Uses Of Noble Metals

Groups of elements on the periodic table of elements have earned nicknames based on shared characteristics. For example, the last group, Group VIII, were nicknamed the noble gases because they did not easily combine with other elements, like nobility refusing to mix with non-nobles. In a similar line of thought, the noble metals earned their nickname by resisting attack by heat and oxygen.

The noble metals consist of silver, gold, platinum, rhodium, iridium, palladium, ruthenium and osmium. Some lists include rhenium as well. The noble metals include those metals that resist oxidation, even when heated. Oxidation means combining with oxygen. In other words, these metals resist rusting. The relatively inert nature of the noble metals makes them particularly useful in many applications.

The precious metals are a subset of the noble metals. While the precious metals gold, silver, platinum, iridium, palladium and sometimes rhodium can be found in jewelry, the most commonly used precious metals are gold, silver and platinum. Gold and silver, along with copper, also are called the coinage or currency metals because of their use in making coins.

Besides resistance to heat and oxidation, gold is malleable (able to be flattened into sheets) and ductile (able to be drawn into wire). These properties make gold very useful in electronics, especially in microelectronics, as contacts, leads and sometimes wires. Gold also resists bacteria, which explains the use of gold alloys in dentistry. However, the high price of gold restricts the use of gold to mostly storing wealth and making coins and jewelry.

Silver is also malleable and ductile, but not quite as much as gold. Like gold, silver is used for jewelry and coins, but silver tarnishes (oxidizes) more than gold. Silver also is less expensive than gold. Despite these limitations, or maybe because of these characteristics, silver has more commercial uses than gold. One of the dental alloy types used for decades consists of silver, copper, zinc and other metals held together by liquid mercury. Silverware once was actually made from silver, but modern silverware is much more likely to be silver-plated, where a thin layer of silver covers less expensive metals.

Silver dissolves much more easily in acids than gold does. Silver reacts with nitric acid to form silver nitrate, which acts as a potent antiseptic, even used as drops in a newborn human's eyes to prevent possible infections from the birth canal. Additional reactions form silver compounds used to plate silver, develop photographs, "silver" the backs of mirrors, and make photosensitive cathodes and alkaline battery cathodes.

Platinum's color and durability makes it an attractive choice for jewelry. Platinum is sometimes alloyed with gold to make "white gold," which is used in dental work as well as jewelry. Platinum's hardness and resistance to reactions with other materials makes platinum useful in making chemical equipment like crucibles and evaporating dishes. Platinum commonly serves as a catalyst (a chemical that triggers but doesn't take part in a reaction) in the petrochemical industry and in the manufacturing of sulfuric acid as well as fuel cells and catalytic converters. Platinum, despite its cost and rarity, is used as a coating for missile cones and jet engine fuel nozzles. Platinum also is used for thermocouple wires, electrical contacts, corrosion-resistant apparatus and platinum resistance thermometers for temperature-controlled furnaces. Even mundane objects like spark plugs, cigarette lighters and hand warmers may contain a tiny amount of platinum. Some cancer treatments use platinum.

The six transition elements in Group VIII of the periodic table are known collectively as the platinum metals (ruthenium, rhodium, palladium, osmium, iridium and platinum). The similar properties of these metals mean they have similar uses. Like platinum, rhodium, iridium and palladium are used for jewelry, although not as often.

Palladium also can be found in vehicle emission systems, electronics and fuel cells. Ruthenium is used as a catalyst and alloy to harden platinum and palladium. Rhodium is used in mammography systems, aircraft spark plugs and fountain pens. Osmium, the heaviest of the naturally occurring elements, appears in surgical implants, electrical contacts and fountain pen tips.

Iridium may be best known to some as the element marking the K-T (Cretaceous-Tertiary) boundary. This iridium layer suggests that a very large meteor may have participated in the extinction of about 80 percent of Earth's animal species at the end of the Mesozoic because asteroids and meteorites contain much higher percentages of iridium than the Earth's crust. Iridium also can be found in X-ray telescopes, rayon fiber manufacturing equipment, deep-water pipes and as crystals in computer memory chips.

Small amounts of rhenium, the last naturally occurring element discovered, are combined with nickel in jet engines. Rhenium isotopes are used to treat liver cancer.