Picric Acid

Picric acid, formally known by its IUPAC name 2,4,6-trinitrophenol (TNP), is a highly nitrated organic compound historically important in industry, medicine, and military technology. With the molecular formula O₂N₃C₆H₂OH, it is one of the most strongly acidic phenols and was among the earliest nitrated aromatic compounds to be recognised for its potent explosive qualities. The term picric derives from the Greek pikros, meaning “bitter”, which reflects the compound’s pronounced bitter flavour. Although primarily used as a high explosive in the late nineteenth and early twentieth centuries, picric acid has also been applied in dyes and antiseptic preparations.

Chemical Characteristics and Properties

Picric acid is distinguished by the presence of three nitro groups attached to an activated aromatic ring, which significantly increases both its acidity and oxidative potential. The electron-withdrawing nitro substituents render it far more acidic than phenol, allowing it to form stable salts with various metals. The compound’s explosive behaviour is associated with its high oxygen content and capacity for rapid decomposition into gaseous products. However, contact with metal surfaces can lead to the formation of metal picrates, which are notably more sensitive to friction and impact, presenting serious safety hazards.
Thermal behaviour also differentiates picric acid from its salts. Whereas picric acid tends to volatilise on heating, metal picrates, which do not sublimate, can accumulate heat more readily, making them more prone to detonation.

Early History and Discovery

Mentions of picric acid appear as early as the seventeenth century in the writings of Johann Rudolf Glauber, who noted products formed by treating organic materials with nitric acid. Prior to the availability of pure phenol, early chemists produced the substance by nitrating organic matter such as horn, silk, and natural resins. In 1771, Peter Woulfe demonstrated the formation of a yellow dye by treating indigo with nitric acid, one of the earliest systematic descriptions of what would later be recognised as picric acid.
During the late eighteenth and early nineteenth centuries, several chemists contributed to clarifying the nature of the compound. Jean-Joseph Welter prepared the substance by treating silk with nitric acid and observed that its potassium salt was explosive. He gave the material the name amer, meaning bitter, in reference to its taste. It was not until 1841 that Jean-Baptiste Dumas formalised the name “picric acid”, while Auguste Laurent succeeded in synthesising it from phenol and determining its correct molecular formula. These developments marked the transition from serendipitous discovery to controlled chemical understanding.

Development as an Explosive

Although Welter and others recognised the explosive nature of picrate salts, picric acid itself was long assumed to be non-detonating. Early theories proposed that the acid contained sufficient internal oxygen to combust completely, but that heat loss during sublimation prevented detonation. The salts, in contrast, did not sublimate and thus could accumulate enough heat to explode. This understanding was refined as experimentation progressed.
A major advance occurred in 1871 when Hermann Sprengel showed that picric acid could indeed be detonated under appropriate conditions. His investigations, patents, and subsequent advocacy convinced military establishments of its potential as a stable yet powerful high explosive. By the 1880s, various nations adopted formulations based on picric acid for artillery shells and blasting charges. French chemist Eugène Turpin patented methods for pressing and casting it, leading to its incorporation into Melinite by France, Lyddite by Britain, and Shimose powder by Japan. Austria-Hungary developed Ecrasite, while Russia also began manufacturing picric acid-filled munitions.
The compound’s suitability for artillery arose from its relative insensitivity to shock compared with nitroglycerine or early nitrocellulose formulations. However, instability caused by metal picrate formation remained a significant risk, demonstrated tragically in accidents such as the Halifax Explosion.
Ammonium picrate, known as Dunnite or explosive D, gained use in the United States from 1906, offering reduced sensitivity for armour-piercing applications.

Industrial Context and Decline in Military Use

Despite its explosive strength exceeding that of trinitrotoluene (TNT), picric acid was gradually replaced by TNT during the early twentieth century due to safety concerns and ease of handling. TNT’s lower sensitivity, despite lower power, made it preferable for mass-production munitions. From around 1902 onwards, Germany and later other nations increasingly utilised TNT as a principal military explosive.
The importance of picric acid and its precursor phenol during the First World War is illustrated by intense industrial efforts to secure phenol supplies. At the time, phenol was largely produced from coal-derived feedstocks. Concerns over shortages and the strategic importance of picric acid led to what became known as the “Great Phenol Plot”, reflecting competitive attempts to control industrial phenol for wartime use. Major chemical companies in the United States, including Monsanto and Dow Chemical, began producing synthetic phenol, while Thomas Edison initiated his own manufacturing to support both wartime needs and his phonograph production. These developments contributed to the expansion of the American chemical industry.

Synthesis and Industrial Manufacture

Modern synthesis of picric acid relies on controlled nitration of phenol. Direct nitration of phenol often results in tar formation due to excessive activation of the aromatic ring. To mitigate this, industrial routes typically involve:

• Sulfonation of anhydrous phenol using fuming sulphuric acid to form phenol-sulphonic acids.
• Subsequent nitration with concentrated nitric acid, which introduces nitro groups and displaces the sulfonic acid substituent.

The process is strongly exothermic, necessitating strict temperature regulation to prevent runaway reactions. Alternative synthetic routes involve nitration of aspirin or salicylic acid, where decarboxylation and removal of acyl groups help limit by-product formation. In all methods, careful control is essential to ensure both yield and safety.

Uses Beyond Explosives

Although best known for its role in military technology, picric acid has also been used in several non-military contexts. Historically, it served as a yellow dye, particularly for silk and wool, owing to its intense colouration. In medical practice, it has been applied as an antiseptic and in burn treatments. Analytical chemistry and histology have employed picric acid as a reagent and fixative. Despite these uses, safety concerns have led to declining reliance on picric acid where safer alternatives exist.