Pyrogallol

Pyrogallol
Names
Preferred IUPAC name
Benzene-1,2,3-triol
Other names
1,2,3-Trihydroxybenzene
Pyrogallic acid
Identifiers
3D model (JSmol)
ChEBI
ChemSpider
ECHA InfoCard 100.001.603
UNII
Properties
C6H6O3
Molar mass 126.11 g/mol
Density 1.45 g/cm3
Melting point 131 to 134 °C (268 to 273 °F; 404 to 407 K)
Boiling point 309 °C (588 °F; 582 K)
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references

Pyrogallol is an organic compound with the formula C6H3(OH)3. It is a white water-soluble solid although samples are typically brownish because of its sensitivity toward oxygen.[1] It is one of three isomeric benzenetriols.

Production, occurrence, reactions

It is produced in the manner it was first prepared by Scheele (1786): heating gallic acid. Presently gallic acid is obtained from tannin. Heating induces decarboxylation:[1]

Because tannin is expensive, many alternative routes have been devised. An alternate preparation involves treating para-chlorophenoldisulphonic acid with potassium hydroxide, a variant on the time-honored route to phenols from sulfonic acids.

The aquatic plant Myriophyllum spicatum produces pyrogallic acid.[2]

When in alkaline solution, it absorbs oxygen from the air, turning brown from a colourless solution. It can be used in this way to calculate the amount of oxygen in air, notably via the use of the Orsat apparatus.

Uses

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.)

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. 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. 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. From 1969 to 1977, Wimberley experimented with the Pyrogallol developing agent. He published his formula for WD2D in 1977 in Petersons Photographic.

PMK and other modern pyro formulations are now used by many black-and-white photographers.

The Film Developing Cookbook has examples.[3]

Safety

Pyrogallol use, e.g. in hair dye formulations, is declining because of concerns about its toxicity.[4] Its LD50 (oral, rat) is 300 mg/kg.[1]

See also

References

  1. 1 2 3 Fiege, Helmut; Heinz-Werner, Voges; Hamamoto, Toshikazu; Umemura, Sumio; Iwata, Tadao; Miki, Hisaya; Fujita, Yasuhiro; Buysch, Hans-Josef; Garbe, Dorothea; Paulus, Wilfried (2014). Ullmann’s Encyclopedia of Industrial Chemistry (7th ed.). Weinheim, Germany: Wiley-VCH. p. 1072. ISBN 9783527334773. doi:10.1002/14356007.a19_313.
  2. ↑ Myriophyllum spicatum-released allelopathic polyphenols inhibiting growth of blue-green algae Microcystis aeruginosa. Satoshi Nakai, Yutaka Inoue, Masaaki Hosomi and Akihiko Murakami, Water Research, Volume 34, Issue 11, 1 August 2000, Pages 3026–3032, doi:10.1016/S0043-1354(00)00039-7
  3. ↑ Stephen G. Anchell and Bill Troop. The Film Developing Cookbook. ISBN 978-0240802770.
  4. ↑ Safety data for 1,2,3-trihydroxybenzene
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