Aqua regia

Aqua regia (/ˈreɪɡiə, ˈriːdʒiə/; from Latin, lit. 'regal water' or 'king's water') is a mixture of nitric acid and hydrochloric acid, optimally in a molar ratio of 1:3. Aqua regia is a yellow-orange (sometimes red) fuming liquid, so named by alchemists because it can dissolve the noble metals gold and platinum, though not all metals.A platinum Soviet commemorative coin being dissolved in aqua regia.4 days later. Aqua regia (/ˈreɪɡiə, ˈriːdʒiə/; from Latin, lit. 'regal water' or 'king's water') is a mixture of nitric acid and hydrochloric acid, optimally in a molar ratio of 1:3. Aqua regia is a yellow-orange (sometimes red) fuming liquid, so named by alchemists because it can dissolve the noble metals gold and platinum, though not all metals. Aqua regia is primarily used to produce chloroauric acid, the electrolyte in the Wohlwill process for refining the highest quality (99.999%) gold. Aqua regia is also used in etching and in specific analytic procedures. It is also used in some laboratories to clean glassware of organic compounds and metal particles. This method is preferred over the more traditional chromic acid bath for cleaning NMR tubes, because no traces of paramagnetic chromium can remain to spoil spectra. While chromic acid baths are discouraged because of the high toxicity of chromium and the potential for explosions, aqua regia is itself very corrosive and has been implicated in several explosions due to mishandling. Due to the reaction between its components resulting in its decomposition, aqua regia quickly loses its effectiveness (yet remains a strong acid), so its components are usually only mixed immediately before use. While local regulations may vary, aqua regia may be disposed of by careful neutralization, before being poured down the sink. If there is contamination by dissolved metals, the neutralized solution should be collected for disposal. Aqua regia dissolves gold, though neither constituent acid will do so alone, because, in combination, each acid performs a different task. Nitric acid is a powerful oxidizer, which will actually dissolve a virtually undetectable amount of gold, forming gold ions (Au3+). The hydrochloric acid provides a ready supply of chloride ions (Cl−), which react with the gold ions to produce tetrachloroaurate(III) anions, also in solution. The reaction with hydrochloric acid is an equilibrium reaction that favors formation of chloroaurate anions (AuCl4−). 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. Appropriate equations are: If the aqua regia solution only contains gold, solid tetrachloroauric acid may be prepared by boiling off excess aqua regia, and removing residual nitric acid by repeatedly heating with hydrochloric acid. That step reduces nitric acid (see decomposition of aqua regia). If elemental gold is desired, it may be selectively reduced with sulfur dioxide, hydrazine, oxalic acid, etc. The equation for the reduction of gold by sulfur dioxide is: Similar equations can be written for platinum. As with gold, the oxidation reaction can be written with either nitric oxide or nitrogen dioxide as the nitrogen oxide product.