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Picric acid
Picric acid
Picric acid
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Picric acid
Names
Preferred IUPAC name
2,4,6-Trinitrophenol[1]
Systematic IUPAC name
2,4,6-Trinitrobenzenol
Other names
Picric acid[1]
Carbazotic acid
Phenol trinitrate
Picronitric acid
Trinitrophenol
2,4,6-Trinitro-1-phenol
2-Hydroxy-1,3,5-trinitrobenzene
TNP
Melinite
Lyddite
Identifiers
3D model (JSmol)
ChEBI
ChEMBL
ChemSpider
DrugBank
ECHA InfoCard 100.001.696 Edit this at Wikidata
RTECS number
  • TJ7875000
UNII
UN number UN1344
  • InChI=1S/C6H3N3O7/c10-6-4(8(13)14)1-3(7(11)12)2-5(6)9(15)16/h1-2,10H checkY
    Key: OXNIZHLAWKMVMX-UHFFFAOYSA-N checkY
  • InChI=1/C6H3N3O7/c10-6-4(8(13)14)1-3(7(11)12)2-5(6)9(15)16/h1-2,10H
    Key: OXNIZHLAWKMVMX-UHFFFAOYAM
  • O=[N+]([O-])c1cc(cc([N+]([O-])=O)c1O)[N+]([O-])=O
Properties
C6H3N3O7
Molar mass 229.104 g·mol−1
Appearance Colorless to yellow solid
Density 1.763 g/cm3, solid
Melting point 122.5 °C (252.5 °F; 395.6 K)[2]
Boiling point Sublimes above MP
12.7 g/L
Solubility in Sulfuric acid
  • 10.18 g/100g sln. (18 °C (64 °F; 291 K))
  • 16.23 g/100g sln. (50 °C (122 °F; 323 K))
  • 25.86 g/100g sln. (80 °C (176 °F; 353 K))
(in 100% H2SO4)[2]
Solubility in Ethanol 7.452 g/100g[2]
Solubility in Diethyl ether 1.08 g/100g (13 °C (55 °F; 286 K))[2]
Solubility in Benzene 5.9 g/100g (15 °C (59 °F; 288 K))[2]
Solubility in Toluene 12.0 g/100ml (20 °C (68 °F; 293 K))[2]
Solubility in Amyl alcohol 1.755 g/100ml (20 °C (68 °F; 293 K))[2]
log P 1.33[3]
Vapor pressure
  • 2 mmHg (0.27 kPa) (195 °C (383 °F; 468 K))
  • 50 mmHg (6.7 kPa) (255 °C (491 °F; 528 K))[2]
Acidity (pKa) 0.38
−84.34×10−6 cm3/mol
Structure[4]
Orthorhombic
Pca21
a = 9.2596 Å, b = 19.138 Å, c = 9.7075 Å
α = 90°, β = 90°, γ = 90°
1720.3 Å3
8
Thermochemistry[2]
−215 kJ/mol
Enthalpy of fusion (ΔfH⦵fus)
 20 kJ/mol
88 kJ/mol
Explosive data[2]
Shock sensitivity
  • 65–93 cm (26–37 in) (Bureau of Mines test)
  • 33 cm (13 in) (Picatinny Arsenal test)
  • 4 g (62 gr) rifle bullet at more than 400 m/s (1,300 ft/s)
Friction sensitivity Very slight (produces weak burning odor)
Detonation velocity 7,480 m/s (24,500 ft/s) at ρ=1.70
RE factor
  • 1.12 (Ballistic mortar test)
  • 1.01 (Trauzl test)
  • 1.07 (Plate dent test)
Hazards[3]
GHS labelling:
GHS01: ExplosiveGHS02: FlammableGHS06: Toxic
Danger
H228, H301+H311, H317, H332
P210, P240, P241, P261, P264, P270, P271, P272, P280, P301+P310+P330, P302+P352, P302+P352+P312, P304+P340+P312, P333+P313, P361, P363, P370+P378, P405, P501
NFPA 704 (fire diamond)
NFPA 704 four-colored diamondHealth 2: Intense or continued but not chronic exposure could cause temporary incapacitation or possible residual injury. E.g. chloroformFlammability 4: Will rapidly or completely vaporize at normal atmospheric pressure and temperature, or is readily dispersed in air and will burn readily. Flash point below 23 °C (73 °F). E.g. propaneInstability 4: Readily capable of detonation or explosive decomposition at normal temperatures and pressures. E.g. nitroglycerinSpecial hazards (white): no code
2
4
4
Flash point 150 °C; 302 °F; 423 K[5]
300–310 °C (572–590 °F; 573–583 K)[2] Burns after melting with sooty flame and characteristic whistling noise. (may explode if confined, contaminated with picrates, or in contact with metals that form picrates)
0.1 mg/m3 (TWA)
Lethal dose or concentration (LD, LC):
  • 100 mg/kg (guinea pig, oral)
  • 250 mg/kg (cat, oral)
  • 120 mg/kg (rabbit, oral)[6]
NIOSH (US health exposure limits):
PEL (Permissible)
 0.1 mg/m3 (TWA, skin)
REL (Recommended)
  • 0.1 mg/m3 (TWA)
  • 0.3 mg/m3 (ST, skin)[5]
IDLH (Immediate danger)
 75 mg/m3[5]
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
checkY verify (what is checkY☒N ?)

Picric acid is an organic compound with the formula (O2N)3C6H2OH. Its IUPAC name is 2,4,6-trinitrophenol (TNP). The name "picric" comes from Greek: πικρός (pikros), meaning "bitter", due to its bitter taste. It is one of the most acidic phenols. Like other strongly nitrated organic compounds, picric acid is an explosive, which is its primary use. It has also been used as medicine (antiseptic, burn treatments) and as a dye.

History

[edit]

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, indigo, and natural resin, the synthesis from indigo first being performed by Peter Woulfe in 1771.[7]

The German chemist Justus von Liebig had named picric acid Kohlenstickstoffsäure (rendered in French as acide carboazotique).[citation needed] Picric acid was given that name by the French chemist Jean-Baptiste Dumas in 1841.[8] Its synthesis from phenol, and the correct determination of its formula, were accomplished during 1841.[9] 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.[10] Not until 1830 did chemists think to use picric acid as an explosive. Before then, chemists assumed that only the salts of picric acid were explosive, not the acid itself.[citation needed]

A theory to explain why picrate salts detonated whereas picric acid itself didn't, was proposed by the French chemists Antoine Fourcroy and Louis Vauquelin in 1806 and reiterated by the French chemist Michel Chevreul in 1809. Picric acid evidently contained enough oxygen within itself — i.e. it was "super-oxygenated" (suroxigéné) — to combust completely even in the absence of air (because even in the absence of air, heat could transform it completely into gases, leaving no carbon). However, when picric acid was burned, the heat that was generated caused some of the acid to evaporate, dissipating so much heat that only burning, not detonation, occurred. In contrast, picrate salts were solids that did not sublimate, thus did not dissipate heat; hence, they did detonate.[11][12]

In 1871 Hermann Sprengel proved it could be detonated at the gunpowder works of John Hall & Sons in Faversham in Kent, England. Sprengel filed patents in Britain for "safety explosives" (i.e., stable explosives) on April 6, 1871 (no. 921),[citation needed] and on October 5, 1871 (no. 2642); in the latter patent, Sprengel proposed using picric acid dissolved in nitric acid as an explosive.[citation needed][13][14][page needed] Afterwards most military powers used picric acid as their main high explosive material.[citation needed] A full synthesis was later found by Leonid Valerieovich Kozakov.[citation needed]

Picric acid was the first strongly explosive nitrated organic compound widely considered suitable to withstand the shock of firing in conventional artillery. Nitroglycerine and nitrocellulose (guncotton) were available earlier, but shock sensitivity sometimes caused detonation 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 shells. [citation needed]

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, 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.[15]

By 1894 Russia was manufacturing artillery shells filled with picric acid. 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.[citation needed]

Workers filling shells with liquid melinite at a French munitions factory during WWI

Picric acid was used in the Battle of Omdurman, the Second Boer War, the Russo-Japanese War, and World War I.[16][17][18][19]

Germany began filling artillery shells with trinitrotoluene (TNT) in 1902. Toluene was less readily available than phenol, and TNT is slightly 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.[16][page needed]

Efforts to control the availability of phenol, the precursor to picric acid, emphasize its importance in World War I. Germans are reported to have bought US supplies of phenol and converted it to acetylsalicylic acid (aspirin) to keep it from the Allies. At the time, phenol was obtained from coal as a co-product of coke ovens and the manufacture of gas for gas lighting. Laclede Gas reports being asked to expand production of phenol (and toluene) to assist the war effort.[20] Both Monsanto and Dow Chemical began manufacturing synthetic phenol in 1915, with Dow being the main producer.[21][22] Dow describes picric acid as "the main battlefield explosive used by the French. Large amounts [of phenol] also went to Japan, where it was made into picric acid sold to the Russians."[22]

Photograph showing the use of picric acid on a farm to remove stumps and rocks.

Synthesis

[edit]

The aromatic ring of phenol 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. During this reaction, nitro groups are introduced, and the sulfonic acid group is displaced. The reaction is highly exothermic, and careful temperature control is required. Synthesis routes that nitrate aspirin or salicylic acid can also be used to mitigate tar formation. Carbon dioxide is lost from the former via decarboxylation, while both acetic acid and carbon dioxide are lost from the latter.[23][better source needed] Another method of picric acid synthesis is direct nitration of 2,4-dinitrophenol with nitric acid.[24][25]

Uses

[edit]

By far the greatest use of picric acid has been in ammunition and explosives.

Derivatives

[edit]

Ammonium picrate (Explosive D, also known as Dunnite), is the ammonium salt of picric acid. It is notably less sensitive to impact than either picric acid or TNT (16–17 in (41–43 cm) initiation drop height using the Picatinny Arsenal apparatus, vs. 12–14 in (30–36 cm) for TNT and 13 in (33 cm) for PA), which allowed its use in armor piercing ammunition. Ammonium picrate was used by the United States army beginning in 1901 and the navy in 1907.[2]

Picramide, formed by aminating picric acid (typically beginning with Dunnite), can be further aminated to produce the very stable explosive TATB.[citation needed] 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.[citation needed]

Optical metallography

[edit]

In metallurgy, a 4% picric acid in ethanol etch, termed "picral", has been commonly used in optical metallography 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 alloys, such as AZ31.[citation needed]

Histology

[edit]

Bouin solution is a common picric-acid–containing fixative solution used for histology specimens.[26] It improves the staining of acid dyes, but it can also result in hydrolysis of any DNA in the sample.[27]

Picric acid is used in the preparation of Picrosirius red, a histological stain for collagen.[28][29]

Blood tests

[edit]

Clinical chemistry laboratory testing utilizes picric acid for the Jaffe reaction to test for creatinine. It forms a colored complex that can be measured using spectroscopy.[30]

Picric acid forms red isopurpurate with hydrogen cyanide (HCN). By photometric measurement of the resulting dye, picric acid can be used to quantify hydrogen cyanide.[31]

During the early 20th century, picric acid was used to measure blood glucose levels. When glucose, picric acid and sodium carbonate 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.[32]

Skin dye

[edit]

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

Antiseptic

[edit]

During the early 20th century, picric acid was stocked in pharmacies as an antiseptic and as a treatment for burns, malaria, herpes, and smallpox. Picric-acid–soaked gauze was commonly stocked in first aid kits from that period as a burn treatment.[citation needed] It was notably used for the treatment of burns suffered by victims of the Hindenburg disaster in 1937.[citation needed]

Picric acid was used as a treatment for trench foot suffered by soldiers on the Western Front during World War I.[33]

Picric acid has been used for many years by 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.[citation needed]

Safety

[edit]

Modern safety precautions recommend storing picric acid wet, to minimize the danger of explosion. Glass or plastic bottles are required, as picric acid forms metal picrate salts that can be more sensitive and hazardous than the acid, and which are powerful enough to initiate detonation in the acid.[2] The buildup of picrates on exposed metal surfaces can constitute an explosion hazard.[34]

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, or it may have formed metal picrates from long storage in a metal first aid case.[35]

Bomb disposal units are often called to dispose of picric acid if it has dried out.[36][37] In the United States there was an effort to remove dried picric acid containers from high school laboratories during the 1980s.[citation needed]

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. The hazard may subside when the shells become corroded enough to admit seawater as these materials are water-soluble.[38] There are fluorescent probes to detect picric acid in very minute quantities.[39]

See also

[edit]

References

[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Picric acid

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from Grokipedia
Picric acid, systematically named 2,4,6-trinitrophenol, is an with the molecular C₆H₃N₃O₇, appearing as odorless, pale yellow crystals that exhibit strong acidity and properties due to the presence of three nitro groups attached to a phenolic ring. It is produced industrially via the of phenol using a of nitric and sulfuric acids, a process that yields the highly nitrated product valued for its chemical reactivity. Historically, picric acid served as a vital high in military applications, notably in French shells during under the trade name mélinite, where its power surpassed that of black powder but was tempered by its sensitivity to impact and friction. Prior to its explosive prominence, it found use as a yellow dye for and , an in wound dressings during the , and even in early photographic processes, though these applications diminished with safer alternatives. Its defining hazards stem from extreme sensitivity when dry, forming shock-sensitive salts with metals that can detonate spontaneously, alongside causing liver and kidney damage upon ingestion, inhalation, or skin absorption, rendering it a restricted substance requiring perpetual moistening for safe storage. Despite these risks, its legacy endures in specialized , such as in Kjeldahl determination, underscoring a balance between utility and peril in nitroaromatic compounds.

Chemical Identity and Properties

Molecular Structure and Formula


Picric acid has the molecular formula C₆H₃N₃O₇ and the IUPAC name 2,4,6-trinitrophenol. The compound contains approximately 18.34% nitrogen by mass, calculated as (3 × 14 / 229) × 100% ≈ 18.34%. Its structure consists of a benzene ring with a hydroxyl group attached to carbon 1 and nitro groups (-NO₂) substituted at the ortho and para positions (carbons 2, 4, and 6).
The three nitro groups exert strong electron-withdrawing inductive and effects, which delocalize the negative charge in the conjugate base, enhancing acidity far beyond that of phenol (pKₐ ≈ 10). Picric acid's pKₐ is 0.42, classifying it as a strong due to this stabilization of the phenolate . These substituents also contribute to its high reactivity in electrophilic and oxidative processes by activating the ring toward certain reactions while deactivating it overall.

Physical Characteristics

Picric acid is an odorless, pale yellow crystalline solid at standard conditions. It exhibits a of 122–123 °C, transitioning to a liquid phase without boiling under normal pressures, as it decomposes at approximately 300 °C. The compound has a density of 1.76 g/cm³ for the solid form. Its solubility in is moderate, at about 12–14 g/L at 20 °C, while it shows high solubility in organic solvents such as (>100 g/L) and . In wetted form, typically with 10–30% content, it presents as a paste or , altering its handling characteristics due to the incorporated phase.
PropertyValue
AppearancePale yellow crystals
OdorNone
Melting point122–123 °C
Density (solid)1.76 g/cm³
Solubility in water (20 °C)~12 g/L
Solubility in ethanolHighly soluble (>100 g/L)
These properties reflect empirical measurements from standardized chemical databases and reflect the material's behavior under ambient conditions.

Chemical Reactivity and Stability

Picric acid, or 2,4,6-trinitrophenol, is a strong monoprotic with a pKa of 0.38 (in ), significantly lower than that of phenol (pKa ≈ 10) due to the electron-withdrawing inductive and effects of the three nitro groups ortho and para to the phenolic hydroxyl, which stabilize the conjugate base phenolate anion. This enhanced acidity enables facile proton donation and salt formation with bases, including metal cations such as (II), lead(II), mercury(II), (II), (II), and iron, yielding salts that exhibit heightened instability from coordination of the metal to the phenolate oxygen and nitro groups, lowering the for . The compound's stability is profoundly influenced by hydration state and physical form: when dry (less than 10-30% ), picric acid displays high sensitivity to initiation by shock, , or (initiation temperatures around 300°C), attributed to the strained aromatic ring and nitro group lability facilitating rapid exothermic to gases like CO, CO₂, N₂, and H₂O; in contrast, the hydrated form is comparatively stable under ambient conditions due to water's moderating effect on energy transfer. Pure dry picric acid detonates at velocities up to 7350 m/s (at 1.7 g/cm³), reflecting the nitro-derived that promotes homolytic bond cleavage under perturbation. Beyond acid-base and explosive behaviors, picric acid engages in redox reactions owing to its nitro groups' oxidizing potential; partial reduction, such as with sodium hydrosulfide or ammonium sulfide, selectively converts one nitro group (typically at the 2-position) to an amino group, producing picramic acid (2-amino-4,6-dinitrophenol) via electron addition and protonation steps. It also forms coordination complexes with transition metals, where the phenolate and nitro oxygens act as ligands, further modulating reactivity but often exacerbating sensitivity in solid salts. These interactions underscore the molecule's inherent electron-deficient character, driven by the cumulative -M and -I effects of the nitro substituents, which lower the energy barrier for reactive pathways without external catalysts.

Historical Development

Discovery and Initial Applications

Picric acid was first prepared in 1771 by British chemist Peter Woulfe, who obtained the compound by treating with , yielding a yellow substance suitable for dyeing . Woulfe documented his experiments in the Philosophical Transactions of the Royal Society, noting the material's intense yellow color but not its full chemical identity or acidic properties. The name "picric acid" derives from pikros, meaning "bitter," reflecting its taste, though Woulfe's initial focus was on its dyeing potential rather than acidity. Early applications centered on its use as a vibrant dye for textiles, particularly , where it provided a stable coloration not easily achieved with natural pigments. By the mid-19th century, production shifted to of phenol, a cheaper precursor derived from , expanding its availability for dyeing fabrics and other materials. This period also saw exploratory uses in non-textile fields, such as surfaces and manufacturing colored glass, leveraging its reactivity for precise material alteration. Toxicity concerns emerged during attempts to employ picric acid derivatives in applications, such as bittering agents or colorants in baked goods, where reports of prompted regulatory restrictions by the late . These incidents underscored its inherent hazards, including bitterness and potential systemic effects, limiting non-industrial adoption despite its dyeing efficacy.

Military and Industrial Expansion

The French military adopted picric acid as a in , designating it melinite for shell bursting charges, marking a significant advancement over black powder due to its higher and , which enabled more destructive effects from smaller payloads. This shift was driven by the scalability of processes applied to abundant phenol derived from , allowing for applications. By , the Allies extensively employed picric acid-based fillings, with Britain using lyddite—a primarily of picric acid—in shells, contributing to the explosive demands of where millions of rounds were fired. However, its tendency to react with metal casings, forming sensitive picrate salts, posed handling risks and contributed to occasional premature detonations. In industrial applications, picric acid's vibrant hue and resistance to fading propelled its expansion in and during the late 19th and early 20th centuries, particularly for , , and goods where color fastness was essential. The compound's production scalability, leveraging the same chemistry as for explosives, supported widespread use in manufacturing colored glass, etching copper, and dye intermediates, integrating it into burgeoning chemical industries. Demand surged with the growth of synthetic dye sectors, as picric acid provided a reliable, bright alternative to natural sources. During , picric acid saw limited but persistent military roles, such as in booster charges for incendiary bombs and certain naval ordnance, despite preferences for less reactive alternatives like TNT, reflecting its enduring utility in specialized high-energy applications where its explosive power outweighed stability concerns. Japanese forces incorporated it into land mines and grenades, valuing its performance metrics over safer options in resource-constrained production. These uses underscored the compound's peak expansion era, where industrial infrastructure enabled dual military-industrial proliferation until superior substitutes curtailed broader adoption.

Decline and Legacy

Following World War I, picric acid's prominence as a military explosive waned due to its inherent instability compared to trinitrotoluene (TNT), which offered greater resistance to shock, friction, and unintended detonation. Picric acid readily formed explosive metal picrate salts in storage, particularly when in contact with brass or copper, exacerbating risks during prolonged warehousing of shells—a problem less prevalent with TNT, which did not corrode casings or require such stringent isolation measures. This shift accelerated post-1918 as armies adopted TNT and amatol (TNT-ammonium nitrate blends) for their superior melt-cast properties and reduced toxicity, evidenced by fewer handling incidents in production scaling. Compounding these factors were high-profile accidents underscoring picric acid's volatility, such as the on December 6, 1917, where 2,366 tons aboard the SS Mont-Blanc detonated after a collision, killing about 1,600 people and injuring 9,000, with the blast's force equivalent to roughly 2.9 kilotons of TNT. Similar storage fires and spontaneous ignitions in munitions factories, like the 1916 Low Moor Explosion in Britain involving picric acid processing, further eroded confidence, prompting empirical preference for alternatives that minimized such causal chains of failure. Regulatory scrutiny intensified, with picric acid classified under strict explosives controls in documents like U.S. federal lists by the mid-20th century, effectively curtailing its civilian applications beyond licensed contexts. Picric acid's legacy endures in specialized analytical roles, serving as a colorimetric reagent for detecting alkaloids, , and certain metals through complex formation, as well as in histological for tissue preservation in . Laboratory incidents, including explosions from desiccated crystals in forgotten vials—such as a 50-gram dry sample during inventory in one documented case—perpetuate awareness of its sensitivity, reinforcing protocols like wet storage to avert friction-induced initiation. The Halifax event, in particular, symbolizes the compound's dual-edged historical impact, referenced in safety literature as a cautionary benchmark for energetic materials handling.

Production Methods

Synthesis from Phenol

Picric acid is synthesized from phenol through a multi-stage process involving initial sulfonation followed by progressive nitration using concentrated sulfuric and nitric acids, enabling the introduction of three nitro groups at the 2,4,6-positions while minimizing oxidative side reactions inherent to direct nitration of phenol. The sulfonation step, conducted by heating phenol with excess concentrated sulfuric acid at approximately 100–120°C for several hours, yields phenol-2,4-disulfonic acid (or trisulfonic under forcing conditions), where the electron-withdrawing sulfonic groups deactivate the ring and direct subsequent electrophilic attack to desired positions, facilitating controlled nitration without excessive oxidation or tar formation. Nitration proceeds in a mixed acid medium of concentrated nitric and sulfuric s, often applied stepwise to manage the highly exothermic reactions: initial cooling to 0–20°C introduces the first nitro group primarily at the ortho or para position relative to the phenolic hydroxyl, followed by gradual temperature elevation to 50–80°C for the second nitro group, and finally to 100–110°C for the third, replacing or displacing sulfonic groups via during prolonged heating or post-reaction treatment with dilute or steam. is critical for industrial scalability, as deviations above 110°C promote side products such as or polymeric tars, reducing yields; modern variants employ automated cooling systems and recycled spent acids to achieve 90–95% yields based on phenol, compared to historical batch processes yielding 80–85% without such optimizations. The reaction mixture, after , undergoes desulfonation by dilution with water and boiling, precipitating crude picric acid, which is then purified via recrystallization from hot water or to remove inorganic salts and unreacted intermediates, ensuring product purity exceeding 98% for high-grade applications. This process, refined since the late for munitions production, balances reaction kinetics with heat dissipation in large-scale reactors, typically processing phenol in 100–500 kg batches to optimize throughput while suppressing the formation of hazardous byproducts like azoxy compounds from over-nitration.

Purification and Variants

Purification of picric acid following nitration typically employs recrystallization from hot water or ethanol to separate it from more soluble impurities such as dinitrophenols and mononitrophenols, leveraging the compound's solubility of approximately 1.4 g/100 mL in boiling water versus much lower in cold conditions. This process yields crystals with purity levels exceeding 99%, essential for applications requiring high stability and performance, such as in ordnance where trace impurities can sensitize the material. Alternative refinement includes solvent extraction with diethyl ether at pH 3, which selectively removes dinitrophenol contaminants while leaving picric acid in the aqueous phase, followed by evaporation and recrystallization for analytical-grade material. In laboratory settings, smaller-scale purification may involve treatment with to form the soluble sodium salt, to eliminate insoluble residues, and subsequent acidification on to regenerate pure picric acid, contrasting that prioritize volume over such meticulous impurity profiling. For specific analytical uses, like creatinine determination, recrystallization has been documented to enhance purity by excluding polar impurities ineffective in aqueous media. Structural variants include picrate salts, formed by neutralizing picric acid with bases; ammonium picrate (C₆H₆N₄O₇), for instance, results from reaction with and displays yellow crystalline form with reduced shock sensitivity relative to the parent acid due to ionic stabilization, though it remains ignitable and decomposes to toxic nitrogen oxides. This salt's of 246.14 g/mol and lower friction sensitivity stem from hydrogen bonding in the lattice, differentiating it from free picric acid's higher volatility to impact. Analogs like (2,4,6-trinitrobenzene-1,3-diol) represent modified structures with an additional hydroxyl group ortho to the existing one, synthesized analogously from , yielding comparable explosive power and sensitivity to picric acid but elevated acidity and around 175–180°C, attributed to enhanced intramolecular hydrogen bonding. These variants exhibit similar pathways, including nitro group loss, but styphnic acid's dibasic nature allows distinct salt formations with altered stability for specialized energetic materials.

Primary Applications

Explosives and Ordnance

Picric acid functioned as a high-explosive bursting charge in shells, torpedoes, and aerial bombs, achieving widespread adoption from the late through the World Wars. British forces designated it lyddite for shell fillings, exploiting its melt-pourable nature at 122–123°C to achieve dense, void-free loads superior to pressed explosives like early black powder variants. This method enhanced loading efficiency for large-caliber projectiles, with variants like shellite (70% picric acid, 30% dinitrophenol) employed in armor-piercing shells to mitigate corrosion. French melinite, a stabilized picric acid formulation, similarly filled naval and field ordnance, powering munitions in conflicts from the onward. Detonation metrics underscored picric acid's efficacy: its relative explosive effectiveness rated 1.20 against TNT's baseline of 1.00, yielding greater for fragmentation and target disruption despite comparable stability under ideal conditions. Compounds derived from or incorporating picric acid consistently demonstrated high , prioritizing shattering power over mere heave. However, picric acid's impact sensitivity (7–8 J) exceeded TNT's (~15 J), rendering it more prone to unintended initiation by shock or friction, though less volatile than primary explosives like mercury fulminate. Storage and handling drawbacks limited longevity: picric acid's acidity corroded and casings, forming hypersensitive metal picrates (e.g., copper picrate) that ignited spontaneously under moisture or friction, as documented in degraded wartime stockpiles. Such reactions contributed to catastrophic magazine detonations, where picric-filled shells were implicated in rapid propagation failures during and II engagements, prompting transitions to or pure TNT by the 1940s for reduced risk. Despite these liabilities, its historical role in ordnance highlighted trade-offs between performance and safety in pre-TNT era high explosives.

Dyes, Stains, and Pigments

Picric acid imparts a bright coloration to textiles due to its nitro groups functioning as , conjugated with the phenolic hydroxyl group acting as an to enhance fiber substantivity. This exhibits good affinity for protein fibers such as and , as well as cellulosic materials like , producing vibrant shades suitable for fabric . Historically, picric acid's dyeing applications emerged following Peter Woulfe's 1771 synthesis via of organic precursors, with solutions applied to achieve yellow hues on and other textiles until the mid- advent of tar-derived alternatives. Its provided durability under exposure, though the color risks fading in alkaline environments due to instability of the structure. Peak usage occurred in the 19th century for and garments, exemplified by preserved yellow artifacts dyed with the compound. Contemporary applications remain niche, largely supplanted by non-toxic synthetic azo dyes offering superior stability and safety profiles, amid picric acid's documented and potential limiting industrial viability. Limited use persists in specialized formulations for inks and paints, where empirical tests indicate moderate UV resistance attributable to the conjugated nitro system, though data on long-term exposure show gradual degradation beyond 1000 hours.

Analytical and Laboratory Uses

Picric acid serves as a precipitating agent for alkaloids in qualitative organic analysis, forming yellow crystalline salts that enable identification through insolubility in and characteristic melting points. This method, employed since the late , exploits the acid's ability to form stable addition compounds with basic nitrogen-containing compounds like , , and , allowing separation from complex mixtures via and recrystallization. In clinical biochemistry, picric acid is a key component of the Jaffé reaction for colorimetric determination of in blood and urine, where it reacts under alkaline conditions to produce a red-orange complex with maximum absorbance at 520 nm, quantifiable by . Developed by Max Jaffé in 1886, this remains a standard in many laboratories despite interferences from acetoacetate, proteins, and , which can elevate readings by up to 20-30%; detection limits typically reach 0.1-1 mg/dL in serum. Enzymatic alternatives, such as those using creatininase, have largely supplanted it in high-precision settings due to greater specificity and reduced non-specific reactivity. For metal ion detection in qualitative inorganic analysis, picric acid forms sparingly soluble picrate salts with cations like lead, , and iron, precipitating as colored complexes distinguishable by solubility in organic solvents or differential thermal stability; sensitivity for lead picrate detection approaches 1-5 μg/mL in aqueous solutions. These precipitates aid in confirmatory tests but are limited by the explosive sensitivity of dry metal picrates, prompting caution in handling. Historically, picric acid was used in laboratory assays for explosive residue characterization, including titration-based purity checks and sensitivity evaluations of nitro compounds via picrate formation. In contemporary analytical protocols, it functions as a calibration standard in reversed-phase high-performance liquid chromatography (RP-HPLC) for quantifying nitroaromatic explosives in environmental samples, with methods achieving detection limits as low as 0.1 μg/L after solid-phase extraction; international standards like ISO 22863-12 specify its use for picrate quantification in pyrotechnics. Safer non-explosive analogs, such as sulfosalicylic acid for protein precipitation or enzymatic kits for creatinine, are increasingly adopted to mitigate risks.

Medical and Pharmaceutical Roles

Picric acid served as an antiseptic agent in the early 20th century, particularly for treating burns and wounds, where diluted solutions applied via soaked bandages exerted bacteriostatic effects to inhibit bacterial growth while providing analgesic relief and preventing pus formation. Its efficacy in burn treatment was discovered accidentally around 1896 by a French medical student, leading to widespread adoption in ointments and dressings that combined picric acid with agents like butyl aminobenzoate during the 1920s and 1930s for surgical applications. During World War I, surgeons employed picric acid solutions as a standard wound antiseptic, capitalizing on its astringent properties to reduce infection rates in battlefield conditions, though its yellow staining of tissues and potential for skin sensitization limited prolonged use. By the 1930s, however, it was largely supplanted by sulfonamide drugs, which offered superior antibacterial action without comparable risks of hypersensitivity or tissue discoloration. In histological applications, picric acid functions as a fixative in Bouin's solution, a mixture of saturated picric acid, , and acetic acid that excels at preserving the architecture of soft and delicate tissues, such as embryos or gonadal structures, for subsequent hematoxylin and staining. This fixative, developed by French biologist Pol Bouin in the early , penetrates tissues rapidly and minimizes shrinkage or hardening, yielding clear morphological detail, though excess picric acid must be removed with 70% washes to avoid interference with downstream processes. Its non-coagulant nature enhances nuclear preservation, making it valuable for biopsies like testicular tissue, despite the need for careful handling due to picric acid's inherent . Pharmaceutical exploration of picric acid for conditions like occurred historically, with the compound itself stocked in early 20th-century pharmacies for such treatments, but development ceased due to documented , including and renal effects upon systemic exposure. While topical formulations demonstrated short-term utility, systemic or applications revealed insufficient therapeutic margins, prompting discontinuation in favor of less hazardous antimalarials.

Risks and Safety Considerations

Explosive Hazards

Picric acid in its dry form constitutes a primary due to its high sensitivity to mechanical shock, , and , capable of initiating with relatively low inputs. Drop-hammer impact tests have demonstrated that dry picric acid can react upon concentrated mechanical force applied to small quantities, confirming its initiation threshold at energies lower than those required for many secondary explosives. This sensitivity arises from the compound's molecular , where the phenolic ring substituted with three nitro groups facilitates rapid and release upon perturbation, leading to high-order velocities exceeding 7,000 m/s in pure form. In contrast, picric acid wetted with at least 10% exhibits markedly reduced risk, as the disrupts direct initiation pathways and stabilizes the material against accidental under standard conditions. However, upon drying—often below 10% content—the inherent instability reemerges, rendering even small quantities capable of violent explosion from minor impacts or frictional forces. Detonation propagation in picric acid can be reliable in dense charges but encounters challenges in porous configurations or mixtures, where the transition from burning to may falter due to discontinuous front advancement, potentially resulting in incomplete or failed high-explosive reactions. This propagation variability underscores the compound's unpredictability in non-ideal packing, where molecular arrangements in the crystal lattice contribute to uneven transmission and heightened sensitivity to initiation flaws. strategies, such as incorporation of agents, temporarily desensitize the material by altering its physical state, yet the underlying chemical predisposition to persists, demanding empirical assessment of levels to avert hazards.

Toxicological Effects

Picric acid exhibits high across exposure routes, with systemic effects stemming from rapid absorption and nitro group metabolism. in rats yields an LD50 of 200 mg/kg for females and 290 mg/kg for males, accompanied by severe , , and organ stress. Dermal contact causes severe , yellow staining of and due to persistent pigmentation, and potential burns, while facilitating percutaneous absorption that amplifies systemic . of dust or vapors irritates respiratory tracts and can induce , reducing oxygen-carrying capacity and manifesting as , , and . Chronic exposure primarily targets the liver and kidneys through into reactive intermediates, leading to , , , and potential from red blood cell destruction. Repeated dermal contact often results in allergic and , with yellow discoloration persisting as a marker of exposure. Gastrointestinal symptoms like , , and may recur with ongoing low-level . Regarding carcinogenicity, picric acid lacks classification by the International Agency for Research on Cancer (IARC), with no epidemiological or robust animal data indicating oncogenic potential; while mutagenic in bacterial assays, mammalian evidence remains weak and inconclusive. Dose-response relationships underscore organ-specific vulnerabilities, with liver and renal endpoints evident at sub-lethal chronic doses in models.

Handling, Storage, and Disposal Protocols

Picric acid must be handled exclusively in a certified chemical to minimize inhalation risks and contain potential spills, with personnel wearing appropriate including , , or gloves, safety goggles, and a laboratory coat to prevent contact and absorption. , impact, shock, and sources should be strictly avoided during manipulation, as dry picric acid is highly sensitive to these stimuli and can detonate spontaneously. Tools and utensils must be non-metallic to prevent formation of metal picrate salts, such as those with or lead. For storage, picric acid requires maintenance in a hydrated state with at least 10-30% water content by volume to desensitize its properties, and containers must be labeled with receipt and opening dates to track potential risks. It should be kept in original, non-metallic containers within a cool, well-ventilated, fireproof cabinet segregated from incompatibles like reducing agents, metals, and alkalis, avoiding proximity to desiccants or high-humidity environments that could alter moisture levels. Disposal protocols mandate submission of hydrated picric acid (without visible crystals) as through institutional safety departments, ensuring excess water is present to suppress explosivity during transport and treatment. Neutralization may involve reduction to non-explosive forms under controlled conditions, such as treatment with or other agents in a , followed by verification of stability before final or in compliance with regulations; dry material requires specialist handling to avoid . In incident response, spills of wet picric acid should be absorbed using water-dampened pads or pillows placed in impervious containers with excess water, while dry spills necessitate immediate evacuation, area isolation, and professional bomb squad or hazardous materials team intervention due to heightened detonation risk from friction or confinement, as evidenced by laboratory cases where desiccated stocks exploded during routine access. Fire involving picric acid demands treatment as an explosive, with responders withdrawing to a safe distance and initiating remote cooling or suppression only if feasible, prioritizing containment over direct engagement.

Modern Context and Regulations

Current Market and Developments

The global picric acid market remains niche but demonstrates resilience post-2020, with demand sustained by high-purity applications in analytical reagents, laboratory staining, and select pharmaceutical intermediates rather than large-scale explosives production. Market estimates vary due to limited public data on this hazardous specialty chemical, but one analysis valued it at approximately USD 520 million in 2023, projecting growth to USD 720 million by 2032 at a CAGR of about 3.7%, driven by advancements in safer formulations and expanded use in trace detection technologies. This counters perceptions of total obsolescence, as production persists for precision needs where alternatives like safer nitroaromatics fall short in compatibility or cost-effectiveness. Key technical developments include 's 2023 launch of enhanced handling protocols and stabilized variants to reduce explosive sensitivity during transport and storage, addressing longstanding safety barriers to broader adoption. Similarly, Odyssey Organics introduced pharmaceutical-grade picric acid in the same year, targeting derivatives and energetic material precursors amid rising demand in regions. Supply chains are concentrated in and , which dominate output through low-cost synthesis from phenol, though regulatory scrutiny on has prompted investments in greener purification methods. Emerging applications in and sensors, such as picric acid-selective luminescent materials for , have spurred R&D, with 2024-2025 studies demonstrating selective detection in aqueous media via zinc-based complexes. However, bulk uses show minimal revival, limited to niche operations in developing markets, while overall growth is tempered by substitution with less sensitive compounds like in ordnance.

Environmental Impacts

Picric acid demonstrates moderate environmental persistence, primarily due to slow microbial degradation under aerobic conditions, with estimated half-lives ranging from 28 to 180 days in aqueous systems. In sediments from munitions production sites, such as the Army Ammunition Plant, initial rapid removal occurs via adsorption to and minerals, followed by slower biotic or abiotic transformation, with overall degradation influenced by sediment organic carbon content and grain size. Its log Koc value of approximately 2.0–2.25 (Koc ≈ 100–180) suggests moderate soil adsorption, which can retard but not prevent leaching into , particularly in neutral or alkaline soils where the anionic form enhances mobility despite relatively high aqueous solubility (about 1.3 g/L at 20°C). Aquatic toxicity arises from direct exposure and degradation byproducts, with acute LC50 values for fish species such as around 170 mg/L over 96 hours, classifying it as harmful to aquatic life. Nitro group reduction during breakdown can release nitrogen oxides (NOx), contributing to localized or toxicity in receiving waters, though chronic effects at sub-lethal concentrations (below 0.001 × LC50) impair growth in fish and without immediate lethality. Remediation strategies leverage adsorption for containment and advanced degradation techniques. Photocatalytic processes using UV irradiation and catalysts like TiO2 effectively mineralize picric acid in from demilitarization, achieving near-complete removal under optimized conditions. Bioreactors employing specialized microbial consortia have shown promise for explosives-contaminated soils, with enhanced rates via co-metabolism or acclimated at former munitions facilities. Field applications at sites like the plant confirm feasibility, though challenges persist in scaling for heterogeneous subsurface plumes due to incomplete mineralization and inhibitory byproduct accumulation. Picric acid is classified by the as an explosive under hazard division 1.1D, indicating a substance or article presenting a mass with a projection , as designated by 0154 for the dry form. In the European Union, picric acid falls under restrictions outlined in REACH Annex XVII, entry 40, which limits its use and handling due to explosive risks, alongside controls in Regulation (EU) 2019/1148 on the marketing and use of explosives precursors to prevent diversion for illicit purposes. The United States Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF) lists picric acid, when manufactured as an explosive, on its annual compilation of explosive materials under 18 U.S.C. 841(d) and 27 CFR 555, subjecting quantities above minimal thresholds—typically requiring permits for possession exceeding 50 pounds in commercial contexts—to federal licensing, storage, and record-keeping requirements. Laboratory applications adhere to OSHA standards under 29 CFR 1910.109, classifying it as a Class A explosive and mandating wet storage (at least 10% water by mass) to avert sensitivity to shock or friction. Disposal of picric acid is governed by the (RCRA) as a characteristic (D003 for ignitability and potential ), requiring generators to ensure hydration prior to treatment via permitted methods such as high-temperature or chemical neutralization, with prohibitions on land disposal without prior processing. Import and export of picric acid are restricted in multiple jurisdictions due to its classification as a dual-use item with applications; for instance, the imposes licensing for intra-community trade under dual-use regulations, while U.S. exports require (BIS) authorization under the (EAR) for items on the Commerce Control List when destined for certain countries.

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