Aqua regia (Latin: royal water) or aqua regis is a highly corrosive, fuming yellow or red solution, also called nitro-hydrochloric acid. The mixture is formed by freshly mixing concentrated nitric acid and concentrated hydrochloric acid, usually in a molar ratio of 1:3 respectively. It was named so because it can dissolve the so-called royal metals, or noble metals, gold and platinum. However, ruthenium, tantalum, iridium, osmium, titanium, rhodium and a few other metals are capable of withstanding chemical attack from it.
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. (See dissolving 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. Furthermore, 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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