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Aqua Regia

Aqua regia (Latin for "royal water") is a highly corrosive, fuming yellow or red solution. The mixture is formed by freshly mixing concentrated nitric acid and concentrated hydrochloric acid, usually in a volumetric ratio of one to three respectively. It is one of the few reagents that dissolves gold and platinum. It was so named because it can dissolve the so-called royal, or noble metals, although tantalum, iridium, and a few other metals are able to withstand it.

Aqua regia is used in etching and in certain 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, ruining acquired spectra. It is important to note that chromic acid baths are discouraged because of chromium toxicity and the potential for explosions. Aqua regia is itself very corrosive and has been implicated in several explosions as well due to mishandling and it should not be used unless gentler cleaning techniques such as the use of brushes, sonication, detergents, or milder oxidisers are inadequate.

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 adsorb the solution on a solid material such as vermiculite before discarding it with solid waste. This practice should not be used when EPA regulated or otherwise toxic metals are present.

Dissolving gold
Aqua regia dissolves gold, even 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 to produce chloraurate anions, also in solution. The reaction with hydrochloric acid is an equilibrium reaction which favors formation of chloraurate anions (AuCl4-). This results in a removal of gold ions from solution and allows further oxidation of gold to take place, and so the gold is dissolved. In addition, gold may be oxidized by the free chlorine present in aqua regia. Appropriate equations are:

Au (s) + 3 NO3- (aq) + 6 H+ (aq) → Au3+ (aq) + 3 NO2 (g) + 3 H2O (l)
Au3+ (aq) + 4 Cl- (aq) → AuCl4- (aq)
The oxidation reaction can also be written with nitric oxide as the product rather than nitrogen dioxide.

Au (s) + NO3- (aq) + 4 H+ (aq) → Au3+ (aq) + NO (g) + 2 H2O (l)

[edit] Dissolving platinum
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.

Pt (s) + 4 NO 3- (aq) + 8 H+ (aq) → Pt4+ (aq) + 4 NO2 (g) + 4 H2O (l)
3Pt (s) + 4 NO 3- (aq) + 16 H+ (aq) → 3Pt4+ (aq) + 4 NO (g) + 8 H2O (l)
The oxidized platinum ion then reacts with chloride ions resulting in the chloroplatinate ion.

Pt4+ (aq) + 6 Cl- (aq) → PtCl62- (aq)
Experimental evidence reveals that the reaction of platinum with aqua regia is considerably more complex. The initial reactions produce a mixture of chloroplatinous acid (H2PtCl4) and nitrosoplatinic chloride ((NO)2PtCl4). The nitrosoplatinic chloride is a solid product. If full dissolution of the platinum is desired, repeated extractions of the residual solids with concentrated hydrochloric acid must be performed.

Pt (s) + 2 HNO3 (aq) + 4 HCl (aq) → (NO)2PtCl4 (s) + 3 H2O (l) + 1/2 O2 (g)
(NO)2PtCl4 (s) + 2 HCl (aq) → H2PtCl4 (aq) + NOCl (g)
The chloroplatinous acid can be oxidized to chloroplatinic acid by saturating the solution with chlorine while heating.

H2PtCl4 (aq) + Cl2 (g) → H2PtCl6 (aq)

[edit] Decomposition of aqua regia
Upon mixing of concentrated hydrochloric acid and concentrated nitric acid, chemical reactions occur. These reactions result in the volatile products nitrosyl chloride and chlorine as evidenced by the fuming nature and characteristic yellow color of aqua regia. As the volatile products escape from solution, the aqua regia loses its potency.

HNO3 (aq) + 3 HCl (aq) → NOCl (g) + Cl2 (g) + 2 H2O (l)
Nitrosyl chloride can further decompose into nitric oxide and chlorine. This dissociation is equilibrium-limited. Therefore, in addition to nitrosyl chloride and chlorine, the fumes over aqua regia contain nitric oxide.

2 NOCl (g) → 2 NO (g) + Cl2 (g)
 

Hydrochloric acid was first discovered around the year 800 by the Iranian alchemist Abu Musa Jabir ebn Hayyan, by mixing common salt with vitriol (sulfuric acid). Jabir's invention of gold-dissolving aqua regia, consisting of hydrochloric acid and nitric acid, contributed to the effort of alchemists to find the philosopher's stone.

When Germany invaded Denmark in World War II, the Hungarian chemist George de Hevesy dissolved the gold Nobel Prizes of Max von Laue and James Franck into aqua regia to prevent the Nazis from stealing them. He placed the resulting solution on a shelf in his laboratory at the Niels Bohr Institute. After the war, he 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 presented new medals to Laue and Franck.