Aqua Regia, the ‘royal water’ was first prepared by a famous alchemist by the name of Abū Mūsā Jābir ibn Ḥayyān in the 8th century A.D. This traditional volatile solution is a mixture of nitric acid and hydrochloric acid in a ratio of 1:3. It is a yellowish/red liquid with a pungent odor with strong oxidizing powers.
It is now called a solution of nitrohydrochloric acid and it is given the title of ‘King’s water’ for its ability to dissolve royal or noble metals like gold (Au), silver (Ag), platinum (Pt), palladium (Pd), iridium (Ir), osmium (Os), etc.
Aqua regia solution is a fuming, extremely corrosive liquid chemical that may result in explosion or skin burn if mishandled.
Preparation and Safety in Handling
Add nitric acid (one part) to hydrochloric acid (three parts) to make a solution. As expected from two strong and concentrated acids i.e. HNO3 (pH ≈ 2.0) and HCl (pH ≈ 2.0), this mixture is incredibly reactive and corrosive. This reactivity makes it such an important chemical because it dissolves even the hardest to dissolve, metals, i.e. gold, platinum, etc.
- Proper lab safety equipment along with acid-resistant gloves are mandatory when handling aqua regia.
- All types of handling must be carried out in a fume hood because it is a highly corrosive and fuming liquid.
- While preparing aqua regia solution, make sure to always add nitric acid to hydrochloric acid, not the other way around.
- Prepare small batches so, in case of a hazard, the damage could be minimized.
- Never let anything organic (with C-H) bond get near aqua regia, as it could cause an explosion.
On Jan 30, 2015, at Texas Tech University, a small level waste bottle explosion occurred due to mishandling of such chemicals. One of the students accidentally poured nitric acid waste into a waste bottle containing hydrochloric acid, dimethyl glyoxime, and methanol. The resultant reaction built-up pressure and exploded the waste bottle but luckily, no one was seriously hurt.
- Never store this mixture as the trapped gases may also cause an explosion. Make a fresh batch every time you have to use it.
Chemistry of Aqua regia
Aqua Regia is a mixture of nitric acid (HNO3) and hydrochloric acid (HCl). The two acids react together to form nitrosyl chloride (NOCl), water vapors (H2O), and chlorine gas (Cl2).
The nitrosyl chloride eventually decomposes to chlorine gas (Cl2) and nitrous oxide gas (NO). The nitrous oxide gas then gets oxidized to nitrogen dioxide gas (NO2), a reddish-brown and poisonous gas upon exposure to air.
HCl (aq) + HNO3 (aq) → NOCl (g) + 2H2O (l) + Cl2 (g)
2NOCl (g) → 2NO (g) + Cl2 (g)
2NO (g) + O2 (g) → 2NO2 (g)
Chemical Reactions
Aqua regia with Gold (Au)
The primary use of aqua regia is the production of chloroauric acid [H(AuCl4).5H2O] which is the primary electrolyte of the Wholwill electrolytic process for the refining of gold. Normally gold is an inert metal and does not dissolve easily, but the magical power of this mixture does so. Nitric acid being a strong oxidizing agent oxidizes Au0 to Au+3 ion. These ions are then acted on by hydrochloric acid, which provides chlorine ions, hence producing tetrachloroaurate (III) ion.
Au (s) + HNO3 (aq) → Au(NO3)3 (aq) + H2 (g)
Au(NO3)3 (aq) + HCl (aq) ⇌ AuCl4–(aq) + HNO3 (aq)
As the solution is well protonated;
AuCl4–(aq) + H+ → [H(AuCl4)]
As the second reaction is reversible, the continual removal of aurate [Au (III)] ions will lead to the dissolution of more and more gold ions. This gold solution can then be used to obtain more pure gold which is 99.99% pure.
Some side reactions continue to occur which produce nitrosyl chloride (NOCl), nitrous oxide (NO), nitrogen dioxide (NO2), and water vapors (H2O) as explained in the section above.
Aqua regia with Platinum (Pt)
The reactivity of aqua regia suggests that it reacts with gold (Au) and platinum (Pt) along with other noble metals in a similar way.
4HCl + 2HNO3+ Pt → PtCl4 + NO2 + NO + 3H2O
Aqua regia with Tin (Sn)
Similarly, it reacts with tin to form tin (IV) chloride, containing tin in its highest oxidation state;
4HCl + 2HNO3+ Sn → SnCl4 + NO2 + NO + 3H2O
Applications of Aqua Regia
Alchemists originally developed it to dissolve metals and today, it’s commonly used in chemistry labs to prepare samples for analysis. In addition, aqua regia can also be used to remove rust from metal objects.
Aqua regia has many uses, such as dissolving gold and silver jewelry.
Storage
As the individual components of this mixture are fuming, NEVER EVER store a stoppered bottle of aqua regia. It should be always made fresh and the excess amount shall be properly disposed of.
Disposal
Excess or waste aqua regia can be discarded into a large quantity of ice, then neutralized by an alkaline solution. For this purpose, NaOH or Na2CO3 solutions (∼10%) can be used. The neutralized solution is now safe to drain off but if the solution contains metal ions such as gold, platinum, lead, chromium, etc, then a standard procedure of chemical waste management and disposal techniques must be used.
If this procedure is not followed properly, the metallic ions will eventually reach back to living beings and will surely have a devastating effect on them.
What to do in case of:
Eye contact;
It is a worst-case scenario and if it somehow happens, flush the eye(s) with a large amount of water for at least 15 mins. Besides this, seek medical attention immediately.
Skin contact;
Its corrosiveness can cause skin burn. Rinse the affected skin and remove the contaminated clothing immediately. Seek medical attention if the burning effect does not lower.
Inhalation;
Immediately move to fresh air and ask for medical attention if symptoms like cough, sneezing, and a sense of tightness in the chest persist.
Ingestion;
It is not a likely route of exposure but rinse your mouth thoroughly in case it happens. Do not induce vomiting, and go for medical help instead.
Spillage;
Spills shall be neutralized on the spot (if there is no human contact) by an alkaline solution of sodium hydroxide or sodium bicarbonate, etc.
Poor man’s Aqua regia
Adding concentrated hydrochloric acid to a nitrate solution (NO3-1 along with a cation), the resultant reaction yields nitrogen dioxide, water, and nitric acid. This mixture is known as poor man’s aqua regia because of its cheap reactants. Although, not effective as the original aqua regia, it can still dissolve gold of lower quality.
Concepts Berg
Can aqua regia precipitate gold out?
Aqua regia only dissolves gold. The dissolved gold can be precipitated out by reducing them with a reducing agent which will only act on gold chloride and would leave the other metal chlorides as it is in the solution.
How to prepare aqua regia acid solution?
This solution mixture is produced by adding three parts of hydrochloric acid (HCl) to one part of nitric acid (HNO3).
What is aqua regia, and what makes it royal?
Aqua regia is a very corrosive and reactive mixture of two acids that dissolves even the most difficult to dissolve i.e. the noble metals. This is why it’s called royal as gold, silver, and platinum metals used by royal families are dissolved in it.
How can aqua regia dissolve gold?
It dissolves gold by first oxidizing it to a tripositive form, and then making a complex with chlorine and hydrogen.
Can aqua regia dissolve diamond?
Although very reactive, it still cannot break the giant crystalline structure present in diamonds. So, it’s not possible to dissolve diamonds in aqua regia.
Is aqua regia the strongest acid?
It is a very strong acid but there are some acids even more strong than this. For example, floroantimonic acid, etc.
What is the best use of aqua regia?
the most important use of aqua regia is to dissolve gold.
Can acid melt a diamond?
Acids along with aqua regia cannot melt or dissolve diamonds.
Which is the king of acids?
Sulfuric acid is generally termed the king of acids.
Can Piranha solution dissolve gold?
Although not properly soluble, freshly prepared piranha solution can significantly damage the gold layer.
Can piranha solution dissolve a human?
Piranha solution dissolves all organic matter that it comes into contact with. Therefore yes, it can dissolve a human carcass.
What acid will dissolve gold?
A mixture of two very strong acids will dissolve gold.
What is the chemical formula of aqua regia?
- HCl + HNO3
What is the symbol of aqua regia
- 3HCl + HNO3
What would happen if I drank aqua regia?
Aqua regia due to its corrosiveness can significantly dissolve organic matter. So, it would be devastating.
What is the pH value of aqua regia
It has a pH value of almost 0 to 1.
What makes aqua regia so strong?
The strongness of acids that make up this mixture makes it very strong.
What is another name for aqua regia?
Aqua regia is called “the royal water” as well as “the king’s water”.
Reference Books
- Aqua Regia solution: A detailed account By the University of Toronto (Lash Miller Laboratories, Saint George Campus)
Reference links
- Aqua Regia (Royal Society of Chemistry)
- Aqua regia (University of Illinois)
- A short account on aqua regia (britannica.com)
- Aqua regia (Princeton University)
