Pyrogallol

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

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

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.

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.)

Pyrogallol use, e.g. in hair dye formulations, is declining because of concerns about its toxicity.^[3] Its LD50 (oral, rat) is 300 mg/kg.^[1]

• Gallic acid
• Catechol