Aqua regia

Applications

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.

Chemistry

Dissolving gold

Aqua regia dissolves gold, although neither constituent acid can dissolve significant quantities alone. Nitric acid is a powerful oxidizer, which will dissolve a very small quantity of gold, forming gold(III) ions (Au³⁺). The hydrochloric acid provides a ready supply of chloride ions (Cl⁻), which react with the gold ions to produce tetrachloroaurate(III) anions ([AuCl₄]⁻), also in solution. The reaction with hydrochloric acid is an equilibrium reaction that favors formation of tetrachloroaurate(III) anions. 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 chlorine present in aqua regia. The equations are:

Au + 3 HNO₃ + 4 HCl ${\displaystyle \stackrel{-\rightharpoonup }{\leftharpoondown }}$ [AuCl₄]⁻ + 3 NO₂ + H₃O⁺ + 2 H₂O

or

Au + HNO₃ + 4 HCl ${\displaystyle \stackrel{-\rightharpoonup }{\leftharpoondown }}$ [AuCl₄]⁻ + NO + H₃O⁺ + H₂O.

Solid tetrachloroauric acid may be isolated by evaporating the excess aqua regia, and decomposing the residual nitric acid by repeatedly heating the solution with additional hydrochloric acid. That step reduces nitric acid (see decomposition of aqua regia). If elemental gold is desired, it may be selectively reduced with reducing agents such as sulfur dioxide, hydrazine, oxalic acid, etc. The equation for the reduction of oxidized gold (Au³⁺) by sulfur dioxide (SO₂) is the following:

2 [AuCl₄]⁻(aq) + 3 SO₂(g) + 6 H₂O(l) → 2 Au(s) + 12 H⁺(aq) + 3 SO2−4(aq) + 8 Cl⁻(aq)

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)

3 Pt(s) + 4 NO−3(aq) + 16 H⁺(aq) → 3 Pt⁴⁺(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 (H₂[PtCl₄]) and nitrosoplatinic chloride ([NO]₂[PtCl₄]). 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:

2 Pt(s) + 2 HNO₃(aq) + 8 HCl(aq) → [NO]₂[PtCl₄](s) + H₂[PtCl₄](aq) + 4 H₂O(l)

and

[NO]₂[PtCl₄](s) + 2 HCl(aq) ⇌ H₂[PtCl₄](aq) + 2 NOCl(g)

The chloroplatinous acid can be oxidized to chloroplatinic acid by saturating the solution with molecular chlorine (Cl₂) while heating:

H₂[PtCl₄](aq) + Cl₂(g) → H₂[PtCl₆](aq)

Dissolving platinum solids in aqua regia was the mode of discovery for the densest naturally-occurring metals, iridium and osmium, both of which are found in platinum ores and are not dissolved by aqua regia, instead collecting as insoluble metallic powder (elemental Ir, Os) on the base of the vessel.

Precipitating dissolved platinum

As a practical matter, when platinum group metals are purified through dissolution in aqua regia, gold (commonly associated with PGMs) is precipitated by treatment with iron(II) chloride. Platinum in the filtrate, as hexachloroplatinate(IV), is converted to ammonium hexachloroplatinate by the addition of ammonium chloride. This ammonium salt is extremely insoluble, and it can be filtered off. Ignition (strong heating) converts it to platinum metal:

3 [NH₄]₂[PtCl₆] → 3 Pt + 2 N₂ + 2 [NH₄]Cl + 16 HCl

Unprecipitated hexachloroplatinate(IV) is reduced with elemental zinc, and a similar method is suitable for small scale recovery of platinum from laboratory residues.

Reaction with tin

Aqua regia reacts with tin to form tin(IV) chloride, containing tin in its highest oxidation state:

4 HCl + 2 HNO₃ + Sn → SnCl₄ + NO₂ + NO + 3 H₂O

Reaction with other substances

Aqua regia can react with iron pyrite to form iron(III) chloride:

FeS₂ + 5 HNO₃ + 3 HCl → FeCl₃ + 2 H₂SO₄ + 5 NO + 2 H₂O

History

According to the British historian of science Joseph Needham, records of nitrate being used to make aqua regia appear in the Hsin Hsiu Pên Tshao by 660 AD.

In Europe, aqua regia first appeared in the De inventione veritatis ("On the Discovery of Truth") by pseudo-Geber (after c. 1300), who produced it by adding sal ammoniac (ammonium chloride) to nitric acid. The preparation of aqua regia by directly mixing hydrochloric acid with nitric acid only became possible after the discovery in the late sixteenth century of the process by which free hydrochloric acid can be produced.

The third of Basil Valentine's keys (c. 1600) shows a dragon in the foreground and a fox eating a rooster in the background. The rooster symbolizes gold (from its association with sunrise and the sun's association with gold), and the fox represents aqua regia. The repetitive dissolving, heating, and redissolving (the rooster eating the fox eating the rooster) leads to the buildup of chlorine gas in the flask. The gold then crystallizes in the form of gold(III) chloride, whose red crystals Basil called "the rose of our masters" and "the red dragon's blood". The reaction was not reported again in the chemical literature until 1895.

Antoine Lavoisier called aqua regia nitro-muriatic acid in 1789.

When Germany invaded Denmark in World War II, Hungarian chemist George de Hevesy dissolved the gold Nobel Prizes of German physicists Max von Laue (1914) and James Franck (1925) in aqua regia to prevent the Nazis from confiscating them. The German government had prohibited Germans from accepting or keeping any Nobel Prize after jailed peace activist Carl von Ossietzky had received the Nobel Peace Prize in 1935. De Hevesy placed the resulting solution on a shelf in his laboratory 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 Franck.