Pyrogallol

Chembox new
Name = Pyrogallol
ImageFile = pyrogallol.svg
ImageName = Pyrogallol
OtherNames = 1,2,3-Trihydroxybenzene
Pyrogallic acid
Section1 = Chembox Identifiers
CASNo = 87-66-1
SMILES = OC1=C(O)C=CC=C1O
Section2 = Chembox Properties
Formula = C₆H₆O₃
MolarMass = 126.11 g/mol
MeltingPt = 131-134 °C
BoilingPt = 309 °C
Density = 1.45 g/cm³

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 water readily, turning purple from a colourless solution. It can also absorb oxygen and is used 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.)

Limited exposure to this compound is recommended. Pyrogallol is not likely to be used as a modern hair dye due to the suspected toxicity of the compound.

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 a good number of black-and-white photographers.

ee also

*Gallic acid
*Catechol

References