Aqua regia

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Aqua regia (Latin for "royal water") is a highly corrosive, fuming yellow or red liquid formed by a fresh mixture of concentrated nitric acid (otherwise known as aqua fortis) and concentrated hydrochloric acid, usually in a volumetric ratio of one to three. It is one of the few reagents able to dissolve 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 extremely passive metals are able to withstand it. Aqua regia is used in etching and in certain analytic procedures. Due to the formation of volatile nitrosyl chloride and chlorine, aqua regia will quickly lose its effectiveness. Therefore, it should be mixed immediately before use.

Contents

1 How it works
2 Decomposition of aqua regia
3 History
4 External links

How it works

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 tiny (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)

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) → PtCl₆^2- (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)

Decomposition of aqua regia

Upon mixing of concentrated hydochloric acid and concentrated nitric acid, chemical reactions will begin to occur. These reactions result in volatile products (nitrosyl chloride and chlorine) that result in the fuming nature and characteristic yellow color of aqua regia. As the volatile products escape from solution, the aqua regia will lose 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, the fumes over aqua regia contain nitrosyl chloride, nitric oxide, and chlorine.

2 NOCl (g) → 2 NO (g) + Cl₂ (g)

History

Jabir ibn Hayyan, mediaeval manuscript drawing, anonymous

Hydrochloric acid was first discovered around the year 800 by the Islamic alchemist Jabir Ibn Hayyan (Gaber), 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. [link]

External links

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