Chloroauric acid

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Chloroauric acid (HAuCl4) is an orange solid commonly encountered in gold refinement, extraction and chemistry.

File:Chloroauric acid.png
Chloroauric acid in solution above unreacted gold

Properties

Chemical

Many reducing agents reduce chloroauric acid to metallic gold, which falls out of solution as a yellow powder. Oxalic acid and sodium metabisulfite are often used in this way to produce the pure metal from dissolved scrap gold.

Like other Au(III) compounds, reaction with gaseous or solutions of ammonia will precipitate the dangerously explosive 'fulminating gold', with explosive properties similar to silver acetylide.

Physical

While hygroscopic, solutions and the solid are stable at standard conditions. 

Availability

As expected, chloroauric acid is expensive and often only marketed for high end chemistry applications. Because of this, it is generally only avaliable from suppliers at a very high purity, driving up the price further. 

Preparation

Chloroauric acid is prepared by dissolving gold in aqua regia, a mix of concentrated hydrochloric and nitric acids. This results in an orange-yellow solution that can be dried in a dessicator. The solid is very hygroscopic, so calculating accurate yields based on weight is difficult. 

Projects

  • Refining gold from scrap

Handling

Safety

The usual precautions when dealing with strong acids should be in place, as not only is chloroauric acid a strong acid, but unless dried, solutions may contain traces of either of the two corrosive mineral acids used to make it. 

Disposal

Due the price of gold, all efforts should obviously be taken to avoid any wastage. Solutions of Au(III) are not overly toxic, but due to the ease at which it can be reduced, solutions of Au(III) should be reduced to the metal before disposal.

References

Relevant Sciencemadness threads