gallic acid or 3,4,5-trihydroxybenzoic acid, C₆H₂(OH)₃CO₂H, colorless crystalline organic acid found in gallnuts, sumach, tea leaves, oak bark, and many other plants, both in its free state and as part of the tannin molecule (see tannin). Since gallic acid has hydroxyl groups and a carboxylic acid group in the same molecule, two molecules of it can react with one another to form an ester, digallic acid. Gallic acid is obtained by the hydrolysis of tannic acid with sulfuric acid. When heated above 220°C;, gallic acid loses carbon dioxide to form pyrogallol, or 1,2,3-trihydroxybenzene, C₆H₃(OH)₃, which is used in the production of azo dyes and photographic developers and in laboratories for absorbing oxygen.

Gallic acid is an organic acid, also known as 3,4,5-trihydroxybenzoic acid, found in gallnuts, sumac, witch hazel, tea leaves, oak bark, and other plants. The chemical formula is C₆H₂(OH)₃COOH. Gallic acid is found both free and as part of tannins. Salts and esters of gallic acid are termed gallates. Despite its name, it does not contain gallium.

Gallic acid is commonly used in the pharmaceutical industry. It is used as a standard for determining the phenol content of various analytes by the Folin-Ciocalteau assay; results are reported in gallic acid equivalents. Gallic acid can also be used to synthesize the hallucinogenic alkaloid mescaline, also known as 3,4,5-trimethoxyphenethylamine.

Gallic acid was one of the substances used by Angelo Mai among other early investigators of palimpsests to clear the top layer of text off and reveal hidden manuscripts underneath. Mai was the first to employ it, but did so "with a heavy hand," often damaging manuscripts for future study.

See also

• Pyrogallol
• Syringol
• Syringaldehyde

References

• LD Reynolds and NG Wilson, “Scribes and Scholars” 3rd Ed. Oxford: 1991. pp193–4.

External links

• King's American Dispensatory (1898) entry on Gallic acid