Pyrogallol is an

organic compound In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen or carbon-carbon bonds. Due to carbon's ability to catenate (form chains with other carbon atoms), millions of organic compounds are known. T ...

with the formula C₆H₃(OH)₃. It is a water-soluble, white solid although samples are typically brownish because of its sensitivity toward oxygen. It is one of three isomers of

benzenetriol The trihydroxybenzenes (or benzenetriols) are organic compounds with the formula C6H3(OH)3. Also classified as polyphenols, they feature three hydroxyl groups substituted onto a benzene ring. They are white solids with modest solubility in water. ...

Production and reactions

It is produced in the manner first reported by

Scheele Scheele is a surname of Germanic origin. Notable people with the surname include: *Carl Wilhelm Scheele (1742–1786), German-Swedish pharmaceutical chemist * George Heinrich Adolf Scheele (1808–1864), German botanist * Karin Scheele (b. 1968) ...

in 1786: heating

gallic acid Gallic acid (also known as 3,4,5-trihydroxybenzoic acid) is a trihydroxybenzoic acid with the formula C6 H2( OH)3CO2H. It is classified as a phenolic acid. It is found in gallnuts, sumac, witch hazel, tea leaves, oak bark, and other plants. I ...

to induce decarboxylation. Gallic acid is also obtained from

tannin Tannins (or tannoids) are a class of astringent, polyphenolic biomolecules that bind to and precipitate proteins and various other organic compounds including amino acids and alkaloids. The term ''tannin'' (from Anglo-Norman ''tanner'' ...

. Many alternative routes have been devised. One preparation involves treating ''para''-chlorophenoldisulfonic acid with

potassium hydroxide Potassium hydroxide is an inorganic compound with the formula K OH, and is commonly called caustic potash. Along with sodium hydroxide (NaOH), KOH is a prototypical strong base. It has many industrial and niche applications, most of which exp ...

, a variant on the time-honored route to phenols from sulfonic acids. When in alkaline solution, pyrogallol undergoes deprotonation of one or more phenolic groups. Such solutions absorb

oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as ...

from the air, turning brown. This conversion can be used to determine the amount of oxygen in a gas sample, notably by the use of the Orsat apparatus. Polyhydroxybenzenes are relatively electron-rich. One manifestation is the easy C-acetylation of pyrogallol.

Uses

One can find its uses in hair dyeing, dyeing of suturing materials and for

absorption in gas analysis. It also has

antiseptic An antiseptic (from Greek ἀντί ''anti'', "against" and σηπτικός ''sēptikos'', "putrefactive") is an antimicrobial substance or compound that is applied to living tissue/skin to reduce the possibility of infection, sepsis, or putre ...

properties. Pyrogallol was also used as a developing agent in black-and-white developers, but its use is largely historical except for special purpose applications.

Hydroquinone Hydroquinone, also known as benzene-1,4-diol or quinol, is an aromatic organic compound that is a type of phenol, a derivative of benzene, having the chemical formula C6H4(OH)2. It has two hydroxyl groups bonded to a benzene ring in a ''para' ...

is more commonly used today. It is also used in isolation of inert gases from a mixture of gases, which requires absorption of oxygen from the mixture.

Use in photography

Though a popular photographic developing agent in the 19th and early 20th centuries, pyrogallol largely fell out of favor around the 1920s, although it was still used by a few notable photographers including

Edward Weston Edward Henry Weston (March 24, 1886 – January 1, 1958) was a 20th-century American photographer. He has been called "one of the most innovative and influential American photographers..." and "one of the masters of 20th century photography." ...

. In those days it had a reputation for erratic and unreliable behavior, due possibly to its propensity for oxidation. It experienced a revival starting in the 1980s due largely to the efforts of experimenters Gordon Hutchings and

John Wimberley John M. Wimberley (born July 1945) is an American photographer and artist. He has photographed extensively in the American west. His body of work also includes figurative work of women underwater, and a large number of photographs of Americ ...

PMK

Hutchings spent over a decade working on pyrogallol formulas, eventually producing one he named PMK for its main ingredients: pyrogallol, Metol, and Kodalk (the trade name of

Kodak The Eastman Kodak Company (referred to simply as Kodak ) is an American public company that produces various products related to its historic basis in analogue photography. The company is headquartered in Rochester, New York, and is incorpor ...

for sodium metaborate). This formulation resolved the consistency issues, and Hutchings found that an interaction between the greenish stain given to film by pyro developers and the color sensitivity of modern variable-contrast photographic papers gave the effect of an extreme compensating developer. From 1969 to 1977, Wimberley experimented with the Pyrogallol developing agent. He published his formula for WD2D in 1977 in Petersen's Photographic. PMK and other modern pyro formulations are now used by many black-and-white photographers. The Film Developing Cookbook has examples.

510-pyro

Another developer mainly based on pyrogallol was formulated by

Jay DeFehr A jay is a member of a number of species of medium-sized, usually colorful and noisy, passerine birds in the Crow family, Corvidae. The evolutionary relationships between the jays and the magpies are rather complex. For example, the Eurasian m ...

. The 510-pyro, is a concentrate that uses

Triethanolamine Triethanolamine, or TEA is a viscous organic compound that is both a tertiary amine and a triol. A triol is a molecule with three alcohol groups. Approximately 150,000 tonnes were produced in 1999. It is a colourless compound although samples m ...

as Alkali, and pyrogallol and

phenidone Phenidone (1-phenyl-3-pyrazolidinone) is an organic compound that is primarily used as a photographic developer. It has five to ten times the developing power as Metol. It also has low toxicity and unlike some other developers, does not cause derm ...

as combined developers. This developer has both staining and tanning properties and negatives developed with it are immune to the callier effect. It can be used for small and large negative formats. The Darkroom Cookbook (Alternative Process Photography) has examples.

Safety

Pyrogallol use, e.g. in hair dye formulations, is declining because of concerns about its toxicity.Safety data for 1,2,3-trihydroxybenzene
/ref> Its (oral, rat) is 300 mg/kg.

References

{{Authority control Antioxidants Catechols Chelating agents Photographic chemicals Reducing agents