Background and Overview of Hazards

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.

Preparation and Safe Handling

• Appropriate Personal Protective Equipment (lab coat, safety glasses, with side shields or splash goggles and gloves compatible with nitric and hydrochloric acid) should always be used when handling aqua regia.When handling large amounts (>500mL) or when splashing is more likely, wear acid resistant gloves with extended cuffs. Check the manufacturer's rating for hydrochloric and nitric acid. 
• All handling of aqua regia MUST be done with glassware inside a fume hood. Never remove a container with aqua regia from the fume hood. Keep the sash as low as possible to capture the toxic fumes. Never raise it above the indicated position (18 inches).
• Perform all work in a secondary container that is clean and free of organic matter.
  Warning: NEVER add any organics to aqua regia solution, it could cause an explosion. Any chemical containing a C-H bond, e.g., acetone, isopropanol, ethanol, photoresist, detergents, is organic. Even small amounts of organics could make the aqua regia solution unstable.
• For best results and for safety reasons, make only small, fresh batches of aqua regia for each use.
• When preparing aqua regia, always add nitric acid to hydrochloric acid, never vice versa.

Emergency Procedures

Accidental Exposure

Skin Contact

Rinse the affected skin immediately with copious amounts of water for about 15 minutes; if necessary, use the safety shower. Remove contaminated clothing.

Eye Contact

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. 

Inhalation

Move into fresh air immediately.

Ingestion

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

Spills should be neutralized immediately with sodium bicarbonate or other acid neutralizer. 

Storage

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.

Disposal

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.

• Aqua regia (with barium, cadmium, lead & selenium waste) – UI# 80574
• Aqua regia (with chromium waste) – UI# 80575
• Aqua regia (with all other wastes) – UI# 586

References

Periodic Videos: Aqua Regia Dissolves Gold. http://www.periodicvideos.com/videos/mv_aqua_regia.htm (accessed Oct 2019).

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.