The process of "washing" gold typically refers to removing impurities and refining it, and this is often achieved using a mixture of nitric and hydrochloric acid. This mixture is called aqua regia.
What is Aqua Regia?
Aqua regia is a highly corrosive mixture of concentrated nitric acid (HNO3) and hydrochloric acid (HCl), optimally in a molar ratio of 1:3. It was named so by alchemists because it can 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]− + 3 NO2 + H3O+ + 2 H2O
or
Au + HNO3 + 4 HCl ⇌ [AuCl4]− + NO + H3O+ + 2 H2O
Nitric Acid in Gold Purification
As referenced in the instructions, nitric acid is used in the purification of gold, but it is not used alone to "wash" gold to dissolve it. It's used in combination with hydrochloric acid (as aqua regia). Some refining processes may use nitric acid to dissolve base metals before applying aqua regia to dissolve the gold itself.
Summary
While nitric acid plays a role in the overall gold refining process, the acid mixture most accurately described as dissolving and therefore "washing" gold is aqua regia, a combination of nitric and hydrochloric acid.
