Aqua regia is a highly corrosive mixture of hydrochloric acid (HCl) and nitric acid (HNO3) in a molar ratio of 3:1 or 4:1. The name “aqua regia” translates from Latin to mean “king’s water” or “royal water,” referring to the acid’s ability to dissolve the noble metals gold, platinum, and palladium. It does not dissolve all noble metals. Tantalum and iridium, for example, won’t dissolve. Freshly-prepared aqua regia is colorless, but darkens to form a fuming yellow-orange or orange-red liquid.
Aqua regia may also be called royal water or nitro-muriatic acid (the name given to it in 1789 by Antoine Lavoisier).
Aqua Regia Synthesis Reaction
When hydrochloric acid and nitric acid are mixed, the initial products are nitrosyl chloride (NOCl), water, and chlorine gas (Cl2):
3HCl + HNO3 → NOCl(g) + 2H2O(l) + Cl2(g)
Nitrosyl chloride decomposes to form more chlorine gas and nitric oxide (NO):
2NOCl(g) → 2NO(g) + Cl(sub>2(g)
Nitric oxide reacts with oxygen to form nitrogen dioxide (NO2):
2NO(g) + O2 → 2NO2(g)
You don’t want to be exposed to any of these compounds. Aqua regia, hydrochloric acid, and nitric acid are strong acids. Chlorine, nitric oxide, and nitrogen dioxide are all toxic gases.
How to Make Aqua Regia
- The preferred molar ratio for aqua regia is 3 moles hydrochloric acid to 1 mole of nitric acid. However, concentrated HCl is typically 35%, while concentrated nitric acid is 65%. It’s easier to use the volume ratio of 4 parts concentrated hydrochloric acid to 1 part nitric acid. Generally, only small volumes of aqua regia are prepared at a time, using 8 mL HCl and 2 mL HNO3 for a total volume of 10 mL.
- Pour the nitric acid into the hydrochloric acid. Do not pour concentrated hydrochloric acid into nitric acid, to reduce the risk of splattering hot acid! Initially, the mixture will be colorless. It will deepen in color to form a fuming golden or orange-red liquid that will smell strongly of chlorine. You shouldn’t smell the chlorine, because the preparation should occur within a functional fume hood!
- Place the mixture in a cool location and use it while it is fresh. Do not stopper aqua regia because pressure buildup could break the container, spilling the corrosive acid.
- Do not store aqua regia, as it will become unstable.
How to Dispose of Aqua Regia (Safely)
Disposal of the pure mixture involves neutralizing aqua regia with a base and then pouring it down the drain. To do this, pour the aqua regia over a large volume of ice to dilute and cool it. Neutralize it with either 10% sodium hydroxide (NaOH) or else saturated sodium bicarbonate (baking soda). This mixture is safe to pour down plumbing.
However, it’s best to consult local regulations because the practical use of the acid mixture is to dissolve other metals. In some cases, it may be preferable to recover the metals (e.g., precious metals) or store the neutralized solution for proper disposal (e.g., toxic metals).
Aqua Regia Safety Tips
Aqua regia is scary stuff! Mixing two strong acids produces heat and releases toxic vapors. Once mixed, the acids continue to react. Even after decomposition occurs, the liquid remains a strong, corrosive acid.
- Only prepare the minimum volume necessary.
- Prepare and use aqua regia within a working fume hood. Close the sash of the hood as much as is practical to avoid the risk of getting splashed.
- Wear full protective gear, including safety goggles, gloves, a lab coat, and covered shoes. Tie back long hair.
- Only prepare and use aqua regia in clean glassware. Organic contaminants react vigorously with the acid. Do not use aqua regia for an application that involves organic compounds.
- In case of a spill, rinse the acid with a large volume of water and neutralize it with baking soda or another weak base. Remove affected clothing. In case of eye contact, use the emergency eye wash and seek immediate medical attention. In case of fume inhalation, remove the victim to fresh air. In case of ingestion, rinse with water, do not induce vomiting, and seek emergency medical attention.
Aqua Regia Facts
IUPAC Name: Nitric acid hydrochloride
Other Names: Aqua regis, Nitrohydrochloric acid
Chemical Formula: HNO3+3 HCl
Appearance: Yellow, orange, or red fuming liquid
Density: 1.01–1.21 g/cm3
Boiling Point: 108 °C (226 °F; 381 K)
Solubility: Miscible in water
Vapor Pressure: 21 mbar
CAS Number: 8007-56-5
- Committee on Prudent Practices for Handling, Storage, and Disposal of Chemicals in Laboratories, National Research Council (1995). Prudent Practices in the Laboratory: Handling and Disposal of Chemicals. National Academies Press. pp. 160–161.
- Principe, Lawrence M. (2012). The Secrets of Alchemy. Chicago: University of Chicago Press. ISBN 0226682951.