Aqua Regia: This reagent is 3 moles HCl and 1 mole HNO3. The chemical equation is

NO3-   +   4H+   +   3e-   -----> NO  +  2 H2O        E=0.957 eV
 

With 2 moles HCl and 1 mole HNO3, the chemical equation is

NO3-   +   3H+   +   2e-   -----> HNO2  +   H2O        E=0.934 eV
 

With equimolar HCl and HNO3, the chemical equation is

2 NO3-   +   4H+   +   2e-   -----> N2O4  +  2 H2O        E=0.803 eV
 

The HCl/H2O2 Method: This method uses HCl and hydrogen peroxide. The chemical equation is

H2O2    +    2H+   +   2e-    -----> 2 H2O         E=1.776 eV
 

The Ozone Method: This method uses HCl and ozone. The chemical equation is

O3    +    2H+   +   2e-    -----> O2    +    H2O     E=2.076 eV
 

The HR Method: This method uses HCl and calcium hypochlorite which give calcium chloride and hypochlorous acid (HClO). The chemical equation is

HClO   +   H+   +   e-   ----->  1/2 Cl2   +   H2O     E=1.611 eV
 

The Perchloric Acid Method: This method uses HCl and HClO4. The chemical equation is

ClO4-   +   8H+   +  7e-   ----->  1/2 Cl2   +   4 H2O     E=1.39 eV
 

When enough HCl is present, chloroauric acid (HAuCl4) is formed. The color of HAuCl4 is yellow. It is soluble in ether and can be extracted from an acid solution with ether.

When there is not enough HCl present, gold trichloride (AuCl3) is formed. The color of AuCl3 is red.

Source: Handbook of Chemistry and Physics, 80th Edition, 1999.