Tannic Acids

Tannic Acids. A number of compounds, all intimately related, are known under the name of tannic acids or tannins. They occur widely diffused in the vegetable kingdom, being found in a large variety of plants - e.g. nutgalls, sumach, tea, coffee, and many woods. They are soluble in water, the solution possessing acidic properties and a strong astringent taste. With solutions of ferric salts they give an intensely deep blue or green coloration or precipitate. They possess the power of combining with skins of animals, and so rendering them incapable of putrefaction. To this quality their usefulness in leather-making is due. Most of them break up when acted on by dilute acids, forming gallic acid and grape sugar with other products. They thus belong to the class of compounds known as glucosides (q.v.). Ordinary tannic acid is obtained chiefly from nutgalls by finely powdering and extraction with commercial alcohol and ether. A solution of the tannin in the water present with these sinks to the bottom and is collected. It is a colourless compound, very soluble, which decomposes into pyrogallic acid if heated. Its reactions prove its constitution to be that of digallic acid, C₁₄H₁₀O₉. It is used to a large extent in dyeing, calico-printing, in tanning, and for the manufacture of ink (q.v.).