The mixture of one mole of conc. HNO3 and 3 mol of conc. HCl is called aqua regia. It is observed practically that, HNO3 or HCl cannot dissolve gold individually. But the mixture of the two acids in the ratio (1 : 3) can dissolve gold, platinum and that’s why it is called aqua regia.
Process of dissolving gold or platinum
Aqua regia is a strong oxidant. In this mixture HCl is oxidized by HNO3 producing nascent chlorine along with nitrosyl chloride (NOCl). Due to the production of nascent chlorine, aqua regia is very active.
HNO3 + 3HCl = 2[Cl] + 2H2O+ NOCl (nitrosyl chloride)
Gold is first oxidized by this nascent chlorine forming auric chloride. Afterwards it reacts with HCl forming soluble chloro auric acid.
Au + 3[Cl] = AuCl3 (Auric chloride)
AuCl3,+ HCl = HAuCl4 (Chloro auric acid)
Platinum is first oxidized by this nascent chlorine forming platinum chloride. Afterwards it reacts with HCl forming soluble chloro platinic acid.
Pt + 4[Cl] = PtCl4 (platinum chloride)
PtCl4 + 2HCl = H2PtCl6 (chloro platinic acid)