Aqua Regia Acid Solution

Pure platinum readily dissolves in aqua regia.
Alexander C. Wimmer

Aqua regia is an extremely corrosive mixture of nitric and hydrochloric acid, used as an etchant, for some analytical chemistry procedures, and to refine gold. Aqua regia dissolves gold, platinum, and palladium, but not the other noble metals. Here's what you need to know to prepare aqua regia and use it safely.

Reaction to Make Aqua Regia

Here is what happens when nitric acid and hydrochloric acid are mixed:

HNO3 (aq) + 3HCl (aq) → NOCl (g) + 2H2O (l) + Cl2 (g)

Over time, nitrosyl chloride (NOCl) will decompose into chlorine gas and nitric oxide (NO). Nitric acid auto-oxidizes into nitrogen dioxide (NO2):

2NOCl (g) → 2NO (g) + Cl2 (g)

2NO (g) + O2 (g) → 2NO2(g)

Nitric acid (HNO3), hydrochloric acid (HCl), and aqua regia are strong acids. Chlorine (Cl2), nitric oxide (NO), and nitrogen dioxide (NO2) are toxic.

Aqua Regia Safety

Aqua regia preparation involves mixing strong acids. The reaction produces heat and evolves poisonous vapors, so it's important to follow safety protocols when making and using this solution:

  • Make and use aqua regia solution inside a fume hood, with the sash down as much as is practical to contain the vapors and protect against injury in case of splashing or glassware breakage.
  • Prepare the minimum volume needed for your application.
  • Make sure your glassware is clean. In particular, you don't want any organic contaminants because they can produce a vigorous or violent reaction. Avoid using any glassware that may be contaminated with a chemical containing a C-H bond. Do not use the finished solution on any material containing an organic.
  • Wear safety goggles.
  • Wear a lab coat.
  • Wear gloves.
  • If you get drops of any of the strong acids on your skin, wipe them off immediately and rinse with lots of water. If you spill acid on clothing, remove it immediately. In ​the case of inhalation, move immediately to fresh air. Use the eye wash and seek emergency medical attention in case of eye contact. In the case of ingestion, rinse mouth with water and do not induce vomiting.
  • Neutralize any spills with sodium bicarbonate or similar compound. Remember, it's best to neutralize a strong acid with a weak base and not a strong base.

Prepare Aqua Regia Solution

  1. The usual molar ratio between concentrated hydrochloric acid and concentrated nitric acid is HCl:HNO3 of 3:1. Keep in mind, concentrated HCl is about 35%, while concentrated HNO3 is about 65%, so the volume ratio is usually 4 parts concentrated hydrochloric acid to 1 part concentrated nitric acid. A typical total final volume for most applications is only 10 milliliters. It's unusual to mix up a large volume of aqua regia.
  2. Add the nitric acid to the hydrochloric acid. Do not add hydrochloric to nitric! The resulting solution with be a fuming red or yellow liquid. It will smell strongly of chlorine (although your fume hood should protect you from this).
  3. Dispose of leftover aqua regia by pouring it over a large amount of ice. This mixture may be neutralized with a saturated sodium bicarbonate solution or 10% sodium hydroxide. The neutralized solution may then be safely poured down the drain. The exception is used solution that contains heavy metals. Heavy metal-contaminated solution needs to be disposed of according to your local regulations.
  1. Once you have prepared aqua regia, it should be used when it's fresh. Keep the solution in a cool location. Do not store the solution for an extended length of time because it becomes unstable. Never store stoppered aqua regia because pressure build-up could break the container.

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Helmenstine, Anne Marie, Ph.D. "Aqua Regia Acid Solution." ThoughtCo, Mar. 15, 2017, Helmenstine, Anne Marie, Ph.D. (2017, March 15). Aqua Regia Acid Solution. Retrieved from Helmenstine, Anne Marie, Ph.D. "Aqua Regia Acid Solution." ThoughtCo. (accessed September 23, 2017).