Aqua regia is a mixture of concentrated hydrochloric (3 parts) and nitric (1 part) acids and has the notoriety of dissolving even gold, palladium and platinum. For best results mix the nitric acid into the hydrochloric acid mixture. Because Aqua Regia is a very powerful oxidizer, never let Aqua Regia come into contact with organic materials. See past Aqua Regia here
HNO3(aq) + 3HCl (aq) → NOCl (g) + 2H2O (l) + Cl2 (g)
Over time generated nitrosyl chloride (NOCl) further decomposes producing chlorine gas and nitric oxide (NO) which auto-oxidizes to nitrogen dioxide (NO2), a poisonous reddish-brown gas.
2NOCl (g) → 2NO (g) + Cl2 (g)
2NO (g) + O2 (g) → 2NO2(g)
Be aware that fresh mixtures of aqua regia evolve oxidizing and toxic gases (chlorine, nitric oxide, nitrogen dioxide), therefore making aqua regia should only be done under good exhaust ventilation (i.e. chemical fume hood). Additionally this evolved gas can cause explosions if containers of aqua regia are capped. Therefore containers should be loosley capped.
Ideally Aqua Regia solutions should be made fresh (for best results), then used and destroyed/quenched/neutralized shortly after use. Batches should also be created in the smallest size needed; avoid making large batches. If quenching or neutralizing Aqua Regia, pour it into a large volume of ice and then start the neutralization process with an appropriate base such as sodium or potassium hydroxide.
