Family: Dual Acids
Formulation: Aqua Regia—“Royal Water” (aka, "Nitrohydrochloric Acid")
Uses:
Typically used for removal of stubborn organic residues from laboratory glassware, or, in semiconductor processing, removal of deposits from inside of quartz furnace tubes.
Introduction:
This is an extremely powerful oxidizing solution prepared from 1 part concentrated Nitric Acid (69% HNO3) and 3 parts concentrated Hydrochloric Acid (38% HCL). It is the only acidic solution that will dissolve gold and will effectively oxidize anything it comes in contact with. Extreme caution must be used when working with aqua regia because it generates chlorine gas and nitrous oxides.
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Background:
Gold is among the so-called “noble metals,” named as such for their lack of reactivity. Gold will not dissolve in concentrated forms of either nitric acid or hydrochloric acid. But mix the two acids together and the combination of chlorides with an oxidizer becomes a very effective medium to dissolve gold, platinum, etc. The colorful name, “aqua regia,” is derived from ancient alchemy. It is from the Latin for “royal water,” and is believed to have been discovered in 800 AD by the alchemist Abu Musa Jabir ibn Hayyan.
Interesting Anecdote:
During World War II, a Hungarian chemist living in Denmark, George de Hevesy, dissolved two fellow scientists’ Nobel Prizes in aqua regia literally as the Nazis stormed into Copenhagen so they wouldn’t be stolen (he assumed, correctly, that the Nazis would just leave the chemicals alone). After the war, he recovered the gold, and the Nobel committee re-coined the prizes.
Formulation:
Aqua regia is a mixture typically of one part concentrated nitric acid (69% HNO3) and three parts concentrated hydrochloric acid (38% HCl).
Aqua regia decays after being mixed, so it must always be made up fresh. The nitric acid slowly works on the chloride ion, releasing highly toxic chlorine gas. Also generated is a reddish-orange gas—the also toxic nitrosyl chloride (NOCl). It is recommended that 1 part H2O be added if the aqua regia will be stored to minimize the generation of chlorine gas.
Precautions:
This cleaning solution can do severe damage to the eyes, skin, mucous membranes and lungs. Extreme caution should be exercised when using this formulation.
Wear butyl gloves (not latex or nitrile exam gloves), eye protection and a lab coat. All work must be conducted under a fume hood.
Note: Do not attempt to store aqua regia in a sealed bottle. The build-up in chlorine gas pressure will inevitably result in an explosion. Only make up what is needed and dispose of it when your work is complete.
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