Editing Aqua regia (section)

===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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