Gold

Gold: the essentials

 Brief description: gold is usually alloyed in jewellery to give it more strength, and the term carat describes the amount of gold present (24 carats is pure gold). It is estimated that all the gold in the world, so far refined, could be placed in a single cube 60 ft. on a side. It is metallic, with a yellow colour when in a mass, but when finely divided it may be black, ruby, or purple.

It is the most malleable and ductile metal; 1 ounce (28 g) of gold can be beaten out to 300 square feet. It is a soft metal and is usually alloyed to give it more strength. It is a good conductor of heat and electricity, and is unaffected by air and most reagents.

Gold is readily available commercially and its price (see the UtiliseGold directory) changes day by day and is one of the most widely tracked commercial prices.

The most common gold compounds are auric chloride (AuCl₃) and chlorauric acid (HAuCl₄). A mixture of one part nitric acid with three of hydrochloric acid is called aqua regia (because it dissolved gold, the King of Metals). It is unaffected by air and most reagents. It is found free in nature and associated with quartz, pyrite and other minerals. Two thirds of the world's supply comes from South Africa, and ²/₃ of USA production is from South Dakota and Nevada. Gold is found in sea water, but no effective economic process has been designed (yet) to extract it from this source.

Cartoon by Nick D Kim (nearingzero.net, used by permission).

Isolation

Isolation: it would not normally be necessary to make gold in the laboratory as it is readily available commercially. The most romantic way to extract gold is by panning it out from a stream in some pleasant valley but most such sources are now depleted. Panning relies upon the density of gold (which is very high) being much greater than that of the sand and other particulates. It therefore settles to the bottom of the pan.

Today, more often than not, gold is extracted from ores. These ores often contain relatively little gold. Some of these processes cause environmental concern. The ore is crushed to a powder so as to expose the small gold particles. These are dissolved by treatment of the rock with cyanide solution in air. The result of this is a gold cyanide complex. Addition of zinc powder to the resulting solution precipitates out the gold.

4Au + 8NaCN + O₂ + 2H₂O → 4Na[Au(CN)₂] + 4NaOH

2Na[Au(CN)₂] + Zn → 2NaCN + Zn(CN)₂ + Au (s)

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