Pyrogallol

Pyrogallol
Other names	1,2,3-Trihydroxybenzene Pyrogallic acid
Identifiers
CAS number	87-66-1 Y
SMILES	OC1=C(O)C=CC=C1O
Properties
Molecular formula	C6H6O3
Molar mass	126.11 g/mol
Density	1.45 g/cm3
Melting point	131-134 °C
Boiling point	309 °C
Y (what is this?)  (verify) Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

Pyrogallol or benzene-1,2,3-triol is a white crystalline powder and a powerful reducing agent. It was first prepared by Scheele 1786 by heating gallic acid. An alternate preparation is heating para-chlorophenoldisulphonic acid with potassium hydroxide.

When in alkaline solution, it absorbs oxygen from the air, turning purple from a colourless solution. It can be used in this way to calculate the amount of oxygen in air.

One can find its uses in hair dying, dying of suturing materials and for oxygen absorption in gas analysis. It also has antiseptic 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 is more commonly used today.)

Harmful if swallowed and repeated or prolonged exposure to this compound is not recommended. Pyrogallol is not likely to be used as a modern hair dye due to the suspected toxicity of the compound. ^[1]

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, though still used by a few notable photographers including Edward Weston. 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 experimenter Gordon Hutchings. Hutchings spent over a decade working on pyrogallol formulas, eventually producing one he named PMK (for its main ingredients, pyrogallol, Metol, and Kodalk [trade name of Kodak 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. PMK and other modern pyro formulations are now used by many black-and-white photographers.

See also

• Gallic acid
• Catechol
• Phloroglucinol (1,3,5-trihydroxybenzene)
• Gallacetophenone (2,3,4-trihydroxyacetophenone)

References