Tannins

Tannins, also tannic acid, common name applied to a group of vegetable products, both amorphous and crystalline, obtained from various plants, and important commercially in the tanning of leather. Tannins have variable composition. Some, called condensed tannins, are phenols of moderately complex structure, and others are esters of glucose or some other sugar with one or more trihydroxybenzoic acids. The empirical formula, C₁₄H₁₄O₁₁, often given for common tannin, is only an average. Tannins occur in many trees, and the best sources include oak galls (see Gall) and the bark of sumac. Extraction with water, or water and alcohol, is the first step in the preparation of tannin. Settling, followed by evaporation at a low temperature, yields the commercial product.

Tannins have a yellow-white to brown color and a faint, characteristic odor. Exposure to light deepens the color. They all taste bitter and are astringent. Water, acetone, and alcohol dissolve tannins readily, but benzene, ether, and chloroform do not. Heating to 210° C (410° F) causes decomposition, accompanied by formation of pyrogallol and carbon dioxide. The chemical property that provides the basis for most uses of tannins is its ready formation of precipitates with albumin, with gelatin, and with many alkaloidal and metallic salts. The ability of tannins to transform proteins into insoluble products resistant to decomposition leads to their use as tanning agents. Ferric salts react with tannins to give bluish-black products that are useful as inks. Tannins are used as mordants for dyeing cloth, as sizes for paper or silk, and as coagulants for rubber (see Dyeing). The precipitating properties of tannins are used in clarifying, or cleaning, wines and beer. Tannic acid is valuable as an external medicine because it is astringent and styptic.