Aqua regia or
aqua regis (
LatinLatin is an Italic language originally spoken in Latium and Ancient Rome. Through the Roman conquest, Latin spread throughout the Mediterranean and a large part of Europe...
for
royal water or
king's water) is a highly corrosive, fuming yellow or red solution, also called
nitro-hydrochloric acid. The
mixtureIn chemistry, a mixture is when two or more different substances are mixed together but not combined chemically. The molecules of two or more different substances are mixed in the form of solutions, suspensions, and colloids....
is formed by freshly mixing concentrated
nitric acidNitric acid , also known as aqua fortis and spirit of nitre, is a highly corrosive and toxic strong acid.Colorless when pure older samples tend to acquire a stronger yellow cast due to the accumulation of oxides of nitrogen. If the solution contains more than 86% nitric acid, it is referred to as...
and concentrated
hydrochloric acidHydrochloric acid is the solution of hydrogen chloride in water. It is a highly corrosive, strong mineral acid and has major industrial uses. It is found naturally in gastric acid....
, usually in a volumetric ratio of 1:3 respectively. It was so named because it can dissolve the so-called "royal metals," or noble metals,
goldGold is a chemical element with the symbol Au and an atomic number of 79. It has been a highly sought-after precious metal for coinage, jewelry, and other arts since the beginning of recorded history. The metal occurs as nuggets or grains in rocks, in veins and in alluvial deposits. Gold is...
and
platinumPlatinum is a chemical element with the chemical symbol Pt and an atomic number of 78. Its name is derived from the Spanish term platina del Pinto, which is literally translated into "little silver of the Pinto River." It is in Group 10 of the periodic table of elements...
. However,
tantalumTantalum is a chemical element with the symbol Ta and atomic number 73. A rare, hard, blue-gray, lustrous transition metal, tantalum is highly corrosion resistant and occurs naturally in the mineral tantalite, always together with the chemically similar niobium...
,
iridiumIridium is the chemical element with atomic number 77, and is represented by the symbol Ir. A very hard, brittle, silvery-white transition metal of the platinum family, iridium is the second densest element and is the most corrosion-resistant metal, even at temperatures as high as 2000 °C...
,
osmiumOsmium is a chemical element that has the symbol Os and atomic number 76. Osmium is a hard, brittle, blue-gray or blue-black transition metal in the platinum family, and is the densest natural element. The density of osmium is , slightly greater than that of iridium, the second densest element....
,
titaniumTitanium is a chemical element with the symbol Ti and atomic number 22. Sometimes called the “space age metal”, it has a low density and is a strong, lustrous, corrosion-resistant transition metal with a silver color.Titanium can be alloyed with iron, aluminium, vanadium, molybdenum, among other...
and a few other metals are capable of withstanding chemical attack from it.
Applications
Aqua regia is primarily used to produce
chloroauric acidChloroauric acid is a inorganic compound with the formula HAuCl4. It is commonly used as a precursor to gold in a variety of purposes. The term chloroauric acid is also sometimes used to describe other gold chlorides....
, the
electrolyteIn chemistry, an electrolyte is any substance containing free ions that behaves as an electrically conductive medium, usually when in a solution...
in the
Wohlwill processThe Wohlwill process is an industrial-scale chemical procedure used to refine gold to the highest degree of purity . The process was invented in 1874 by Emil Wohlwill. This electrochemical process involves using a cast dore ingot, often called a dore bar, of 95%+ gold to serve as an anode...
. This process is used for refining highest quality (99.999%) gold. (See dissolving gold)
Aqua regia is also used in
etchingIn industry, etching, also known as chemical milling, is the process of using acids, bases or other chemicals to dissolve unwanted materials such as metals, semiconductor materials or glass. This process has been used on a wide variety of metals with depths of metal removal as large as 12mm...
and in specific
analytic proceduresAnalytical chemistry is the study of the chemical composition of natural and artificial materials. Properties studied in analytical chemistry include geometric features such as molecular morphologies and distributions of species, as well as features such as composition and species identity...
. It is also used in some laboratories to clean
glasswareLaboratory glassware refers to a variety of equipment, traditionally made of glass, used for scientific experiments and other work in science, especially in chemistry and biology laboratories...
of
organic compoundAn organic compound is any member of a large class of chemical compounds whose molecules contain carbon. For historical reasons discussed below, a few types of compounds such as carbonates, simple oxides of carbon and cyanides, as well as the allotropes of carbon, are considered inorganic...
s and metal particles. This method is preferred over the "traditional"
chromic acidChromic acid generally refers to a collection of compounds generated by the acidification of solutions containing chromate and dichromate anions or the dissolving of chromium trioxide in sulfuric acid. Often the species are assigned the formulas H2CrO4 and...
bath for cleaning
NMR tubeAn NMR tube is a thin glass walled tube used to contain samples in nuclear magnetic resonance spectroscopy. Typically NMR tubes come in 5 mm diameters but 10 mm and 3mm samples are known. It is important that the tubes are uniformly thick and well-balanced to ensure that NMR tube spins at a regular...
s, because no traces of paramagnetic
chromiumChromium is a chemical element which has the symbol Cr and atomic number 24, first element in Group 6. It is a steely-gray, lustrous, hard metal that takes a high polish and has a high melting point. It is also odourless, tasteless, and malleable...
can remain to later spoil acquired spectra. Furthermore,
chromic acidChromic acid generally refers to a collection of compounds generated by the acidification of solutions containing chromate and dichromate anions or the dissolving of chromium trioxide in sulfuric acid. Often the species are assigned the formulas H2CrO4 and...
baths are discouraged because of the high toxicity of chromium and the potential for explosions. Aqua regia is itself very corrosive and has been implicated in several explosions due to mishandling.
Due to the reaction between its components resulting in its decomposition, aqua regia quickly loses its effectiveness. As such, its components should only be mixed immediately before use. While local regulations may vary, aqua regia may be disposed of by carefully neutralizing with an appropriate agent—such as
sodium bicarbonateSodium bicarbonate or sodium hydrogen carbonate is the chemical compound with the formula NaHCO3. Sodium bicarbonate is a white solid that is crystalline but often appears as a fine powder. It has a slight alkaline taste resembling that of washing soda . It is a component of the...
—before pouring down the sink. If there is a large amount of metal in solution with the acid, it may be preferable to carefully neutralize it, and absorb the solution with a solid material such as
vermiculiteVermiculite is a natural mineral that expands with the application of heat. The expansion process is called exfoliation and it is routinely accomplished in purpose-designed commercial furnaces. Vermiculite is formed by hydration of certain basaltic minerals...
before discarding it with solid waste.
Dissolving gold
Aqua regia dissolves gold, though neither constituent acid will do so alone, because, in combination, each acid performs a different task. Nitric acid is a powerful oxidizer, which will actually dissolve a virtually undetectable amount of gold, forming gold ions (Au
3+). The hydrochloric acid provides a ready supply of chloride ions (Cl
-), which react with the gold ions to produce chloroaurate anions, also in solution. The reaction with hydrochloric acid is an equilibrium reaction which favors formation of chloroaurate anions (AuCl
4-). This results in a removal of gold ions from solution and allows further oxidation of gold to take place. The gold dissolves to become
chloroauric acidChloroauric acid is a inorganic compound with the formula HAuCl4. It is commonly used as a precursor to gold in a variety of purposes. The term chloroauric acid is also sometimes used to describe other gold chlorides....
. In addition, gold may be dissolved by the free chlorine present in aqua regia. Appropriate equations are
- Au (s) + 3 NO3- (aq) + 6 H+ (aq) → Au3+ (aq) + 3 NO2 (g) + 3 H2O (l) and
- Au3+ (aq) + 4 Cl- (aq) → AuCl4- (aq).
The oxidation reaction can also be written with
nitric oxideNitric oxide or nitrogen monoxide is a chemical compound with chemical formula NO. This gas is an important signaling molecule in the body of mammals, including humans, and is an extremely important intermediate in the chemical industry...
as the product rather than
nitrogen dioxideNitrogen dioxide is the chemical compound with the formula . It exists as a radical in nature. One of several nitrogen oxides, is an intermediate in the industrial synthesis of nitric acid, millions of tons of which are produced each year. This reddish-brown toxic gas has a characteristic sharp,...
:
- Au (s) + NO3- (aq) + 4 H+ (aq) → Au3+ (aq) + NO (g) + 2 H2O (l).
Dissolving platinum
Similar equations can be written for
platinumPlatinum is a chemical element with the chemical symbol Pt and an atomic number of 78. Its name is derived from the Spanish term platina del Pinto, which is literally translated into "little silver of the Pinto River." It is in Group 10 of the periodic table of elements...
. As with gold, the oxidation reaction can be written with either nitric oxide or nitrogen dioxide as the nitrogen oxide product.
- Pt (s) + 4 NO3− (aq) + 8 H+ (aq) → Pt4+ (aq) + 4 NO2 (g) + 4 H2O (l)
- 3Pt (s) + 4 NO3− (aq) + 16 H+ (aq) → 3Pt4+ (aq) + 4 NO (g) + 8 H2O (l)
The oxidized platinum ion then reacts with chloride ions resulting in the chloroplatinate ion.
- Pt4+ (aq) + 6 Cl− (aq) → PtCl62− (aq)
Experimental evidence reveals that the reaction of platinum with aqua regia is considerably more complex. The initial reactions produce a mixture of chloroplatinous acid (H
2PtCl
4) and nitrosoplatinic chloride ((NO)
2PtCl
4). The nitrosoplatinic chloride is a solid product. If full dissolution of the platinum is desired, repeated extractions of the residual solids with concentrated hydrochloric acid must be performed.
- Pt (s) + 2 HNO3 (aq) + 4 HCl (aq) → (NO)2PtCl4 (s) + 3 H2O (l) + 1/2 O2 (g)
2PtCl
4 (s) + 2 HCl
(aq) → H
2PtCl
4 (aq) + 2 NOCl
(g)
The chloroplatinous acid can be oxidized to chloroplatinic acid by saturating the solution with chlorine while heating.
- H2PtCl4 (aq) + Cl2 (g) → H2PtCl6 (aq)
Dissolving platinum solids in aqua regia was the mode of discovery for the most dense metals.
IridiumIridium is the chemical element with atomic number 77, and is represented by the symbol Ir. A very hard, brittle, silvery-white transition metal of the platinum family, iridium is the second densest element and is the most corrosion-resistant metal, even at temperatures as high as 2000 °C...
and
osmiumOsmium is a chemical element that has the symbol Os and atomic number 76. Osmium is a hard, brittle, blue-gray or blue-black transition metal in the platinum family, and is the densest natural element. The density of osmium is , slightly greater than that of iridium, the second densest element....
, both of which are found in platinum ore and will not be dissolved by the acid, instead collecting on the base of the vessel.
Decomposition of aqua regia
Upon mixing of concentrated hydrochloric acid and concentrated nitric acid, chemical reactions occur. These reactions result in the volatile products
nitrosyl chlorideNitrosyl chloride is the chemical compound NOCl. It is a yellow gas that is most commonly encountered as a decomposition product of aqua regia, a mixture of hydrochloric acid and nitric acid...
and
chlorineChlorine Chlorine Chlorine ( , from the Greek word 'χλωρóς' (khlôros, meaning 'pale green'), is the chemical element with atomic number 17 and symbol Cl. It is a halogen, found in the periodic table in group 17 (formerly VII, VIIa, or VIIb). As the chloride ion, which is part of common salt and...
as evidenced by the fuming nature and characteristic yellow color of aqua regia. As the volatile products escape from solution, the aqua regia loses its potency.
- HNO3 (aq) + 3 HCl (aq) → NOCl (g) + Cl2 (g) + 2 H2O (l)
Nitrosyl chloride can further decompose into
nitric oxideNitric oxide or nitrogen monoxide is a chemical compound with chemical formula NO. This gas is an important signaling molecule in the body of mammals, including humans, and is an extremely important intermediate in the chemical industry...
and chlorine. This dissociation is equilibrium-limited. Therefore, in addition to nitrosyl chloride and chlorine, the fumes over aqua regia contain nitric oxide.
- 2 NOCl (g) → 2 NO (g) + Cl2 (g)
History
Hydrochloric acid was first discovered around the year 800 by the
alchemistAlchemy is both a philosophy and a practice with an aim of achieving ultimate wisdom as well as immortality, involving the improvement of the alchemist as well as the making of several substances described as possessing unusual properties...
Abu Musa Jabir ibn HayyanGeber is the Latinized form of "Jabir", with the full name of Abu Musa Jābir ibn Hayyān al azdi , a prominent polymath: a chemist and alchemist, astronomer and astrologer, engineer, geologist, philosopher, physicist, and pharmacist and physician...
(Geber) by mixing
common saltSodium chloride, also known as salt, common salt, table salt, or halite, is an ionic compound with the formula NaCl. Sodium chloride is the salt most responsible for the salinity of the ocean and of the extracellular fluid of many multicellular organisms...
with vitriol (
sulfuric acidSulfuric acid, , is a strong mineral acid. It is soluble in water at all concentrations. Sulfuric acid has many applications, and is one of the top products of the chemical industry. World production in 2001 was 165 million tonnes, with an approximate value of US$8 billion...
). Jabir's invention of gold-dissolving aqua regia, consisting of hydrochloric acid and
nitric acidNitric acid , also known as aqua fortis and spirit of nitre, is a highly corrosive and toxic strong acid.Colorless when pure older samples tend to acquire a stronger yellow cast due to the accumulation of oxides of nitrogen. If the solution contains more than 86% nitric acid, it is referred to as...
, contributed to the effort of alchemists to find the
philosopher's stoneThe philosopher's stone is a legendary alchemical substance, supposedly capable of turning base metals, especially lead, into gold; it was also sometimes believed to be an elixir of life, useful for rejuvenation and possibly for achieving immortality....
.
When Germany invaded Denmark in World War II, the Hungarian chemist
George de HevesyGeorge Charles de Hevesy, Georg Karl von Hevesy, was a Hungarian radiochemist and Nobel laureate, recognized in 1943 for his key role in the development of radioactive tracers to study chemical processes such as in the metabolism of animals.- Early years :Hevesy György was born in Budapest,...
dissolved the gold
Nobel PrizeThe Nobel Prize is a Sweden-based international monetary prize. The award was established by the 1895 will and estate of Swedish chemist and inventor Alfred Nobel. It was first awarded in Physics, Chemistry, Physiology or Medicine, Literature, and Peace in 1901...
s of
Max von LaueMax Theodor Felix von Laue was a German physicist who won the Nobel Prize in Physics in 1914 for his discovery of the diffraction of X-rays by crystals. He was strongly opposed to National Socialism...
and
James FranckJames Franck was a German physicist and Nobel laureate .-Biography :Franck completed his Ph.D. in 1906 and received his venia legendi for physics in 1911, both at the University of Berlin, where he lectured and taught until 1918, having reached the position of extraordinarius professor...
into aqua regia to prevent the Nazis from stealing them. He placed the resulting solution on a shelf in his laboratory at the
Niels Bohr InstituteThe Niels Bohr Institute is a research institute at the University of Copenhagen. The research of the institute spans astronomy, geophysics, nanotechnology, particle physics, quantum mechanics and biophysics....
. It was subsequently ignored by the Nazis who thought the jar—one of perhaps hundreds on the shelving—contained common chemicals. After the war, de Hevesy returned to find the solution undisturbed and precipitated the gold out of the acid. The gold was returned to the Royal Swedish Academy of Sciences and the Nobel Foundation who recast the medals and again presented them to Laue and Franck.
Literature
- Cryptonomicon
Cryptonomicon is a 1999 novel by American author Neal Stephenson. It concurrently follows both the exploits of World War II-era Allied codebreakers and tactical-deception operatives affiliated with the Government Code and Cypher School at Bletchley Park as well as their present day descendants'...
, by Neal StephensonNeal Town Stephenson is an American writer, known for his speculative fiction works, which have been variously categorized science fiction, historical fiction, cyberpunk, and postcyberpunk. He has also written under the pseudonym of Stephen Bury.Stephenson explores areas such as mathematics,...
- The fuel for the "Galvanick Lucipher" (a sort of specialized lantern) used by the butler Ghnxh on QwghlmQwghlm is a fictional location, featured in the books Cryptonomicon and The Baroque Cycle by Neal Stephenson. Qwghlm consists of a pair of islands, Inner Qwghlm and Outer Qwghlm, off the northwestern coast of Great Britain. The islands are geographically similar to the Hebrides...
.
- The Crying of Lot 49
The Crying of Lot 49 is a novel by Thomas Pynchon. The shortest of Pynchon's novels and often considered his most accessible, the book is about a woman, Oedipa Maas, possibly unearthing the centuries-old conflict between two mail distribution companies, Thurn und Taxis and the Trystero...
, by Thomas PynchonThomas Ruggles Pynchon, Jr. is an American novelist based in New York City and noted for his dense and complex works of fiction. Hailing from Long Island, Pynchon spent two years in the United States Navy and earned an English degree from Cornell University...
- During The Courier's Tragedy, the faithful servant Ercole pours aqua regia into a steel box around the traitor Domenico's head.
- Octopussy
Octopussy is the thirteenth spy film in the James Bond series, and the sixth to star Roger Moore as the fictional MI6 agent James Bond. The film's title is taken from Ian Fleming's 1966 short story "Octopussy". However, the film's story is original and the short story is narrated as a flashback by...
, a James BondJames Bond 007 is a fictional character created in 1953 by writer Ian Fleming, who featured him in twelve novels and two short story collections. The character has also been used in the longest running and most financially successful English language film franchise to date, starting in 1962 with Dr...
film; Bond is provided by QQ is a fictional character in the James Bond novels and films. Q , like M, is a job title rather than a name. He is the head of Q Branch , the fictional research and development division of the British Secret Service...
with a fountain pen containing a mixture of hydrochloric and nitric acids, which Bond utilizes to cut his way through metal prison bars.
- In the children's novel Pigeon Post
Pigeon Post is the sixth book in Arthur Ransome's Swallows and Amazons series of children's books, published in 1936. It won the first Carnegie Medal awarded for children's literature....
, by Arthur RansomeArthur Mitchell Ransome was an English author and journalist, best known for writing the Swallows and Amazons series of children's books. These tell of school-holiday adventures of children, mostly in the Lake District and the Norfolk Broads. Many of the books involve sailing; other common...
, Dick Callum dissolves the ore they have discovered in Aqua Regia, and thinks it is gold, not an ore of copper. But copper is what Captain Flint and Timothy wanted to find.
- In "The Devil In The Dark," an episode of Star Trek
Star Trek is an American science fiction entertainment series.The original Star Trek was an American television series, created by Gene Roddenberry, which debuted in 1966 and ran for three seasons, following the interstellar adventures of Captain James T. Kirk and the crew of the Federation...
's first season in the Original Series, as James BlishJames Benjamin Blish was an American author of fantasy and science fiction...
wrote in his script novelizations, the silicon-based horta exudes aqua regia naturally as it tunnels through rock.
See also
- Aqua fortis
Aqua fortis, or "strong water," in alchemy, is a corrosive solution of nitric acid in water. It is made from saltpeter. It was used in alchemy as a solvent for dissolving silver and most other metals with notable exceptions of gold and platinum that can only be dissolved using aqua regia...
- Aqua vitae
Aqua vitae is an archaic name for a concentrated aqueous solution of ethanol. The term was in wide used during the Middle Ages, although its origin is undoubtedly much earlier having been used by St Patrick and his fellow monks to refer to both the alcohol and the waters of baptism...
- Chromic acid
Chromic acid generally refers to a collection of compounds generated by the acidification of solutions containing chromate and dichromate anions or the dissolving of chromium trioxide in sulfuric acid. Often the species are assigned the formulas H2CrO4 and...
- Digger gold
Digger gold is the common slang term for gold recovered from electronics components such as board fingers, CPUs, and connector pins. For the gold fingers on boards or circuits, often a stripping solution is used to remove the gold from the board material, nitric acid also works well in this regard...
- Sulfuric acid
Sulfuric acid, , is a strong mineral acid. It is soluble in water at all concentrations. Sulfuric acid has many applications, and is one of the top products of the chemical industry. World production in 2001 was 165 million tonnes, with an approximate value of US$8 billion...
- Nitric acid
Nitric acid , also known as aqua fortis and spirit of nitre, is a highly corrosive and toxic strong acid.Colorless when pure older samples tend to acquire a stronger yellow cast due to the accumulation of oxides of nitrogen. If the solution contains more than 86% nitric acid, it is referred to as...
External links