Peter Woulfe obtained a solution of picric acid by the action of nitric acid on indigo. The first artificial dye, the substance transformed the textile industry, but its more significant use was as an explosive, adding significant new weapons to the military arsenals of the late eighteenth century.
As the Industrial Revolution gained momentum, chemical experimentation began to yield discoveries that benefited industry. Peter Woulfe was an experimenter and a believer in alchemy. By modern standards, he had little fundamental understanding of his results. Nevertheless, he made a number of interesting observations and gained enough respect from the scientific community that he was made a fellow of the Royal Society in 1767.
Woulfe had become interested in nitric acid (possibly through his friendship with Joseph Priestley) and tested its effects on a number of substances, including indigo. Indigo was an important dye derived from a plant and used for centuries for its blue color. At this time, the textile industry was almost entirely dependent on natural products for dyes, including indigo, madder, cochineal, woad, and a few others. Some of the natural dyes exhibited color changes when subjected to acid treatment: Thus, Johannes Barth in 1744 treated indigo with sulfuric acid and obtained a blue dye.
Possibly these considerations inspired Woulfe to treat indigo with nitric acid. In any case, he did so and obtained a yellow solution, noting that this solution imparted a yellow color to wool or silk. This discovery is often cited as the first synthesis of an artificial dye, even though Woulfe started with a natural product, because the application of the acid changed the color of the original substance. (There is some reason to believe that potassium picrate may have been present in a potion called “tincturianitri Glauberi” made by Johannes Glauber in 1647, but details are hazy.)
Picric acid is a yellow solid, melting at 122 degrees Celsius, and is strongly acidic. Its solutions can be used to dye wool and cotton and were used for this purpose until the advances in coal tar dyes provided better yellow colors. The explosive properties of picric acid were noted by several investigators in the late eighteenth century, and it was recognized as an acid. Slowly, its relationship to other chemicals began to emerge. In addition, picric acid was found to result from the action of nitric acid on silk and other materials. It was noted that indigo was probably converted first to aniline by the nitric acid, and that the aniline was converted to picric acid in a second step. Countless chemists over the years have experienced yellow stains on their fingers after working with nitric acid. This color change has been suggested as a test for proteins and is called the xanthoproteic reaction. Probably the color is due to picric acid or related nitro compounds formed from amino acids in proteins of the skin when attacked by nitric acid.
Woulfe’s discovery attracted the interest of other scientists, particularly in France, where his experiments would be repeated by (among others) the nineteenth century chemist Jean-Baptiste-André Dumas, who originated the name “picric acid,” based on the Greek pikros, meaning bitter. Dumas also completed an elemental analysis of picric acid and determined its empirical formula. In 1841, Auguste Laurent found that picric acid could easily be made by nitration of phenol (a coal tar derivative) using nitric and sulfuric acids. This became the modern method for synthesizing the chemical.
The eighteenth century investigators noted that picric acid decomposed suddenly when heated above its melting point. The decomposition was what might today be described as a deflagration rather than an explosion, unless the heating was rapid. In 1873, Hermann Sprengel showed that picric acid in combination with other substances could be detonated by a blasting cap. By 1880, it was understood that picric acid was a powerful explosive, and moves were made to make it useful in warfare. For centuries, black powder (sulfur, charcoal, and potassium nitrate) had been the only military explosive. Picric acid was much more powerful than black powder and began to find use as a bursting charge in mines, grenades, and artillery shells. It was used in the Boer War, the Russian-Japanese War, and World War I.
Picric acid is slightly more powerful than TNT (trinitrotoluene), but it suffers from two disadvantages: As a strong acid, it is corrosive to metals, such as those forming shell casings, and its melting point is too high to enable it to be conveniently melted and poured into place as TNT is poured. Moreover, the metal picrates that are formed by corrosion of shell casings are extremely shock sensitive, leading to the strong possibility of accidental detonations. Ammonium picrate is an exception. Although a powerful explosive when detonated, it is relatively insensitive to mechanical shock and is valued as a bursting charge in armor piercing shells. These useful properties of ammonium picrate were discovered and explored at the Frankford Arsenal near Philadelphia, Pennsylvania, early in the twentieth century.
The use of picric acid as a textile dye decreased after the discovery of the coal tar dyes, but its use continues in histology, where it finds application as a counter stain and fixative, and in the demonstration of hemoglobin. Misguided attempts were made to use it in beer as a substitute for hops, until it was found to be quite toxic. As a derivative of phenol, the bactericidal properties of which are well known, picric acid was also found to kill bacteria and has been used in salves for the treatment of burns. It exerts a pain-relieving effect by paralyzing the nerve endings in the skin.
Picric acid was manufactured in a number of countries as an explosive and was used by itself or in combination with various additives. In England, picric acid was mixed with petroleum jelly to make an explosive called lyddite, named for the village of Lydd in Suffolk. The French had another formulation known as mèlinite, while the Japanese called their version shimose. Picric acid was produced in large quantities in the United States during World War I by several manufacturers, including E. I. DuPont de Nemours. However, in the later stages of World War I, picric acid was beginning to be replaced by TNT. Other explosives, such as nitrocellulose (“gun cotton”) and nitroglycerin, were discovered in the 1840’s and became the materials of choice for commercial blasting (dynamite) and propellants for bullets (smokeless powder, cordite, and so on). Some picric acid was still being used by the Japanese in World War II.
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Beilsteins Handbuch Der Organischen Chemie (Beilstein’s handbook of organic chemistry). 4th ed. Vol. 6. Berlin: Julius Springer, 1923. Provides a historical account of picric acid, with references to the researches of Dumas, Welter, and others. -
Brown, G. I. The Big Bang: A History of Explosives. Stroud, Gloucestershire, England: Sutton, 1998. Contains a short account of the use of picric acid munitions in warfare from 1885 to 1918. -
Meyer, R., J. Koehler, and A. Homburg. Explosives, 5th ed. Weinheim: Wiley-VCH, 2002. Technical details are given for the properties of many explosives, including picric acid and its derivatives. -
Sprengel, H. “On a New Class of Explosives Which Are Non-Explosive During Their Manufacture, Storage, and Transport.” Journal of the Chemistry Society 26 (1873). Picric acid is shown to detonate when set off by a cap. A fundamental discussion is given of the explosion process and its relationship to the chemical composition of the explosives. -
Urbanski, T. The Chemistry and Technology of Explosives. Oxford, England: Pergamon, 1964. Methods of manufacture of picric acid and other explosives are discussed. -
Van Gelder, A. P., and H. Schlatter. History of the Explosives Industry in America. New York: Arno Press, 1972. Discusses the research on picric acid derivatives for use in artillery shells, and the manufacture of picric acid in the United States. -
Woulfe, Peter. “Experiments to Show the Nature of Aurum Mosaicum.” Philosophical Transactions of the Royal Society 61 (1771): 114. The experiments with nitric acid and indigo are reported here along with many unrelated matters.
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