Aqua regia

Aqua regia (/ˈreɪɡiə, ˈriːdʒiə/; from Latin, lit. "regal water" or "Royal water") is a mixture of nitric acid and hydrochloric acid, optimally in a molar ratio of 1:3.[note 2] Aqua regia is a yellow-orange (sometimes red) fuming liquid, so named by alchemists because it can dissolve the noble metals gold and platinum, though not all metals.

Aqua regia[note 1]
Names
IUPAC name
nitric acid hydrochloride
Other names
  • Aqua regis
  • Nitrohydrochloric acid
  • Royal water
Identifiers
3D model (JSmol)
UNII
Properties
HNO3+3 HCl
Appearance Red, yellow or gold fuming liquid
Density 1.01–1.21 g/cm3
Melting point −42 °C (−44 °F; 231 K)
Boiling point 108 °C (226 °F; 381 K)
Miscible
Vapor pressure 21 mbar
Hazards
NFPA 704 (fire diamond)
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Infobox references
Freshly prepared aqua regia to remove metal salt deposits
Freshly prepared aqua regia is colorless, but it turns orange within seconds. Here, fresh aqua regia has been added to these NMR tubes to remove all traces of organic material.

Manufacture and decomposition

Upon mixing of concentrated hydrochloric acid and concentrated nitric acid, chemical reactions occur. These reactions result in the volatile products nitrosyl chloride and chlorine gas:

HNO3 (aq) + 3 HCl (aq) → NOCl (g) + Cl2 (g) + 2 H2O (l)

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. Nitrosyl chloride can further decompose into nitric oxide and chlorine:

2 NOCl (g) → 2 NO (g) + Cl2 (g)

This dissociation is equilibrium-limited. Therefore, in addition to nitrosyl chloride and chlorine, the fumes over aqua regia contain nitric oxide. Because nitric oxide reacts readily with atmospheric oxygen, the gases produced also contain nitrogen dioxide, NO2:

2 NO (g) + O2 (g) → 2 NO2 (g)

Applications

Aqua regia is primarily used to produce chloroauric acid, the electrolyte in the Wohlwill process for refining the highest quality (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.[1] 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.[2]

Due to the reaction between its components 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.

While local regulations may vary, aqua regia may be disposed of by careful neutralization, before being poured down the sink. If there is contamination by dissolved metals, the neutralized solution should be collected for disposal.[3][4]

Chemistry

Dissolving gold
Pure gold precipitate produced by the aqua regia chemical refining process

Aqua regia dissolves gold, 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 ions to produce tetrachloroaurate(III) anions, also in solution. The reaction with hydrochloric acid is an equilibrium reaction that favors formation of chloroaurate anions (AuCl4−). 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. Appropriate equations are:

Au + 3 HNO
3 + 4 HCl [AuCl
4]−
 + 3 [NO
2] + [H
3O]+
 + 2 H
2O
or
Au + HNO
3 + 4 HCl [AuCl
4]−
 + [NO] + [H
3O]+
 + H
2O.

If the aqua regia solution only contains gold, solid tetrachloroauric acid may be prepared by boiling off excess aqua regia, and removing residual nitric acid by repeatedly heating with hydrochloric acid. That step reduces nitric acid (see decomposition of aqua regia). If elemental gold is desired, it may be selectively reduced with sulfur dioxide, hydrazine, oxalic acid, etc.[5] The equation for the reduction of gold by sulfur dioxide is:

2 AuCl−
4 (aq) + 3 SO
2(g) + 6 H
2O (l) → 2 Au (s) + 12 H+
 (aq) + 3 SO2−
4(aq) + 8 Cl−
 (aq).


Dissolution of gold by aqua regia.
Initial state of the transformation.
Intermediate state of the transformation.
Final state of the transformation.

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) → PtCl2−
6 (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:

2Pt (s) + 2HNO3 (aq) + 8 HCl (aq) → (NO)2PtCl4 (s) + H2PtCl4 (aq) + 4 H2O (l)

and

(NO)2PtCl4 (s) + 2 HCl (aq) ⇌ H2PtCl4 (aq) + 2 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).

Dissolving platinum solids in aqua regia was the mode of discovery for the densest metals, iridium and osmium, both of which are found in platinum ore and will not be dissolved by the acid, instead collecting on the base of the vessel.


Dissolution of platinum[note 3] by aqua regia.
Initial state of the transformation.
Intermediate state of the transformation.
Final state of the transformation
(four days later).

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:[6]

3 (NH4)2PtCl6 → 3 Pt + 2 N2 + 2 NH4Cl + 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.[7]

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

It can react with iron pyrite to form Iron(III) chloride:

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

History
The fox in Basil Valentine's Third Key represents aqua regia, Musaeum Hermeticum, 1678

Aqua regia first appears in the De inventione veritatis ("On the Discovery of Truth") by pseudo-Geber (after c. 1300).[8] The third of Basil Valentine's keys (ca. 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 were known as dragon's blood. The reaction was not reported again in the chemical literature until 1890.[9]

Antoine Lavoisier called aqua regia nitro-muriatic acid in 1789.[10]

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.[11][12]


See also

Notes
  1. The information in the infobox is specific to a molar ratio of 1:3 between nitric acid and hydrochloric acid.
  2. 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 actual HNO3:HCl mass ratio is less than 1:2,
  3. A platinum Soviet commemorative coin to be precise.

References
  1. Hoffman, R. (10 March 2005) How to make an NMR sample, Hebrew University. Accessed 31 October 2006.
  2. American Industrial Hygiene Association, Laboratory Safety Incidents: Explosions. Accessed 8 September 2010.
  3. Committee on Prudent Practices for Handling, Storage, and Disposal of Chemicals in Laboratories, National Research Council (1995). Prudent Practices in the Laboratory: Handling and Disposal of Chemicals (free fulltext). National Academies Press. pp. 160–161.CS1 maint: uses authors parameter (link)
  4. "Aqua Regia". Laboratory Safety Manual. Princeton University.
  5. Renner, Hermann; Schlamp, Günther; Hollmann, Dieter; Lüschow, Hans Martin; Tews, Peter; Rothaut, Josef; Dermann, Klaus; Knödler, Alfons; et al. "Gold, Gold Alloys, and Gold Compounds". Ullmann's Encyclopedia of Industrial Chemistry. Weinheim: Wiley-VCH. doi:10.1002/14356007.a12_499.
  6. Hunt, L. B.; Lever, F. M. (1969). "Platinum Metals: A Survey of Productive Resources to industrial Uses" (PDF). Platinum Metals Review. 13 (4): 126–138.
  7. Kauffman, George B.; Teter, Larry A.; Rhoda, Richard N. (1963). Recovery of Platinum from Laboratory Residues. Inorg. Synth. Inorganic Syntheses. 7. p. 232. doi:10.1002/9780470132388.ch61. ISBN 9780470132388.
  8. 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.
  9. Principe, Lawrence M. (2012). The secrets of alchemy. Chicago: University of Chicago Press. ISBN 978-0226682952.
  10. Lavoisier, Antoine (1790). Elements of Chemistry,. in a New Systematic Order, Containing All the Modern Discoveries. Edinburgh: William Creech. p. 116. ISBN 978-0486646244..
  11. "Adventures in radioisotope research", George Hevesy
  12. Birgitta Lemmel (2006). "The Nobel Prize Medals and the Medal for the Prize in Economics". The Nobel Foundation.
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