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Tannic acid or gallotannin is a commercial prepared form of tannin, a collective term applied to a group of complex polyphenolic secondary metabolites. These are present in localized structures associated with different plant parts. Tannins are distributed in both Angiosperms and Gymnosperms and are more common in dicotyledons than monocotyledons. Some important sources of tannins are oak (Quercus sps), wattle (Acacia sps), Eucalyptus species, willow (Salix caprea), pine (Pinus species) and Myrobalans (Terminalia chebula and Terminalia belerica).

The ability of these plant-derived polyphenolic substances to convert animal skin to leather or their use in ‘tanning’ led to these being referred to as tannins. They are rarely representative of a single chemical entity as their composite structure is composed of acid units and their derivatives such as gallic acid, ellagic acid, chebulic acid, catechu-tannic acid etc. Tannins are high-molecular-weight compounds ranging between 1,000 and 5,000 and rarely give sharp melting points due to their complex nature. The chemistry of tannins has been most extensively investigated and they are said to be constituted of an accumulation of a substantial number of phenolic groups within a moderately sized molecule. Associated with a number of 0-dihydroxy and 0-trihydroxy orientation within a phenyl ring, tannins are classified as hydrolysable tannins, condensed tannins and pseudotannins.

Commercial tannic acid is extracted from many sources including gall nuts of the oak tree (Quercus infectoria), pods of Caesalpinia spinosa, leaves of Rhus coriaria among others. Indigenous sources of tannins representing different types of tannic acid are T. chebula, T. belerica, Embelica officinalis, Cassia auriculata, Azadirachta indica, Acacia catechu, Casurina equistifolia etc.

Tannic acid or gallotannic acid is found to the extent of 50% to 70% in nut galls (Turkish galls), vegetable outgrowths formed on the young branches of the oak tree. Nut galls are used in the tanning and dyeing industry and earlier in the manufacture of ink. Myrobalans which contain 20% to 40% tannic acid are principally constituted of ellagitannins. Several plants of medicinal interest are found to contain ellagitannins and these are being actively investigated.