Picric Acid
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Picric acid is an
organic compound Some chemical authorities define an organic compound as a chemical compound that contains a carbon–hydrogen or carbon–carbon bond; others consider an organic compound to be any chemical compound that contains carbon. For example, carbon-co ...
with the formula (O2N)3C6H2OH. Its IUPAC name is 2,4,6-trinitrophenol (TNP). The name "picric" comes from (''pikros''), meaning "bitter", due to its bitter taste. It is one of the most
acid An acid is a molecule or ion capable of either donating a proton (i.e. Hydron, hydrogen cation, H+), known as a Brønsted–Lowry acid–base theory, Brønsted–Lowry acid, or forming a covalent bond with an electron pair, known as a Lewis ...
ic
phenols In organic chemistry, phenols, sometimes called phenolics, are a class of chemical compounds consisting of one or more hydroxyl groups (− O H) bonded directly to an aromatic hydrocarbon group. The simplest is phenol, . Phenolic compounds ar ...
. Like other strongly
nitrate Nitrate is a polyatomic ion with the chemical formula . salt (chemistry), Salts containing this ion are called nitrates. Nitrates are common components of fertilizers and explosives. Almost all inorganic nitrates are solubility, soluble in wa ...
d organic compounds, picric acid is an
explosive An explosive (or explosive material) is a reactive substance that contains a great amount of potential energy that can produce an explosion if released suddenly, usually accompanied by the production of light, heat, sound, and pressure. An ex ...
, which is its primary use. It has also been used as medicine (
antiseptic An antiseptic ( and ) is an antimicrobial substance or compound that is applied to living tissue to reduce the possibility of sepsis, infection, or putrefaction. Antiseptics are generally distinguished from ''antibiotics'' by the latter's abil ...
, burn treatments) and as a dye.


History

Picric acid was probably first mentioned in the 17th-century alchemical writings of Johann Rudolf Glauber. Initially, it was made by nitrating substances such as animal horn,
silk Silk is a natural fiber, natural protein fiber, some forms of which can be weaving, woven into textiles. The protein fiber of silk is composed mainly of fibroin and is most commonly produced by certain insect larvae to form cocoon (silk), c ...
,
indigo InterGlobe Aviation Limited (d/b/a IndiGo), is an India, Indian airline headquartered in Gurgaon, Haryana, India. It is the largest List of airlines of India, airline in India by passengers carried and fleet size, with a 64.1% domestic market ...
, and natural
resin A resin is a solid or highly viscous liquid that can be converted into a polymer. Resins may be biological or synthetic in origin, but are typically harvested from plants. Resins are mixtures of organic compounds, predominantly terpenes. Commo ...
, the synthesis from indigo first being performed by Peter Woulfe in 1771. The German chemist
Justus von Liebig Justus ''Freiherr'' von Liebig (12 May 1803 – 18 April 1873) was a Germans, German scientist who made major contributions to the theory, practice, and pedagogy of chemistry, as well as to agricultural and biology, biological chemistry; he is ...
had named picric acid (rendered in French as ). Picric acid was given that name by the French chemist
Jean-Baptiste Dumas Jean Baptiste André Dumas (; 14 July 180010 April 1884) was a French chemist, best known for his works on organic analysis and synthesis, as well as the determination of atomic weights (relative atomic masses) and molecular weights by measuri ...
in 1841. Its synthesis from
phenol Phenol (also known as carbolic acid, phenolic acid, or benzenol) is an aromatic organic compound with the molecular formula . It is a white crystalline solid that is volatile and can catch fire. The molecule consists of a phenyl group () ...
, and the correct determination of its formula, were accomplished during 1841. In 1799, French chemist Jean-Joseph Welter (1763–1852) produced picric acid by treating silk with nitric acid; he found that potassium picrate could explode. Not until 1830 did chemists think to use picric acid as an
explosive An explosive (or explosive material) is a reactive substance that contains a great amount of potential energy that can produce an explosion if released suddenly, usually accompanied by the production of light, heat, sound, and pressure. An ex ...
. Before then, chemists assumed that only the
salt In common usage, salt is a mineral composed primarily of sodium chloride (NaCl). When used in food, especially in granulated form, it is more formally called table salt. In the form of a natural crystalline mineral, salt is also known as r ...
s of picric acid were explosive, not the acid itself. In 1871 Hermann Sprengel proved it could be detonated and afterwards most
military A military, also known collectively as armed forces, is a heavily armed, highly organized force primarily intended for warfare. Militaries are typically authorized and maintained by a sovereign state, with their members identifiable by a d ...
powers used picric acid as their main
high explosive An explosive (or explosive material) is a reactive substance that contains a great amount of potential energy that can produce an explosion if released suddenly, usually accompanied by the production of light, heat, sound, and pressure. An exp ...
material. A full synthesis was later found by Leonid Valerieovich Kozakov. Picric acid was the first strongly explosive nitrated
organic compound Some chemical authorities define an organic compound as a chemical compound that contains a carbon–hydrogen or carbon–carbon bond; others consider an organic compound to be any chemical compound that contains carbon. For example, carbon-co ...
widely considered suitable to withstand the shock of firing in conventional
artillery Artillery consists of ranged weapons that launch Ammunition, munitions far beyond the range and power of infantry firearms. Early artillery development focused on the ability to breach defensive walls and fortifications during sieges, and l ...
. Nitroglycerine and
nitrocellulose Nitrocellulose (also known as cellulose nitrate, flash paper, flash cotton, guncotton, pyroxylin and flash string, depending on form) is a highly flammable compound formed by nitrating cellulose through exposure to a mixture of nitric acid and ...
(guncotton) were available earlier, but shock sensitivity sometimes caused
detonation Detonation () is a type of combustion involving a supersonic exothermic front accelerating through a medium that eventually drives a shock front propagating directly in front of it. Detonations propagate supersonically through shock waves with ...
in an artillery barrel at the time of firing. In 1885, based on research of Hermann Sprengel, French chemist Eugène Turpin patented the use of pressed and cast picric acid in blasting charges and
artillery shell A shell, in a modern military context, is a projectile whose payload contains an explosive, incendiary device, incendiary, or other chemical filling. Originally it was called a bombshell, but "shell" has come to be unambiguous in a military ...
s. In 1887 the French government adopted a mixture of picric acid and guncotton with the name Melinite. In 1888, Britain started manufacturing a very similar mixture in
Lydd Lydd is a town and electoral ward in Kent, England, lying on Romney Marsh. It is one of the larger settlements on the marsh, and the most southerly town in Kent. Lydd reached the height of its prosperity during the 13th century, when it was a ...
, Kent, with the name Lyddite. Japan followed with an alternative stabilization approach known as Shimose powder which, instead of attempting to stabilize the material itself, removed its contact with metal by coating the inside of the shells with layer(s) of resin and wax. In 1889, a mixture of ammonium cresylate with trinitrocresol, or an ammonium salt of trinitrocresol, started to be manufactured with the name Ecrasite in
Austria-Hungary Austria-Hungary, also referred to as the Austro-Hungarian Empire, the Dual Monarchy or the Habsburg Monarchy, was a multi-national constitutional monarchy in Central Europe#Before World War I, Central Europe between 1867 and 1918. A military ...
. By 1894 Russia was manufacturing artillery shells filled with picric acid. Ammonium picrate (known as Dunnite or explosive D) was used by the United States beginning in 1906. However, shells filled with picric acid become unstable if the compound reacts with the metal shell or fuze casings to form metal picrates which are more sensitive than the parent phenol. The sensitivity of picric acid was demonstrated by the Halifax Explosion. Picric acid was used in the Battle of Omdurman, the
Second Boer War The Second Boer War (, , 11 October 189931 May 1902), also known as the Boer War, Transvaal War, Anglo–Boer War, or South African War, was a conflict fought between the British Empire and the two Boer republics (the South African Republic and ...
, the
Russo-Japanese War The Russo-Japanese War (8 February 1904 – 5 September 1905) was fought between the Russian Empire and the Empire of Japan over rival imperial ambitions in Manchuria and the Korean Empire. The major land battles of the war were fought on the ...
, and
World War I World War I or the First World War (28 July 1914 – 11 November 1918), also known as the Great War, was a World war, global conflict between two coalitions: the Allies of World War I, Allies (or Entente) and the Central Powers. Fighting to ...
. Germany began filling artillery shells with
trinitrotoluene Troponin T (shortened TnT or TropT) is a part of the troponin complex, which are proteins integral to the contraction of skeletal and heart muscles. They are expressed in skeletal and cardiac myocytes. Troponin T binds to tropomyosin and help ...
(TNT) in 1902.
Toluene Toluene (), also known as toluol (), is a substituted aromatic hydrocarbon with the chemical formula , often abbreviated as , where Ph stands for the phenyl group. It is a colorless, water Water is an inorganic compound with the c ...
was less readily available than phenol, and TNT is less powerful than picric acid, but the improved safety of munitions manufacturing and storage caused the replacement of picric acid by TNT for most military purposes between the World Wars.Brown, G.I. (1998), ''The Big Bang: a History of Explosives'', Sutton Publishing pp.151–163 Efforts to control the availability of
phenol Phenol (also known as carbolic acid, phenolic acid, or benzenol) is an aromatic organic compound with the molecular formula . It is a white crystalline solid that is volatile and can catch fire. The molecule consists of a phenyl group () ...
, the precursor to picric acid, emphasize its importance in
World War I World War I or the First World War (28 July 1914 – 11 November 1918), also known as the Great War, was a World war, global conflict between two coalitions: the Allies of World War I, Allies (or Entente) and the Central Powers. Fighting to ...
. Germans are reported to have bought US supplies of phenol and converted it to
acetylsalicylic acid Aspirin () is the Generic trademark, genericized trademark for acetylsalicylic acid (ASA), a nonsteroidal anti-inflammatory drug (NSAID) used to reduce pain, fever, and inflammation, and as an antithrombotic. Specific inflammatory conditions ...
(
aspirin Aspirin () is the genericized trademark for acetylsalicylic acid (ASA), a nonsteroidal anti-inflammatory drug (NSAID) used to reduce pain, fever, and inflammation, and as an antithrombotic. Specific inflammatory conditions that aspirin is ...
) to keep it from the Allies. At the time, phenol was obtained from
coal Coal is a combustible black or brownish-black sedimentary rock, formed as rock strata called coal seams. Coal is mostly carbon with variable amounts of other Chemical element, elements, chiefly hydrogen, sulfur, oxygen, and nitrogen. Coal i ...
as a co-product of coke ovens and the manufacture of gas for
gas lighting Gas lighting is the production of artificial light from combustion of a fuel gas such as methane, propane, butane, acetylene, ethylene, hydrogen, carbon monoxide, coal gas (town gas) or natural gas. The light is produced either directly by ...
. Laclede Gas reports being asked to expand production of phenol (and
toluene Toluene (), also known as toluol (), is a substituted aromatic hydrocarbon with the chemical formula , often abbreviated as , where Ph stands for the phenyl group. It is a colorless, water Water is an inorganic compound with the c ...
) to assist the war effort.Beck, Bill (2007) ''Laclede Gas and St. Louis: 150 Years Working Together, 1857–2007'', Laclede Gas Company, p. 64 Both MonsantoForrestal, Dan J. (1977), ''Faith, Hope & $5000: The Story of Monsanto'', Simon & Schuster, p. 24 and Dow ChemicalBrandt, E.N. (1997), ''Growth Company: Dow Chemical's First Century'', Michigan State University, p. 77, 97 and 244 began manufacturing synthetic phenol in 1915, with Dow being the main producer. Dow describes picric acid as "the main battlefield explosive used by the French. Large amounts f phenolalso went to Japan, where it was made into picric acid sold to the Russians."
Thomas Edison Thomas Alva Edison (February11, 1847October18, 1931) was an American inventor and businessman. He developed many devices in fields such as electric power generation, mass communication, sound recording, and motion pictures. These inventions, ...
needed phenol to manufacture
phonograph A phonograph, later called a gramophone, and since the 1940s a record player, or more recently a turntable, is a device for the mechanical and analogue reproduction of sound. The sound vibration Waveform, waveforms are recorded as correspond ...
records. He responded by undertaking production of phenol at his Silver Lake, New Jersey, facility using processes developed by his chemists. He built two plants with a capacity of six tons of phenol per day. Production began the first week of September, one month after hostilities began in Europe. He built two plants to produce the raw material
benzene Benzene is an Organic compound, organic chemical compound with the Chemical formula#Molecular formula, molecular formula C6H6. The benzene molecule is composed of six carbon atoms joined in a planar hexagonal Ring (chemistry), ring with one hyd ...
at
Johnstown, Pennsylvania Johnstown is the largest city in Cambria County, Pennsylvania, United States. The population was 18,411 as of the 2020 United States census, 2020 census. Located east of Pittsburgh, it is the principal city of the Metropolitan statistical area ...
, and
Bessemer, Alabama Bessemer is a city in Jefferson County, Alabama, Jefferson County, Alabama, United States and a southwestern suburb of Birmingham, Alabama, Birmingham. The population was 26,019 at the 2020 United States census, 2020 census. It is within the Bi ...
, replacing supplies previously from Germany. Edison manufactured aniline dyes, which had previously been supplied by the German dye trust. Other wartime products included xylene, p-phenylenediamine,
shellac Shellac () is a resin secreted by the female Kerria lacca, lac bug on trees in the forests of India and Thailand. Chemically, it is mainly composed of aleuritic acid, jalaric acid, shellolic acid, and other natural waxes. It is processed and s ...
, and pyrophyllite. Wartime shortages made these ventures profitable. In 1915, his production capacity was fully committed by midyear.


Synthesis

The aromatic ring of
phenol Phenol (also known as carbolic acid, phenolic acid, or benzenol) is an aromatic organic compound with the molecular formula . It is a white crystalline solid that is volatile and can catch fire. The molecule consists of a phenyl group () ...
is activated towards electrophilic substitution reactions, and attempted nitration of phenol, even with dilute nitric acid, results in the formation of high molecular weight tars. In order to minimize these side reactions, anhydrous phenol is sulfonated with fuming sulfuric acid, and the resulting sulfonic acid is then nitrated with concentrated
nitric acid Nitric acid is an inorganic compound with the formula . It is a highly corrosive mineral acid. The compound is colorless, but samples tend to acquire a yellow cast over time due to decomposition into nitrogen oxide, oxides of nitrogen. Most com ...
. During this reaction,
nitro Nitro may refer to: Chemistry *Nitrogen, a chemical element and a gas except at very low temperatures, with which many compounds are formed: **Nitro compound, an organic compound containing one or more nitro functional groups, -NO2 **Nitro ligand ...
groups are introduced, and the
sulfonic acid In organic chemistry, sulfonic acid (or sulphonic acid) refers to a member of the class of organosulfur compounds with the general formula , where R is an organic alkyl or aryl group and the group a sulfonyl hydroxide. As a substituent, it is kn ...
group is displaced. The reaction is highly exothermic, and careful temperature control is required. Synthesis routes that nitrate
aspirin Aspirin () is the genericized trademark for acetylsalicylic acid (ASA), a nonsteroidal anti-inflammatory drug (NSAID) used to reduce pain, fever, and inflammation, and as an antithrombotic. Specific inflammatory conditions that aspirin is ...
or
salicylic acid Salicylic acid is an organic compound with the formula HOC6H4COOH. A colorless (or white), bitter-tasting solid, it is a precursor to and a active metabolite, metabolite of acetylsalicylic acid (aspirin). It is a plant hormone, and has been lis ...
can also be used to mitigate tar formation. Carbon dioxide is lost from the former via decarboxylation, while both
acetic acid Acetic acid , systematically named ethanoic acid , is an acidic, colourless liquid and organic compound with the chemical formula (also written as , , or ). Vinegar is at least 4% acetic acid by volume, making acetic acid the main compone ...
and carbon dioxide are lost from the latter. Another method of picric acid synthesis is direct nitration of 2,4-dinitrophenol with nitric acid. It crystallizes in the orthorhombic space group ''Pca''21 with a = 9.13 Å, b = 18.69 Å, c = 9.79 Å and α = β = γ = 90°.V. Bertolasi, P. Gilli, G. Gilli: ''Hydrogen Bonding and Electron Donor-Acceptor (EDA) Interactions Controlling the Crystal Packing of Picric Acid and Its Adducts with Nitrogen Bases. Their Rationalization in Terms of the p''K''a Equalization and Electron-Pair Saturation Concepts.'' In: '' Cryst. Growth Des.'' 2011, 11, 2724–2735, doi:10.1021/cg101007a.


Uses

By far the greatest use of picric acid has been in ammunitions and explosives. Explosive D, also known as Dunnite, is the
ammonium Ammonium is a modified form of ammonia that has an extra hydrogen atom. It is a positively charged (cationic) polyatomic ion, molecular ion with the chemical formula or . It is formed by the protonation, addition of a proton (a hydrogen nucleu ...
salt of picric acid. Dunnite is more powerful but less stable than the more common explosive TNT (which is produced in a similar process to picric acid but with toluene as the feedstock). Picramide, formed by aminating picric acid (typically beginning with Dunnite), can be further aminated to produce the very stable explosive TATB. It has found some use in organic chemistry for the preparation of crystalline salts of organic bases (picrates) for the purpose of identification and characterization.


Optical metallography

In
metallurgy Metallurgy is a domain of materials science and engineering that studies the physical and chemical behavior of metallic elements, their inter-metallic compounds, and their mixtures, which are known as alloys. Metallurgy encompasses both the ...
, a 4% picric acid in ethanol etch, termed "picral", has been commonly used in optical
metallography Metallography is the study of the physical structure and components of metals, by using microscopy. Ceramic and polymeric materials may also be prepared using metallographic techniques, hence the terms ceramography, plastography and, collecti ...
to reveal prior austenite grain boundaries in ferritic steels. The hazards associated with picric acid have meant it has largely been replaced with other chemical etchants. However, it is still used to etch
magnesium alloy Magnesium alloys are mixtures of magnesium (the lightest structural metal) with other metals (called an alloy), often aluminium, zinc, manganese, silicon, copper, rare earths and zirconium. Magnesium alloys have a hexagonal lattice structur ...
s, such as AZ31.


Histology

Bouin solution is a common picric-acid–containing fixative solution used for
histology Histology, also known as microscopic anatomy or microanatomy, is the branch of biology that studies the microscopic anatomy of biological tissue (biology), tissues. Histology is the microscopic counterpart to gross anatomy, which looks at large ...
specimens. It improves the staining of acid dyes, but it can also result in
hydrolysis Hydrolysis (; ) is any chemical reaction in which a molecule of water breaks one or more chemical bonds. The term is used broadly for substitution reaction, substitution, elimination reaction, elimination, and solvation reactions in which water ...
of any DNA in the sample. Picric acid is used in the preparation of Picrosirius red, a histological stain for
collagen Collagen () is the main structural protein in the extracellular matrix of the connective tissues of many animals. It is the most abundant protein in mammals, making up 25% to 35% of protein content. Amino acids are bound together to form a trip ...
.


Blood tests

Clinical chemistry laboratory testing utilizes picric acid for the Jaffe reaction to test for
creatinine Creatinine (; ) is a breakdown product of creatine phosphate from muscle and protein metabolism. It is released at a constant rate by the body (depending on muscle mass). Biological relevance Serum creatinine (a blood measurement) is an impor ...
. It forms a colored complex that can be measured using spectroscopy. Picric acid forms red isopurpurate with
hydrogen cyanide Hydrogen cyanide (formerly known as prussic acid) is a chemical compound with the chemical formula, formula HCN and structural formula . It is a highly toxic and flammable liquid that boiling, boils slightly above room temperature, at . HCN is ...
(HCN). By photometric measurement of the resulting dye, picric acid can be used to quantify hydrogen cyanide. During the early 20th century, picric acid was used to measure blood
glucose Glucose is a sugar with the Chemical formula#Molecular formula, molecular formula , which is often abbreviated as Glc. It is overall the most abundant monosaccharide, a subcategory of carbohydrates. It is mainly made by plants and most algae d ...
levels. When glucose, picric acid and
sodium carbonate Sodium carbonate (also known as washing soda, soda ash, sal soda, and soda crystals) is the inorganic compound with the formula and its various hydrates. All forms are white, odourless, water-soluble salts that yield alkaline solutions in water ...
are combined and heated, a characteristic red color forms. With a calibrating glucose solution, the red color can be used to measure the glucose levels added. This is known as the Lewis and Benedict method of measuring glucose.


Skin dye

Much less commonly, wet picric acid has been used as a skin dye, or temporary branding agent. It reacts with proteins in the skin to give a dark brown color that may last as long as a month.


Antiseptic

During the early 20th century, picric acid was stocked in pharmacies as an
antiseptic An antiseptic ( and ) is an antimicrobial substance or compound that is applied to living tissue to reduce the possibility of sepsis, infection, or putrefaction. Antiseptics are generally distinguished from ''antibiotics'' by the latter's abil ...
and as a treatment for
burn A burn is an injury to skin, or other tissues, caused by heat, electricity, chemicals, friction, or ionizing radiation (such as sunburn, caused by ultraviolet radiation). Most burns are due to heat from hot fluids (called scalding), soli ...
s,
malaria Malaria is a Mosquito-borne disease, mosquito-borne infectious disease that affects vertebrates and ''Anopheles'' mosquitoes. Human malaria causes Signs and symptoms, symptoms that typically include fever, Fatigue (medical), fatigue, vomitin ...
, herpes, and
smallpox Smallpox was an infectious disease caused by Variola virus (often called Smallpox virus), which belongs to the genus '' Orthopoxvirus''. The last naturally occurring case was diagnosed in October 1977, and the World Health Organization (W ...
. Picric-acid–soaked gauze was commonly stocked in first aid kits from that period as a burn treatment. It was notably used for the treatment of burns suffered by victims of the Hindenburg disaster in 1937. Picric acid was used as a treatment for trench foot suffered by soldiers stationed on the Western Front (World War I), Western Front during
World War I World War I or the First World War (28 July 1914 – 11 November 1918), also known as the Great War, was a World war, global conflict between two coalitions: the Allies of World War I, Allies (or Entente) and the Central Powers. Fighting to ...
. Picric acid has been used for many years by Fly tying, fly tyers to dye mole skins and feathers a dark olive green for use as fishing lures. Its popularity has been tempered by its toxic nature.


Safety

Modern safety precautions recommend storing picric acid wet, to minimize the danger of explosion. Dry picric acid is relatively sensitive to Shock (mechanics), shock and friction, so laboratories that use it store it in bottles under a layer of water (molecule), water, rendering it safe. Glass or plastic bottles are required, as picric acid can easily form metal picrate salts that are even more sensitive and hazardous than the acid itself. Industrially, picric acid is especially hazardous because it is volatile and slowly sublimes even at room temperature. Over time, the buildup of picrates on exposed metal surfaces can constitute an explosion hazard. Picric acid gauze, if found in antique first aid kits, presents a safety hazard because picric acid of that vintage (60–90 years old) will have become crystallized and unstable, and may have formed metal picrates from long storage in a metal first aid case. Bomb disposal units are often called to dispose of picric acid if it has dried out. In the United States there was an effort to remove dried picric acid containers from high school laboratories during the 1980s. Munitions containing picric acid may be found in sunken warships. The buildup of metal picrates over time renders them shock-sensitive and extremely hazardous. It is recommended that shipwrecks that contain such munitions not be disturbed in any way.Albright, p.78 The hazard may subside when the shells become corroded enough to admit seawater as these materials are water-soluble. Currently there are various fluorescent probes to sense and detect picric acid in very minute quantities.


See also

*Shellite (explosive), an explosive containing picric acid, formerly used in naval shells. *Table of explosive detonation velocities *RE factor *Verhoeff's stain *Styphnic acid


References


Further reading

* * * Cooper, Paul W., ''Explosives Engineering'', New York: Wiley-VCH, 1996.

{{Authority control Explosive chemicals Nitrophenols Organic acids