View source for Aqua regia

{{Short description|Mixture of nitric acid and hydrochloric acid in a 1:3 molar ratio}}
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{{Chembox
| ImageFile      = [[File:Aqua regia.svg|220px|class=skin-invert]]
| ImageSize      = 
| ImageAlt       = 
| Name           = 
| Reference      = {{efn|The information in the infobox is specific to a molar ratio of 1:3 between [[nitric acid]] and [[hydrochloric acid]].}}
| IUPACName      = Nitric acid trihydrochloride
| OtherNames     = {{ubl|Aqua regis|Nitrohydrochloric acid|Royal water}}
| SystematicName = 
| Section1       = {{Chembox Identifiers
| CASNo = 8007-56-5
| CASNo_Ref = {{cascite|correct|CAS}}
| UNII_Ref = {{fdacite|correct|FDA}}
| UNII = X3TT5X989E
| PubChem = 90477010
| SMILES = [N+](=O)(O)[O-].Cl.Cl.Cl}}
| Section2       = {{Chembox Properties
| Formula = {{chem2|HNO3 + 3 HCl}}
| Appearance = Fuming liquid. Freshly prepared aqua regia is colourless, but it turns yellow, orange or red within seconds. 
| Density = 1.01–1.21 g/cm<sup>3</sup>
| MeltingPtC = −42
| MeltingPt_notes = 
| BoilingPtC = 108
| BoilingPt_notes = 
| VaporPressure = 21 mbar
| Solubility = [[Miscible]]
  }}
| Section3       = {{Chembox Hazards
| MainHazards =
| NFPA-H = 3
| NFPA-F = 0
| NFPA-R = 0
| NFPA-S = OX
| FlashPt =
| AutoignitionPt = 
  }}
| Section4       = 
| Section5       = 
| Section6       = 
}}
[[File:Aqua regia in Davenport Laboratories.jpg|thumb|Freshly prepared ''aqua regia'' to remove metal salt deposits]]
[[File:Aqua regia in NMR tubes.jpg|thumb|upright|Freshly prepared aqua regia is colorless, but it turns orange within seconds. Here, fresh aqua regia has been added to these [[NMR tube]]s to remove all traces of organic material.]]
'''Aqua regia''' ({{IPAc-en|ˈ|r|eɪ|ɡ|i|ə|,_|ˈ|r|iː|dʒ|i|ə}}; from [[Latin]], "regal water" or "royal water") is a [[mixture]] of [[nitric acid]] and [[hydrochloric acid]], optimally in a [[molar concentration|molar]] ratio of 1:3.{{efn|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 {{chem2|HNO3}}:HCl mass ratio is less than 1:2.}} Aqua regia is a fuming liquid. Freshly prepared aqua regia is colorless, but it turns yellow, orange, or red within seconds from the formation of [[nitrosyl chloride]] and [[nitrogen dioxide]]. It was so named by [[alchemy|alchemists]] because it can dissolve [[noble metal]]s, such as [[gold]] and [[platinum]], while leaving many other metals unaffected. It has been used to process or conceal gold into the modern era.

== Preparation 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:

:{{chem2|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 into [[nitric oxide]] (NO) and elemental chlorine ({{chem2|Cl2}}):

:{{chem2|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 atmospheric [[oxygen]], the gases produced also contain [[nitrogen dioxide]], {{chem2|NO2}} (red fume):

:{{chem2|2 NO + O2 → 2 NO2}}

==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 [[Chemical milling|etching]] and in specific [[Analytical chemistry|analytic procedures]]. It is also used in some laboratories to clean [[laboratory glassware|glassware]] of [[organic compound]]s and metal particles. This method is preferred among most over the more traditional [[chromic acid]] bath for cleaning [[NMR tube]]s, because no traces of paramagnetic [[chromium]] can remain to spoil spectra.<ref>Hoffman, R. (10 March 2005) [http://chem.ch.huji.ac.il/nmr/preparation/preparation.html How to make an NMR sample], [[Hebrew University]]. Accessed 31 October 2006.</ref> While chromic acid baths are discouraged because of the [[Chromium toxicity|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.<ref>[[American Industrial Hygiene Association]], [https://web.archive.org/web/20110718093018/http://www.aiha.org/insideaiha/volunteergroups/labHandScommittee/Pages/IncidentsExplosions.aspx Laboratory Safety Incidents: Explosions]. Accessed 8 September 2010.</ref>

Because its components react quickly, resulting in its [[#Manufacture and decomposition|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===
[[File:Golddust.jpg|thumb|Pure gold [[Precipitation (chemistry)|precipitate]] produced by the aqua regia chemical refining process]]

Aqua regia dissolves [[gold]], 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) [[ion]]s ({{chem2|Au(3+)}}). The hydrochloric acid provides a ready supply of chloride ions ({{chem2|Cl−}}), which react with the gold ions to produce tetrachloroaurate(III) [[anion]]s ({{chem2|[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. Appropriate [[Chemical equation|equations]] are:

: Au + 3&nbsp;{{Chem|HNO|3}} + 4&nbsp;HCl {{EqmR}} {{Chem|[AuCl|4|]|−}} + 3&nbsp;{{Chem|NO|2|}} + {{Chem|H|3|O|+}} + 2&nbsp;{{Chem|H|2|O}}
or
: Au + {{Chem|HNO|3}} + 4&nbsp;HCl {{EqmR}} {{Chem|[AuCl|4|]| −}} + NO + {{Chem|H|3|O|+}} + {{Chem|H|2|O}}.

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 [[redox|reduces]] nitric acid (see [[Aqua regia#Preparation and decomposition|decomposition of aqua regia]]). If elemental gold is desired, it may be selectively [[redox|reduced]] with reducing agents such as [[sulfur dioxide]], [[hydrazine]], [[oxalic acid]], etc.<ref name = ullgold>{{Ullmann | doi = 10.1002/14356007.a12_499 | title = Gold, Gold Alloys, and Gold Compounds | first1 = Hermann | last1 = Renner | first2 = Günther | last2 = Schlamp | first3 = Dieter | last3 = Hollmann | first4 = Hans Martin | last4 = Lüschow | first5 = Peter | last5 = Tews | first6 = Josef | last6 = Rothaut | first7 = Klaus | last7 = Dermann | first8 = Alfons | last8 = Knödler | first9 = Christian | last9 = Hecht | display-authors=8}}</ref> The [[Chemical equation|equation]] for the [[redox|reduction]] of [[oxidized]] gold ({{chem2|Au(3+)}}) by sulfur dioxide ({{chem2|SO2}}) is the following:

:{{chem2|2 [AuCl4]−(aq) + 3 SO2(g) + 6 H2O(l) → 2 Au(s) + 12 H+(aq) + 3 SO4(2−)(aq) + 8 Cl−(aq)}}

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{{multiple image
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| image1 = Dissolution of gold in aqua regia (I).JPG
| alt1 = 
| caption1 = Initial state of the transformation.
| image2 = Dissolution of gold in aqua regia (II).JPG
| alt2 = 
| caption2 = Intermediate state of the transformation.
| image3 = Dissolution of gold in aqua regia (III).JPG
| alt3 = 
| caption3 = Final state of the transformation.
| header_align = center
| header = '''Dissolution of gold by aqua regia.'''
}}
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===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:

:{{chem2|Pt(s) + 4 NO3-(aq) + 8 H+(aq) → Pt(4+)(aq) + 4 NO2(g) + 4 H2O(l)}}
:{{chem2|3 Pt(s) + 4 NO3-(aq) + 16 H+(aq) → 3 Pt(4+)(aq) + 4 NO(g) + 8 H2O(l)}}

The oxidized platinum ion then reacts with chloride ions resulting in the chloroplatinate ion:

:{{chem2|Pt(4+)(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]] ({{chem2|H2[PtCl4]}}) and nitrosoplatinic chloride ({{chem2|[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:

:{{chem2|2 Pt(s) + 2 HNO3(aq) + 8 HCl(aq) → [NO]2[PtCl4](s) + H2[PtCl4](aq) + 4 H2O(l)}}
and
:{{chem2|[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 ({{chem2|Cl2}}) while heating:

:{{chem2|H2[PtCl4](aq) + Cl2(g) → H2[PtCl6](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 ores and are not dissolved by aqua regia, instead collecting as insoluble metallic powder (elemental Ir, Os) on the base of the vessel.

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{{multiple image
| align = center
| direction = horizontal
| total_width = 760
| caption_align = center
| image1 = Plaatina reageerimine kuningveega 01.JPG
| alt1 = 
| caption1 = Initial state of the transformation.
| image2 = Plaatina reageerimine kuningveega 02.JPG
| alt2 = 
| caption2 = Intermediate state of the transformation.
| image3 = Plaatina reageerimine kuningveega 03.JPG
| alt3 = 
| caption3 = Final state of the transformation <br/>(four days later).
| header_align = center
| header = '''Dissolution of platinum{{efn|A [[Commemorative coins of the Soviet Union#Platinum coins|platinum Soviet commemorative coin]] to be precise.}} by aqua regia.'''
}}
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===Precipitating dissolved platinum===
As a practical matter, when platinum group metals are purified through dissolution in aqua regia, gold (commonly associated with PGMs) is [[Precipitation (chemistry)|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:<ref>{{cite journal|first1 = L. B.|last1 = Hunt|last2 = Lever |first2= F. M.|journal = Platinum Metals Review|volume = 13|issue = 4|year = 1969|pages = 126–138|title = Platinum Metals: A Survey of Productive Resources to industrial Uses| doi=10.1595/003214069X134126138 |url = http://www.platinummetalsreview.com/pdf/pmr-v13-i4-126-138.pdf}}</ref>

:{{chem2|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.<ref>{{cite book | author1 = Kauffman, George B. | author2 = Teter, Larry A. | doi = 10.1002/9780470132388.ch61 | year = 1963 | last3 = Rhoda | first3 = Richard N. | chapter = Recovery of Platinum from Laboratory Residues | title = Inorganic Syntheses | isbn = 978-0-470-13238-8 | volume = 7 | pages = 232–236}}</ref>

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

:{{chem2|4 HCl + 2 HNO3 + Sn → SnCl4 + NO2 + NO + 3 H2O}}

===Reaction with other substances===
It can react with [[Pyrite|iron pyrite]] to form [[Iron(III) chloride]]:

:{{chem2|FeS2 + 5 HNO3 + 3 HCl → FeCl3 + 2 H2SO4 + 5 NO + 2 H2O}}

==History==
Aqua regia first appeared in the ''De inventione veritatis'' ("On the Discovery of Truth") by [[pseudo-Geber]] (after {{circa|1300}}), who produced it by adding [[sal ammoniac]] ([[ammonium chloride]]) to nitric acid.<ref>{{Cite journal|last1=Karpenko|first1=Vladimír|last2=Norris|first2=John A.|year=2002|title=Vitriol in the History of Chemistry|journal=Chemické listy|volume=96|issue=12|pages=997–1005|url=http://www.chemicke-listy.cz/ojs3/index.php/chemicke-listy/article/view/2266}} 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.</ref>{{efn|However, [[Ahmad Y. Al-Hassan]] (2005) claimed that Islamic texts dated to before the 13th century, including the works of [[Jabir ibn Hayyan]] and [[Abu Bakr al-Razi]], did in fact contain detailed descriptions of {{lang|la|aqua regia}}.<ref>Ahmad Y. Al-Hassan, ''Cultural contacts in building a universal civilisation: Islamic contributions'', published by [https://books.google.com/books?id=3iQXAQAAIAAJ O.I.C. Research Centre for Islamic History, Art and Culture in 2005] and available [http://www.history-science-technology.com/articles/articles%2072.html online at History of Science and Technology in Islam]</ref>}} 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.<ref>{{cite book|last=Multhauf|first=Robert P.|author-link=Robert P. Multhauf|year=1966|title=The Origins of Chemistry|location=London|publisher=Oldbourne|isbn=}} p. 208, note 29; cf. p. 142, note 79.</ref>

[[File:Musaeum Hermeticum 1678 p 398 III. Clavis AQ27.tif|thumb|The fox in Basil Valentine's Third Key represents aqua regia, ''Musaeum Hermeticum'', 1678]]
The third of [[Basil Valentine|Basil Valentine's]] [[The Twelve Keys of Basil Valentine|keys]] ({{circa|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".<ref name="Principe 2012">{{cite book|last=Principe|first=Lawrence M.|author-link=Lawrence M. Principe|year=2013|title=The Secrets of Alchemy|publisher=University of Chicago Press|location=Chicago|isbn=978-0-226-68295-2}} pp. 149-153.</ref> The reaction was not reported again in the chemical literature until 1895.<ref>{{cite journal|last=Rose|first=Thomas Kirke|year=1895|title=The Dissociation of Chloride of Gold|journal=Journal of the Chemical Society|volume=67|pages=881–904|doi=10.1039/CT8956700881|url=https://zenodo.org/record/1784163}} Cf. {{harvnb|Principe|2013|p=152}}.</ref>

[[Antoine Lavoisier]] called aqua regia nitro-muriatic acid in 1789.<ref>{{Cite book | author = Lavoisier, Antoine | date = 1790 | url = https://archive.org/details/elementschemist00kerrgoog | title = Elements of Chemistry,. in a New Systematic Order, Containing All the Modern Discoveries | publisher = Edinburgh: William Creech | page = 116 | isbn = 978-0-486-64624-4}}.</ref>

When [[Operation Weserübung|Germany invaded Denmark]] in World War II, Hungarian chemist [[George de Hevesy]] dissolved the gold [[Nobel Prize]]s 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 [[Precipitation (chemistry)|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.<ref>[https://archive.org/stream/adventuresinradi01heve#page/27/mode/1up/search/medals "Adventures in radioisotope research"], George Hevesy</ref><ref>{{Cite web| publisher = The Nobel Foundation | url = http://nobelprize.org/nobel_prizes/about/medals/ | author = Birgitta Lemmel | title = The Nobel Prize Medals and the Medal for the Prize in Economics | year = 2006}}</ref>

== See also ==
* {{Annotated link|Green death}}
* {{Annotated link|Piranha solution}} sometimes also used to clean glassware

==Notes==
{{Notelist}}

==References==
{{Reflist|2}}

==External links==
* [http://jchemed.chem.wisc.edu/JCESoft/CCA/CCA3/MAIN/AQREGIA/PAGE1.HTM Chemistry Comes Alive! ''Aqua Regia'']
* [http://www.periodicvideos.com/videos/mv_aqua_regia.htm Aqua Regia] at ''[[The Periodic Table of Videos]]'' (University of Nottingham)
* [https://www.youtube.com/watch?v=XoqU1GfIOkI Demonstration of ''Gold Coin Dissolving in Acid (Aqua Regia)'']

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[[Category:Gold]]
[[Category:Alchemical substances]]
[[Category:Oxidizing mixtures]]
[[Category:Oxidizing acids]]
[[Category:Mineral acids]]