Gold

Chemical Formula

Background

Known and highly valued since ancient times, gold is found in nature as the free metal and in tellurides. It is widely distributed and is almost always associated with quartz or pyrite. It occurs in veins and alluvial deposits, and is often separated from rocks and other minerals by sluicing and panning operations. The metal is recovered from its ores by cyaniding, amalgamating and smelting processes, while refining is carried out by electrolysis.

Gold in its pure state is considered the most beautiful and hence most valuable of the pure metals. It is metallic, having a yellow colour when in mass, though when finely divided it may be black, ruby or purple.

Gold is soft, ductile and the most malleable of metals, and because of this is usually alloyed to give improved strength and durability. Gold’s reflectivity of ultraviolet and visual light rays is low, however it has high reflectivity of infrared and red wavelengths.

Gold is a good conductor of heat and electricity, and is unaffected by air, nitric, hydrochloric, or sulphuric acid and most other reagents. A mixture of one part nitric acid with three parts hydrochloric acid, called aqua regia, dissolves gold. It is also dissolved by solutions of azoimide. Further, it is attacked by sodium, and potassium cyanide plus oxygen.

The most common gold compounds are auric chloride (AuCl₃) and chlorauric acid (HAuCl₄).

Gold has eighteen isotopes.

Applications

Gold is chiefly used for:

· Coinage

· Ornaments

· Jewellery

· Gilding

Green gold is generally used for jewellery. Green gold is an alloy of gold, silver and copper, and is graded from 14 to 18 karats. Coinage gold (USA) or standard gold (UK) contains gold and copper in slightly varied amounts, while Australian gold (Australia) contains silver instead of copper.

Other uses for gold include:

· Textile industry

· Gold flake is used for a radiation-control coating for spacecraft

· In electronic tubes, as gold-plated grid wire, to give high conductivity and suppressing secondary emissions

· Gold powder and gold sheet is used for soldering semiconductors, with gold having a good ability to wet silicon at 371°C (725°F)

· Gold is used as a plating material, where sodium gold cyanide [NaAu(CN)₂] is used as a gold plating solution. The plating has good chemical resistance and electrical properties, however the plating lacks wear resistance, in which case gold-indium plate is utilised.

Gold alloys also have a number of applications such as:

· Gold-gallium and gold-antimony are used in electronic industry (primarily as wire)

· Gold is used for dental applications and is rightly termed dental gold, where gold is alloyed with silver, platinum and on occasion palladium. It is sometimes alloyed with iridium for hardening.