Pyrogallol
[CAS 87-66-1]

Identification

• Classification: Natural product >> Natural phenols
• Name: Pyrogallol
• Synonyms: 1,2,3-Benzenetriol; 1,2,3-Trihydroxybenzene; C.I. 76515
• Molecular Formula: C₆H₆O₃
• Molecular Weight: 126.11
• CAS Registry Number: 87-66-1
• EC Number: 201-762-9
• SMILES: C1=CC(=C(C(=C1)O)O)O

Properties

• Density: 1.5±0.1 g/cm³ Calc.^*, 1.453 g/mL (Expl.)
• Melting point: 132 - 134 °C (Expl.)
• Boiling point: 309.0 °C 760 mmHg (Calc.)^*, 309 °C (Expl.)
• Flash point: 164.3±16.9 °C (Calc.)^*
• Index of refraction: 1.677 (Calc.)^*, Refraction index Data source (Expl.)

^* Calculated using Advanced Chemistry Development (ACD/Labs) Software.

Safety Data

• Hazard Symbols: GHS07;GHS08 Warning
• Risk Statements: H302-H312-H332-H341-H412
• Safety Statements: P203-P261-P264-P270-P271-P273-P280-P301+P317-P302+P352-P304+P340-P317-P318-P321-P330-P362+P364-P405-P501

Hazard Classification

Hazard	Class	Category Code	Hazard Statement
Germ cell mutagenicity	Muta.	2	H341
Acute toxicity	Acute Tox.	4	H332
Acute toxicity	Acute Tox.	4	H312
Chronic hazardous to the aquatic environment	Aquatic Chronic	3	H412
Acute toxicity	Acute Tox.	4	H302
Skin irritation	Skin Irrit.	2	H315
Eye irritation	Eye Irrit.	2	H319
Specific target organ toxicity - single exposure	STOT SE	3	H336
Carcinogenicity	Carc.	2	H351
Skin sensitization	Skin Sens.	1B	H317
Acute toxicity	Acute Tox.	3	H301
Chronic hazardous to the aquatic environment	Aquatic Chronic	4	H413
Skin sensitization	Skin Sens.	1	H317

• Transport Information: UN 1814; UN 2811

chemBlink Chemical Story

Pyrogallol, benzene-1,2,3-triol, is a small aromatic molecule with three adjacent hydroxyl groups. That dense arrangement makes it easy to oxidize, and this reactivity has shaped almost every chapter of its history. Pyrogallol was obtained historically by heating gallic acid, which loses carbon dioxide during decarboxylation. The old name preserves that connection: "pyro" signaled a substance produced through heating.

Its appetite for oxygen became a classic analytical tool. In alkaline solution pyrogallol rapidly absorbs oxygen while being oxidized to complex colored products. Before electronic oxygen sensors, alkaline pyrogallol was widely used in gas analysis to remove oxygen from gas mixtures. The reaction is not a neat one-product transformation, which is exactly why the reagent is effective as a scavenger: oxidation proceeds readily through radical and quinone-like chemistry.

The same oxidation chemistry gave pyrogallol a place in early photography. Developers must reduce exposed silver halide crystals to metallic silver, and polyhydroxy aromatic compounds can act as reducing agents. Pyrogallol, often called pyro by photographers, became one of the important nineteenth-century photographic developers. Its oxidation products can also stain gelatin, producing image characteristics that some specialized photographic processes still value.

Modern chemistry sees another side of the molecule. Adjacent phenolic OH groups can bind metals, participate in hydrogen bonding, and generate reactive oxygen species under oxidative conditions. Pyrogallol has therefore been used as a biochemical reagent and as a model compound in antioxidant/pro-oxidant studies. The label "antioxidant" is context dependent: a molecule that readily donates electrons can quench some oxidants but can also undergo redox cycling and promote oxidative chemistry under other conditions.

Pyrogallol matters because one structural feature - three neighboring phenolic groups - produced a remarkable sequence of applications. The same easy oxidation allowed nineteenth-century chemists to measure oxygen, photographers to develop silver images, and modern researchers to probe redox chemistry. Its history is a reminder that reactivity itself is neither beneficial nor harmful in isolation. What matters is whether chemistry places that reactivity in a controlled analytical, technological, or biological context.

References:
NIST Chemistry WebBook. Pyrogallol, CAS 87-66-1.
Marklund S, Marklund G. European Journal of Biochemistry. 1974;47:469-474. DOI: 10.1111/j.1432-1033.1974.tb03714.x.
Historical analytical and photographic literature on alkaline pyrogallol and pyrogallol developers.

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