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An acid test is a qualitative chemical or metallurgical assay utilizing acid. Historically, it often involved the use of a robust acid to distinguish gold from base metals . Figuratively, the term represents any definitive test for attributes, such as gauging a person's character or evaluating a product's performance.
The reaction with hydrochloric acid is an equilibrium reaction that favors formation of tetrachloroaurate(III) anions. This results in a removal of gold ions from solution and allows further oxidation of gold to take place. The gold dissolves to become chloroauric acid. In addition, gold may be dissolved by the chlorine present in aqua regia.
Chloroauric acid is the product formed when gold dissolves in aqua regia. [15] On contact with water, AuCl 3 forms acidic hydrates and the conjugate base [AuCl 3 (OH)] −. A Fe 2+ ion may reduce it, causing elemental gold to be precipitated from the solution. [1] [16] Other chloride sources, such as KCl, also convert AuCl 3 into [AuCl 4] −.
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A model of a late 19th-century Canadian seal used to certify the quality of assayed gold. A metallurgical assay is a compositional analysis of an ore, metal, or alloy, usually performed in order to test for purity or quality. Some assay methods are suitable for raw materials; others are more appropriate for finished goods.
This method has been used since ancient times. In modern times, additional tests can be done. The trace will react in different ways to specific concentrations of nitric acid or aqua regia, thereby identifying the quality of the gold: 24 karat gold is not affected but 14 karat gold will show chemical activity.
Colloidal gold is a sol or colloidal suspension of nanoparticles of gold in a fluid, usually water. [1] The colloid is coloured usually either wine red (for spherical particles less than 100 nm ) or blue-purple (for larger spherical particles or nanorods ). [ 2 ]
Apart from X-ray spectrometer technique, other older traditional methods are using the TouchStone and Acid to test the gold purity. But TouchStone and Acid are destructive testing - a tiny sample of gold is cut and then tested. The sample is rubbed on TouchStone and a drop of acid is put on it and the goldsmith observes the residue using a ...
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