Gold reacts with acid
From a chemical point of view, aqua regia - the aqua regia - is a mixture of the two mineral acids hydrochloric acid and nitric acid. It was named because of its properties of being able to dissolve the otherwise so resistant royal precious metals gold and platinum.
The production of aqua regia is done by carefully mixing three parts of 37% hydrochloric acid HCl and one part 65% nitric acid HNO3; the result is a yellow to reddish-brown, smoking, pungent smelling solution. The use of less concentrated acids is possible.
In the strongly corrosive mixture, the two acids and water are initially present side by side and unchanged. However, aqua regia cannot be kept for long, as it slowly decomposes due to contamination by the acids or under the influence of the vessel walls, releasing chlorine, nitrosyl chloride and nitrous gases.
The strong oxidizing effect of aqua regia is not based on the interaction of the acids involved, which are already strong, but on the reaction
ENT3 + 3 HCl → NOCl + 2 Clnasc. + 2 H.2O,
in which, in addition to water, the compounds responsible for the oxidizing effect nitrosyl chloride (NOCl) and particularly nasal chlorine (see status nascendi) are formed.
* Gold - reaction to tetrachloridogold (III) acid:
2 Au + 2 NOCl + 6 Clnasc. + 2 ENT3 → 2 HAuCl4 + 4 NO2.
* Platinum - formation of hexachloroplatinic acid (yellow), reaction equation:
3 Pt + 4 ENT3 + 18 HCl → 3 H2(PtCl6) + 4 NO + 8 H2O.
* Tin - reaction to tin tetrachloride:
Sn + 2 ENT3 + 4 HCl → SnCl4 + NO2 + NO + 3H2O.
Aqua regia digestion
In analytical chemistry, aqua regia is used to digest inorganic as well as organic samples where other digestion methods fail. According to DIN EN 13346, aqua regia digestion is used to determine the metals Cd, Cr, Cu, Hg, Ni, Pb and Zn in sewage sludge and soils.
The method fails with some metal oxides and with metals that form oxide layers resistant to aqua regia. Silicon dioxide (sand), Ag (silver chloride layer), Ir, Os, Rh, Nb, Ta. Ru dissolves in the presence of oxygen.
Hazard warnings and safety advice
Aqua regia is highly corrosive (H 314) and oxidising (H 272).
There is also a general ban on transportation of the acid. Disposal should therefore take place directly in the laboratory under the fume cupboard through careful dilution (warming up!) And neutralization with sodium hydroxide solution (1 molar).
Further safety data: See Gestis substance database, entry .
Sources and further information
 - Sir Humphry Davy:
About the aqua regia.
Annals of Physics, (1817), DOI 10.1002 / andp.18170571104.
 - NN .:
The active ingredient in aqua regia, or hydrochloric acid.
Pharmacy Archives, (1844), DOI 10.1002 / ardp.18440870316.
 - A. Baudrimont:
Research on aqua regia and a special product to which it owes its properties.
Justus Liebig's Annals of Chemistry, (1846), DOI 10.1002 / jlac.18460590113.
 - Gay-Lussac:
About the aqua regia.
Justus Liebig's Annals of Chemistry, (1848), DOI 10.1002 / jlac.18480660208.
 - NN .:
About the solubility of sulfur in aqua regia.
Pharmacy Archives, (1870), DOI 10.1002 / ardp.18701910111.
 - C. J. van Nieuwenburg, J. W. L. van Luchten:
Analysis of the residue insoluble in aqua regia.
Qualitative chemical analysis, (1959), DOI 10.1007 / 978-3-7091-7647-4_8.
Updated on 06 February 2019.
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