Properties of tannins
Tannins are non-crystallisable, amorphous compounds. They are soluble in water, ethyl alcohol, glycerol, acetone
and in dilute alkalis. Their aqueous solution shows an acidic reaction and a sharp astringent taste. Most of the tannin
compounds cause precipitation of solutions of alkaloids, glycosides, gelatine and heavy metal salts of copper, lead
and tin. Tannins produce a deep red colour with potassium ferricyanide and ammonia. With freshly prepared ferric
chloride solution hydrolysable tannins produce blue-black precipitate while condensed tannins show brownish green precipitate.