Aqua+Regia

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Aqua Regia, also know as "King's Water" or "Royal Water", is a mixture of hydrochloric acid and nitric acid, which in turn make a combine to make a highly corrosive mixture that can dissolve gold. It is a fuming yellow liquid prepared by mixing one volume of concentrated nitric acid with three to four volumes of concentrated hydrochloric acid.===== Aqua regia and other mixtures similar to it are used in procedures for the solution of iron ores, phosphate rocks,, nickel-chromium alloys, antimony, selenium. It is also used in less water soluble sulfides such as those of mercury, cobalt, mercury, and lead.

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The fumes and yellow color are caused by reaction of nitric acid, HNO3, with hydrogen chloride, HCl, to form nitrosyl chloride, NOCl, chlorine, Cl2, and water; both chlorine and nitrosyl chloride are yellow-colored and volatile. The nitrosyl chloride further decomposes to nitric oxide, NO, and chlorine. Nitric acid is a powerful oxidizing agent, but the chemical equilibrium for its reaction with gold, Au, only permits formation of a tiny amount of Au+3 ion, so the amount of gold dissolved in pure nitric acid is undetectable. The presence of chloride=====

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ion, Cl-, allows formation of AuCl4-. Because of the high concentration of chloride ion in aqua regia, the Au+3 is reacted almost as soon as it is formed, keeping its concentration low; this allows oxidation of more Au to Au+3, and the gold is dissolved. The gold may also react directly with the free chlorine in aqua regia, since chlorine is a powerful oxidizing agent.=====

Aqua regia is primarily used to produce chloroauric acid, the electrolyte in the Wohlwill process. This process is used for refining 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 "traditional" chromic acid bath for cleaning NMR tubes, because no traces of paramagnetic chromium can remain to later spoil acquired 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. As such, its components should only be mixed immediately before use. While local regulations may vary, aqua regia may be disposed of by carefully neutralizing with an appropriate agent — such as sodium bicarbonate — before pouring down the sink. If there is a large amount of metal in solution with the acid, it may be preferable to carefully neutralize it, and absorb the solution with a solid material such as vermiculite before discarding it with solid waste.

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In World War II, George de Hevesy, a Hungarian chemist dissolved the gold Noble Prizes of Max von Laue and James Franck in aqua regia to prevent the Nazis from stealing them when Germany invaded Denmark. De Hevesy kept the melted gold solution on a shelf in his lab at the Niels Bohr Institute. It was subsequently ignored by the Nazis who thought the jar—one of perhaps hundreds on the shelving—contained common chemicals. After the war, de Hevesy returned to find the solution undisturbed and precipitated the gold out of the acid. The gold was returned to the Royal Swedish Academy of Sciences and the Nobel Foundation. They re-cast the medals and again presented them to Laue and F=====

Gold precipitate produced by the aqua regia refining process.

tin to form tin(IV) chloride, containing tin in its highest oxidation state: 4 HCl + 2 HNO3 + Sn → SnCl4 + NO2 + NO + 3 H2O
 * Reactions with other metals: **

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 chloroaurate anions, also in solution. The reaction with hydrochloric acid is an equilibrium reaction which 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 free chlorine present in aqua regia.