The Aqua Regia formula does not really exist because Aqua Regia is a mixture of concentrated acids and is not a compound; only chemical compounds have chemical formulae. However, as Aqua Regia is a mixture of one part concentrated Nitric Acid (HNO3) and three parts concentrated Hydrochloric Acid (HCl), a chemical formula of 3HCl + HNO3 could be argued. An alternative Aqua Regia formula could be written as HCl + HNO3.
Aqua Regia is a powerful cleaning substance whose name is derived from the Latin words ‘aqua’ and ‘regia’ which translate as ‘royal water’. Sometimes referred to as Aqua Regis or ‘King’s Water’ this powerful cleaning substance has the ability to dissolve Gold and Platinum, the so-called Noble Metals. On an individual basis, neither Nitric Acid not Hydrochloric Acid has this ability. Aqua Regia appears as a fuming yellow or red liquid which is classified as Highly Corrosive. It is widely used in metallurgy and is also known as Nitro-hydrochloric Acid.
Early alchemists used Aqua Regia as far back as the 800’s as it is capable of dissolving Gold. Today, the primary use of Aqua Regia is to produce Chloroauric Acid, an inorganic compound which is a common precursor to Gold. Aqua Regia used in this way facilitates the testing of very high purity Gold of up to 99.999% purity. It is also used as a specialist cleaner and etcher for certain pieces of laboratory glassware and to clean NMR tubes.
In addition to being a Highly Corrosive substance, when the two chemicals are mixed it forms a powerful oxidising medium. Brown fumes of NO2 can be seen as the Hydrochloric Acid is oxidised and the Nitric Acid is reduced. Keep away from organic solvents and reducing agents.
If you are looking for an Aqua Regia supplier it is worth noting that it is difficult to buy Aqua Regia ‘off the shelf’. The material decomposes so quickly after mixing that it needs to be made fresh each time. Also it is so volatile, toxic and explosive with other substances that it should really only be made by a qualified person who knows exactly what they are doing, having already carried out a full Risk Assessment on the procedure.
Aqua Regia should be made fresh for use and should not be placed in a sealed container as a build-up of gases may cause the container to explode. Try to use a less hazardous chemical if possible and ensure that only qualified persons handle the substance. Read the MSDS (Material Safety Data Sheet) provided by your Aqua Regia supplier BEFORE commencing work.
Ensure operators wear full personal protective clothing including approved eye protection, safety overalls, protective footwear and safety gloves. Work under fume extraction in a clean, well-ventilated area. The liquid itself and its vapours and mists can cause serious burns to all body tissue.
Inhalation of vapours can lead to permanent damage to the soft mucous membranes of the nasal passages, lungs and respiratory tract which may result in pneumonia and pulmonary oedema. Inhalation can prove fatal. Ingestion of the liquid causes serious burns to the mouth, throat, gastro-intestinal tract and stomach. Accidental skin contact will result in pain, redness and severe burns whilst eye contact will cause severe irritation and may result in permanent eye damage.