 100, with copper at 97 and gold at 76. As such, it has a number of important applications in the electronics industry.
• Silver has excellent antimicrobial properties, and is commonly used in various health and environmental applications, including wound treatments and water purification.
• Like many other metals, silver is an effective chemical catalyst. It is central to the production of ethylene oxide, which is the chemical ‘building block’ of a range of plastics. Hundreds of millions of ounces of silver reside in chemical plants around the world for this reason.
• Silver pastes are critical in the manufacture of photovoltaic cells, which are used for the production of solar energy.
• Many lightweight, high-capacity batteries employ silver oxides or silver-zinc alloys in their design.

Platinum

(courtesy of Johnson Matthey)

• One of the densest metals (ca. 20 g cm−3).
• Genuinely rare, annual production is less than a tenth of that of gold.
• Precious: it was used by the ancient Incas to make ornaments and today is very popular for wedding rings, particularly in Asian countries.
• Its melting point is so high (1,768ºC) that it was impossible to melt until the invention of the oxygen flame and it was melted for the first time in 1782 in France.
• It is very inert: biomedical devices used inside a living body, from pacemakers to very simple bone pins to help broken bones heal correctly, use platinum so that they do not harm the tissues around them.
• Platinum does not corrode even at high temperatures, so it is used in equipment to make fibreglass and to make extremely pure glass for flat screen TVs and mobile phones.
• The largest use of platinum in the present day is in automobile exhaust catalysts, which remove harmful pollutants from vehicles.
• A cure for cancer: cisplatin contains platinum and is still one of the most effective anticancer agents of modern times.
• A platinum rod 10 cm long and 1 cm in diameter can be drawn into a wire approximately 2,777 km long – making platinum theoretically seven times more ductile than gold.

Palladium

(courtesy of Johnson Matthey)

• A precious metal that does not tarnish and is naturally ‘white’.
• Named after Pallas, the ancient Greek goddess of wisdom.
• Russia is the biggest producer and it is also produced in North America and South Africa.
• It is about half as dense as platinum or gold, which means that it can be used to make larger but lighter jewellery.
• Palladium can absorb an astonishing 900 times its own volume of hydrogen – making it great for storing hydrogen safely.
• Silver-palladium tubes are used in hydrogen generators and utilised for such diverse applications as the manufacture of semiconductors, annealing of stainless steel and cooling of power station alternators.
• The 2010 Nobel Prize in Chemistry was for palladium-catalysed cross-coupling, a process used today all over the world to manufacture products from edible oils to painkillers and novel drugs.
• Dental alloys with palladium contents of between 40% and 60% are used to make crowns and bridges with a ‘white’ colour that are compatible with currently used dental porcelains.
• Used in three-way catalysts for super ultra low emission vehicles (SULEVs), with astonishing performance and durability using relatively low loadings.