Aqua Regia


aqua regia is a 3:1 mixture of hydrochloric acid and nitric acid. It is a strong oxidizing mixture that dissolves many base metal sulfide minerals as well as gold and platinum. However, it is a partial leach that leaves an undigested silicate and alumina residue as well as refractory minerals such as garnet and spinel. It is possible to vary the sample size from less than 1g up to 20 to 50g depending on the target elements and their expected concentration. Sub-ppb detection limits are possible and base metal detection limits of low ppb are common.

Its digestion can result in the partial loss of volatile arsenic and antimony. Insoluble barium and lead sulfates can precipitate when sulfide is oxidized to sulfate. This is common since its a strong oxidizing agent.

Decomposition Of Aqua regia
Upon mixing of concentrated hydrochloric acid and concentrated nitric acid, chemical reactions occur. These reactions result in the volatile products nitrosyl chloride and chlorine as evidenced by the fuming nature and characteristic yellow color of aqua regia. As the volatile products escape from solution, the aqua regia loses its potency.

HNO3 (aq) + 3 HCl (aq) → NOCl (g) + Cl2 (g) + 2 H2O (l)

Nitrosyl chloride can further decompose into nitric oxide and chlorine. This dissociation is equilibrium-limited. Therefore, in addition to nitrosyl chloride and chlorine, the fumes over aqua regia contain nitric oxide.

2 NOCl (g) → 2 NO (g) + Cl2 (g
      Aqua regia is used in etching and in certain analytic procedures. It is also used in some laboratories to clean glassware of organic compounds and metal particles. In practice Aqua regia is often used as a solvent in  the processing of gold metal, and chemicals are often used as a gold refining.