Two Explosions Involving Aqua Regia

Key Learning Points

  • Use a reagent that is milder than aqua regia for cleaning glassware if it will suffice.

  • Do not take aqua regia out of the fume hood in which it was prepared, and do not store it there either; make only what you need and destroy the residue.  Aqua regia can be destroyed by cautious and careful dilution with water - talk to your supervisor or your safety office for a detailed procedure. If necessary, the solution can then be neutralized and disposed of in the approved manner.

  • Never put aqua regia in a closed container or near flammables.

There have been explosions involving aqua regia ( a mixture of hydrochloric acid and nitric acid) reported at two universities. Both of the incidents took place in chemistry laboratories.

In the first incident, a graduate student was using aqua regia for the cleaning of NMR tubes. When he was finished, he placed the residues (about 50-60 ml) in a four-liter waste bottle, capped it tightly and placed it in a flammable storage cabinet. Approximately one hour after the bottle was placed in the cabinet, it burst, breaking an adjacent bottle of pyridine. Luckily, the pyridine did not ignite and other nearby bottles containing flammable solvents did not become involved. The pyridine leaked onto the floor, where it dissolved floor tiles and created a lingering bad smell.

The second incident occurred in a fume hood in a synthetic chemistry laboratory.   A tightly closed waste bottle containing used aqua regia exploded, most probably due to  pressure buildup inside the bottle.

Since the sash was not completely closed the broken waste bottle was not contained. Broken glass as well as some liquid acid waste were thrown out of the hood.  Since nobody was near the hood at that moment, there were no injuries.   Moreover, a nearby bottle of mercury nitrate waste was also broken as well as the secondary container, so that a small spill (less than 1 liter) of liquid acid and solid mercury nitrate occurred inside the hood. 

What is aqua regia?

Aqua regia has been used by chemists for centuries, especially as a medium for dissolving noble metals but also for other purposes. It is a mixture of concentrated hydrochloric and nitric acid which forms a powerful oxidizing medium. Mixing an oxidizer with organic materials may result in a highly exothermic reaction.  Even without other materials present, a chemical reaction occurs slowly and brown fumes of NO2 can be observed (in freshman chemistry terms, nitric acid is reduced and hydrochloric acid is oxidized). The activity as a dissolving agent decreases slowly and so, by definition, the solution is unstable - it should be used "freshly prepared". 

Rules for using aqua regia

Aqua regia is often used as a substitute for chromic sulfuric acid cleaning solutions.   However, aqua regia is also corrosive and strongly oxidizing.  It  is essential for some purposes but should not be used for routine cleaning of glassware.  If a milder reagent will suffice avoid using aqua regia.  Alternatives include ultrasonic baths, alconox or similar detergents, Pierce RBS-35 (available from VWR) or similar detergents or biodegradable surfactants.  
Be aware that sufficient pressure can build up in a  short amount of  time to burst the container,  even from a very small volume of aqua regia.

If it is decided that aqua regia is needed, wear protective clothing (goggles, gloves, coat) and work in a clean well-ventilated fume hood. Keep the sash down when reactions are in progress. Never take aqua regia out of the hood.

Prepare it, use it, and destroy any excess in the hood in which it was prepared.

Only prepare the amount of aqua regia you need for immediate use. Never store it and never put it in a closed vessel, since evolved gases will cause a pressure build-up and possible explosion.

Aqua regia is a strong oxidizer. It is incompatible with organic solvents, flammables and any reducing agents.