Editing Aqua regia (section)

==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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| image1 = Dissolution of gold in aqua regia (I).JPG
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| caption1 = Initial state of the transformation.
| image2 = Dissolution of gold in aqua regia (II).JPG
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| caption2 = Intermediate state of the transformation.
| image3 = Dissolution of gold in aqua regia (III).JPG
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| 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}}