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Aqua regia (ˈreɪɡiə,_ˈriːdʒiə; from Latin, literally "regal water" or "royal water") is a mixture of nitric acid and hydrochloric acid, optimally in a molar ratio of 1:3. Aqua regia is a fuming liquid. Freshly prepared aqua regia is colorless, but it turns yellow, orange or red within seconds from the formation of nitrosyl chloride and nitrogen dioxide. It was named by alchemists because it can dissolve the noble metals gold and platinum, though not all metals. Upon mixing of concentrated hydrochloric acid and concentrated nitric acid, chemical reactions occur. These reactions result in the volatile products nitrosyl chloride and chlorine gas: as evidenced by the fuming nature and characteristic yellow color of aqua regia. As the volatile products escape from solution, aqua regia loses its potency. Nitrosyl chloride (NOCl) can further decompose into nitric oxide (NO) and elemental chlorine (): This dissociation is equilibrium-limited. Therefore, in addition to nitrosyl chloride and chlorine, the fumes over aqua regia also contain nitric oxide (NO). Because nitric oxide readily reacts with atmospheric oxygen, the gases produced also contain nitrogen dioxide, (red fume): Aqua regia is primarily used to produce chloroauric acid, the electrolyte in the Wohlwill process for refining the highest purity (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 among most 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. Because its components react quickly, 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.

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Aqua regia

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