Aqua regia is a mixture of three parts concentrated hydrochloric acid to one part concentrated nitric acid, mixed by volume. The name is Latin for “royal water” because it’s one of the few chemical mixtures capable of dissolving gold and platinum. It’s used in laboratories for cleaning glassware, dissolving precious metals, and etching certain materials.
The Ratio and Mixing Order
The standard preparation is a 3:1 volume ratio of concentrated hydrochloric acid (HCl) to concentrated nitric acid (HNO₃). For example, 75 mL of hydrochloric acid combined with 25 mL of nitric acid produces 100 mL of aqua regia. Always add the nitric acid to the hydrochloric acid slowly, not the other way around. This order matters for controlling the reaction.
As the two acids combine, they react to produce nitrosyl chloride and chlorine gas, along with water. This reaction is what gives aqua regia its distinctive fuming yellow color and its power. Both chlorine and nitrosyl chloride are volatile, so the mixture produces visible yellow-tinted fumes almost immediately. If the solution turns yellow and begins fuming, that’s the expected result.
How It Dissolves Gold
Neither hydrochloric acid nor nitric acid can dissolve gold on its own. Together, they accomplish something neither can do alone. The nitric acid acts as a powerful oxidizer, stripping electrons from the gold’s surface. The hydrochloric acid then provides chloride ions that bind to the oxidized gold atoms, pulling them into solution and exposing fresh metal underneath. This cycle continues until the gold is fully dissolved.
George de Hevesy, a Hungarian chemist working in Copenhagen, famously used this property during World War II. When German forces occupied Denmark in April 1940, two Nobel Prize medals made of gold were sitting in Niels Bohr’s institute. The medals belonged to German physicists Max von Laue and James Franck, who had sent them to Copenhagen to prevent confiscation by the Nazi regime. Having your name engraved on gold smuggled out of Germany was dangerous. De Hevesy dissolved both medals in aqua regia while German troops marched through the streets outside. The jar of orange solution sat on a shelf, unnoticed, for the entire war. Afterward, the gold was recovered from the solution and the Nobel Foundation recast both medals. De Hevesy later noted that dissolving them hadn’t been easy, since gold is “exceedingly unreactive and difficult to dissolve.”
Why It Cannot Be Stored
Aqua regia loses its potency quickly. The reactive compounds that give it dissolving power break down over time, and the mixture continuously produces gas even after the initial reaction. This is the critical safety issue with storage: the ongoing decomposition releases nitrogen dioxide gas. In a sealed container, that gas builds pressure with no way to escape.
NASA documented a case where a tightly capped bottle of dilute aqua regia waste, stored in a fume hood, exploded from exactly this kind of pressure buildup. The lesson is straightforward: never put aqua regia in a sealed container, and never store it for later use. Prepare only the amount you need, use it promptly, and dispose of the spent solution the same day.
Fume and Inhalation Hazards
The gases produced during mixing and use are seriously dangerous. Nitrosyl chloride and chlorine gas are both toxic to the respiratory tract. Inhaling the fumes can cause coughing, choking, and irritation of the nose and throat in the short term. Higher or prolonged exposure can lead to breathing difficulties, pneumonia, and fluid buildup in the lungs (pulmonary edema), which can be fatal. Long-term exposure to the vapors causes erosion of tooth enamel and permanent lung damage.
This is not a mixture you can safely prepare in an open room, a garage, or outdoors. It requires a properly functioning chemical fume hood that pulls the gases away from you and vents them. The liquid itself is equally corrosive: it causes severe chemical burns to skin, eyes, and any tissue it contacts. Splashes to the eyes can cause permanent damage in seconds.
Required Safety Equipment
Working with aqua regia requires, at minimum, chemical splash goggles (not regular safety glasses), a face shield worn over the goggles, acid-resistant gloves, a lab coat or chemical-resistant apron, and closed-toe shoes. All mixing and use should happen inside a fume hood. If you don’t have access to a fume hood, you don’t have the setup needed to work with this mixture safely.
Keep the acids and the prepared mixture away from organic solvents, flammable materials, and metals you don’t intend to dissolve. Aqua regia reacts violently with many organic compounds and can ignite or explode on contact. Glass and certain fluoropolymer containers are appropriate; metal containers are not.
Practical Considerations
Because aqua regia degrades quickly, prepare it fresh each time. Start with clean, dry glassware. Pour the measured volume of hydrochloric acid into the container first, then slowly add the nitric acid. Expect an immediate color change to yellow or orange and visible fume production. Allow the mixture to react for a few minutes before introducing the material you want to dissolve.
The speed at which aqua regia dissolves gold depends on the surface area. Thin foils and fine powders dissolve faster than solid chunks. Gentle heating can accelerate the process, but increases fume production significantly and should only be done inside a fume hood with the sash lowered.
Spent aqua regia still contains strong acids and potentially dissolved metals. It needs to be neutralized and disposed of according to your local hazardous waste regulations. Pouring it down a drain is not safe and is illegal in most jurisdictions. If you’re working in a university or commercial lab, your environmental health and safety office will have specific procedures for collection and disposal.