Editing Aqua regia (section)

===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.
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| header = '''Dissolution of gold by aqua regia.'''
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