Aqua regia is a highly corrosive, fuming yellow or red solution. In a mixing between Nitric Acid (HNO3) and hydrochloric acid (HCl) is 3: 1, That is the volume of a solution of 3 parts hydrochloric acid (HCl) was added to 1 volume part of the solution of Nitric Acid (HNO3) .
Aqua regia or in Latin is more known as royal water is a chemical solvent all the metals. It is one of the few reagents that dissolves as gold, platinum and palladium from substrates, particularly in microfabrications and microelectronics labs.
It was so named because it can dissolve the so-called royal, or noble metals, although tantalum, iridium, and a few other metals are able to withstand it. Glassware may also be washed with aqua regia to remove organic compounds only in trace amounts. Aqua regia solutions are extremely corrosive and may result in explosion or skin burns if not handled with extreme caution.
Aqua regia dissolves gold, even though neither constituent acid will do so alone, because, in combination, each acid performs a different task. Nitric acid is a powerful oxidizer, which will actually dissolve a virtually undetectable amount of gold, forming gold ions (Au3 ).
The hydrochloric acid provides a ready supply of chloride ions (Cl-), which react with the gold to produce chloraurate anions, also in solution. The reaction with hydrochloric acid is an equilibrium reaction which favors formation of chloraurate anions (AuCl4-).
This results in a removal of gold ions from solution and allows further oxidation of gold to take place, and so the gold is dissolved. In addition, gold may be oxidized by the free chlorine present in aqua regia
Chemical Reactions Aqua Regia Gold
Au (s) + 3 NO3-(aq) + 6 H + (aq) → Au3 + (aq) + 3 NO2 (g) + 3 H2O (l) and
After the reaction is complete then the next step is the process of settling and separation of the deposition solution. The next solution in neutral with the addition of urea to reduce the residual HNO3 As happened in the following chemical reaction
3Pt (s) + 4 NO 3- (aq) + 16 H+ (aq) → 3Pt4+ (aq) + 4 NO (g) + 8 H2O (l)
(NO)2PtCl4 (s) + 2 HCl (aq) → H2PtCl4 (aq) + NOCl (g)