Aqua Regia (Latin for “royal water”) is an acidic, corrosive, and oxidative mixture of three parts concentrated hydrochloric acid (HCl) and one part concentrated nitric acid (HNO3). It is called aqua regia because it is one of the few acid mixtures that can dissolve the “noble” metals: gold (Au), platinum (Pt), and palladium (Pd).
HNO3 (aq) + 3HCl (aq) → NOCl (g) + 2H2O (l) + Cl2 (g)
Nitrosyl chloride (NOCl) decomposes over time, producing chlorine gas and nitric oxide (NO) that auto-oxidizes to nitrogen dioxide (NO2), a poisonous reddish-brown gas.
2NOCl (g) → 2NO (g) + Cl2 (g)
2NO (g) + O2 (g) → 2NO2(g)
WARNING: Chlorine (Cl2), nitric oxide (NO), and nitrogen dioxide (NO2) are poisonous!
The gas evolution will lead to pressure build-up and container rupture if the container is closed.
Rinse the affected skin immediately with copious amounts of water for about 15 minutes; if necessary, use the safety shower. Remove contaminated clothing.
Use the eye wash to rinse the eye thoroughly for at least 15 minutes, occasionally lifting the upper and lower eyelids and rolling the eyeballs.
Move into fresh air immediately.
Do not induce vomiting. Rinse mouth with water.
If any symptoms persist after the first aid procedures, seek medical attention. Provide the medical team with the Safety Data Sheets (SDS) for hydrochloric acid and nitric acid.
Spills should be neutralized immediately with sodium bicarbonate or other acid neutralizer.
WARNING! Never store a stoppered bottle of aqua regia—it may explode! Aqua regia should be made fresh before every use and excess amounts neutralized shortly after use.
Pour excess and waste aqua regia into a large quantity of ice (500 grams of ice per 100 mL of aqua regia). Neutralize the mixture with an aqueous basic solution, such as 1M or 10% sodium hydroxide (NaOH) or saturated sodium bicarbonate (NaHCO3) in water until pH is neutral. The neutralized solution may then be poured down drain. If the neutralized mixture contains heavy metals (e.g., gold, platinum, lead, chromium), the solution should be labeled and disposed of through the chemical waste management system.
Periodic Videos: Aqua Regia Dissolves Gold. http://www.periodicvideos.com/videos/mv_aqua_regia.htm (accessed Dec 2013).
American Industrial Hygiene Association: Two Explosions Involving Aqua Regia. http://www.aiha.org/get-involved/VolunteerGroups/LabHSCommittee/Pages/Two-Explosions-Involving-Aqua-Regia.aspx (accessed Dec 2013).
Pitt, M. J. In Bretherick’s Handbook of Reactive Chemical Hazards 6th ed.; Urben, P. G. Ed.; Butterworth-Heinemann Ltd: Oxford, 1999; Vol. 2, pp 307-312.