Aqua regia is the chemical that destroys (dissolves) gold.
Aqua regia, named by alchemists because of its ability to dissolve noble metals like gold and platinum, is a highly corrosive mixture of concentrated nitric acid and hydrochloric acid, optimally in a molar ratio of 1:3. The name means "royal water" or "king's water," referring to its ability to dissolve the noble metals gold and platinum, though not all metals.
How Aqua Regia Dissolves Gold
Neither nitric acid nor hydrochloric acid can dissolve gold alone. Nitric acid will act as an 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 ( [AuCl4]− ), also in solution. The reaction with hydrochloric acid is an equilibrium reaction that favors the formation of tetrachloroaurate(III) anions. This reaction results in the removal of gold ions from solution and allows the further oxidation of gold to take place. The appropriate reaction equation is:
Au + 3 HNO3 + 4 HCl ⇌ [AuCl4]− + 3NO2 + H3O+ + 2 H2O
Why Aqua Regia?
While nitric acid is a powerful oxidizer that can form gold ions (Au3+), the concentration of gold ions remains low in the presence of nitric acid alone, preventing the complete dissolution of gold. Hydrochloric acid significantly enhances the gold dissolution process. By reacting with the gold ions to form tetrachloroaurate(III) anions ([AuCl4]-), it effectively removes gold ions from the solution, which enables further oxidation of gold. This synergistic effect is crucial for dissolving gold completely.
In summary, aqua regia's unique combination of nitric and hydrochloric acids enables it to dissolve gold by forming gold ions and then removing them from the solution as tetrachloroaurate(III) anions.
