Aqua Regia (HNO3 + 3HCl) is used to etch gold or platinum. It is a mixture of Nitric acid and Hydrochloric acid that is freshly mixed before use. Optimally it is in a molar ratio of 1 part nitric acid to 3 parts hydrochloric acid. When freshly mixed is clear, turns yellow very quickly and eventually red. Aqua regia was named by alchemists because of it's ability to dissolve noble metals.
The mechanism of aqua regia happens upon mixing the concentrated hydrochloric acid and the nitric acid. The result is in volatile products, nitrosyl chloride and chlorine gas:
HNO3 + 3HCl --> NOCl + Cl2 + 2H2O
This is shown by the fuming nature of aqua regia. As the volatile products are removed via the exhaust, the nitrosyl chloride can further decompose into nitric oxide and more chlorine gas:
2NOCl --> 2NO + Cl2
The nitric oxide can then react with atmospheric oxygen to form nitrogen dioxide.
Aqua regia is used to dissolve gold. Neither nitric acid or hydrochloric acid can do this alone. Nitric acid will act as the oxidizer and is used to form gold ions (Au3+). The hydrochloric acid is then used to react with the gold ions to form tetrachloroaurate(III) anions. The gold ions can then be removed from the solution which further allows the oxidation of gold. The gold is then dissolved to form chloroauric acid.
Au + 3HNO3 + 4HCl <-->[AuCl4]- + 3[NO2] + [HNO3}+ + 2H2O
A similar reaction can be written for platinum.
Pt + 4NO3- + 8H+ --> Pt4+ + 4NO2 + 4H2O
The oxidized platinum ion then can react with chlorine ions
Pt4+ + 6Cl- --> PtCl6-
There are many hazards with working with aqua regia, please be aware of these before using any chemicals
The following chemicals make up aqua regia:
Aqua Regia has the following properties:
- form: liquid
- colour: clear-colourless --> yellow --> red
- odour: strong
- pH: < 1
HCl is very corrosive and reactive. Please handle HCl with extreme care. HNO3 is corrosive and reactive as well. Read over the SDS on both HCl and HNO3
- Etching Gold
- Etching Platinum
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