Freshly preparedaqua regia to remove metal salt depositsFreshly prepared aqua regia is colorless, but it turns orange within seconds. Here, fresh aqua regia has been added to theseNMR tubes to remove all traces of organic material.
Aqua regia (/ˈreɪɡiə,ˈriːdʒiə/; fromLatin, "regal water" or "royal water") is amixture ofnitric acid andhydrochloric acid, optimally in amolar ratio of 1:3.[b] Aqua regia is a fuming liquid. Freshly prepared aqua regia is colorless, but it turns yellow, orange, or red within seconds from the formation ofnitrosyl chloride andnitrogen dioxide. It was so named byalchemists because it can dissolvenoble metals, such asgold andplatinum, while leaving many other metals unaffected. It has been used to process or conceal gold into the modern era.
Upon mixing of concentrated hydrochloric acid and concentrated nitric acid, chemical reactions occur. These reactions result in the volatile productsnitrosyl chloride andchlorine gas:
HNO3 + 3 HCl → NOCl + Cl2 + 2 H2O
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 intonitric oxide (NO) and elemental chlorine (Cl2):
2 NOCl → 2 NO + Cl2
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 atmosphericoxygen, the gases produced also containnitrogen dioxide,NO2 (red fume):
Aqua regia is also used inetching and in specificanalytic procedures. It is also used in some laboratories to cleanglassware oforganic compounds and metal particles. This method is preferred among most over the more traditionalchromic acid bath for cleaningNMR tubes, because no traces of paramagneticchromium can remain to spoil spectra.[1] While chromic acid baths are discouraged because of thehigh toxicity of chromium and the potential for explosions, aqua regia is itself very corrosive and has been implicated in several explosions due to mishandling.[2]
Because its components react quickly, resulting in itsdecomposition, aqua regia quickly loses its effectiveness (yet remains a strong acid), so its components are usually only mixed immediately before use.
Pure goldprecipitate produced by the aqua regia chemical refining process
Aqua regia dissolvesgold, although neither constituent acid will do so 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 becomechloroauric acid. In addition, gold may be dissolved by the chlorine present in aqua regia. Appropriateequations are:
Au + 3 HNO 3 + 4 HCl[AuCl 4]− + 3 NO 2 +H 3O+ + 2 H 2O
Similar equations can be written forplatinum. As with gold, the oxidation reaction can be written with either nitric oxide or nitrogen dioxide as the nitrogen oxide product:
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 ofchloroplatinous 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:
The chloroplatinous acid can be oxidized tochloroplatinic 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 metals,iridium andosmium, 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.
As a practical matter, when platinum group metals are purified through dissolution in aqua regia, gold (commonly associated with PGMs) isprecipitated by treatment withiron(II) chloride. Platinum in the filtrate, as hexachloroplatinate(IV), is converted toammonium hexachloroplatinate by the addition ofammonium chloride. This ammonium salt is extremely insoluble, and it can be filtered off. Ignition (strong heating) converts it to platinum metal:[4]
Unprecipitated hexachloroplatinate(IV) is reduced with elementalzinc, and a similar method is suitable for small scale recovery of platinum from laboratory residues.[5]
In Europe, aqua regia first appeared in theDe inventione veritatis ("On the Discovery of Truth") bypseudo-Geber (afterc. 1300), who produced it by addingsal ammoniac (ammonium chloride) to nitric acid.[6][d] 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.[8]
According to Needham, records of nitrate being used to make aqua regia appear in theHsin Hsiu Pên Tshao by 660 AD.[9]
The fox in Basil Valentine's Third Key represents aqua regia,Musaeum Hermeticum, 1678
WhenGermany invaded Denmark in World War II, Hungarian chemistGeorge de Hevesy dissolved the goldNobel Prizes of German physicistsMax von Laue (1914) andJames 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 activistCarl von Ossietzky had received the Nobel Peace Prize in 1935. De Hevesy placed the resulting solution on a shelf in his laboratory at theNiels 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 andprecipitated 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.[13][14]
^The relative concentrations of the two acids in water differ; values could be 65% w/v for nitric acid and 35% w/v for hydrochloric acid – that is, the actualHNO3:HCl mass ratio is less than 1:2.
^Karpenko, Vladimír; Norris, John A. (2002)."Vitriol in the History of Chemistry".Chemické listy.96 (12):997–1005. p. 1002. As Karpenko & Norris note, the uncertain dating of the pseudo-Geber corpus (which was probably written by more than one author) renders the dating of aqua regia equally uncertain.
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