Aqua Regia


Aqua regia (Latin for "Royal Water") is a solution of nitrohydrochloric acid. The traditional solution is comprised of a 3:1 mixture of hydrochloric acid and nitric acid, respectively.  It is commonly used to remove noble metals such as gold, platinum and palladium from substrates, particularly in microfabrications and microelectronics labs.  Glassware may also be washed with aqua regia to remove organic compounds only in trace amounts. Aqua regia solutions are extremely corrosive and may result in explosion or skin burns if not handled with extreme caution.

Emergency Procedures

In case of skin contact: May cause skin burns.  Flush the skin with copious amounts of water for at least 15 minutes.  Seek medical attention.

In case of eye contact:  Aqua Regia is corrosive and irritating to the eyes.  Flush contaminated eye(s) immediately with copious quantities of water for at least 15 minutes.  Seek medical attention immediately.

In case of inhalation: May irritate the respiratory tract.  Conscious persons should be assisted to an area with fresh, uncontaminated air.  Seek medical attention in the event of respiratory irritation, cough, or tightness in the chest.  Symptoms may be delayed.

In case of ingestion: Not a likely route of exposure.


  • Always use glass (preferably Pyrex) containers.  Aqua regia will melt some plastics and corrode most metals. 
  • Never store aqua regia solutions. Mix up only what you need, then destroy after each use.
  • Mix the solution in a hood with the sash between you and the solution.  Wear chemical splash goggles, faceshield, labcoat and appropriate gloves. 
  • When preparing the aqua regia solution, always add the nitric acid to the hydrochloric acid slowly. 
  • Dissolving metals in aqua regia releases toxic gases, always work with aqua regia in a fumehood.
  • Aqua regia solution is very energetic and potentially explosive.  It is very likely to become hot, more than 100oC.  Handle with care. 
  • Adding any acids or bases to aqua regia or spraying it with water will accelerate the exothermic reaction.
  • Leave the hot aqua regia solution in an open container until cool. 
  • Never store aqua regia in a closed container. It will oxidize over time to form toxic nitrosyl chloride, nitrogen dioxide and chlorine gases. This will pressize the container, likely causing an explosion. 
  • Mixing aqua regia with organic compounds may cause an explosion. 


Do not store aqua regia.  Aqua regia quickly loses it effectiveness due to oxidation of its reactive components. Mix a fresh solution for each use.  Excess solutions should be neutralized with sodium bicarbonate and disposed via the drain, followed by flushing with copious amounts of water.


After the material has cooled, the spent solution should be neutralized with sodium bicarbonate and disposed via the drain, followed by flushing with copious amounts of water. If the solution is contaminated with heavy metals (i.e. silver, chromium), the neutralized solution should be collected as hazardous waste.


Stanley Howell
Program Manager - Chemical Safety

Steve Elwood
Associate Director for Laboratory Safety