Aqueous solution of gallic acid gives the following reactions:--Solution of ferric chloride gives a deep blue coloration which is destroyed by boiling. Ferrous sulphate, if free from ferric salt, gives no reaction in dilute solutions, but a white precipitate in strong ones. The mixture rapidly darkens by oxidation. In alkaline solution gallic acid absorbs oxygen from the air and darkens from the formation of tannomelanic acid. Lime-water produces a white precipitate which rapidly becomes blue from oxidation. The same reaction is produced by baryta-water, or by the chlorides of barium or calcium on addition of ammonia (distinction from pyrogallol). It is distinguished from gallotannic acid by the following:--It does not precipitate gelatin, except in the presence of gum. It does not precipitate tartar emetic in presence of ammonic chloride, though both tannin and gallic acid are precipitated by tartar emetic alone. It precipitates lead acetate but not lead nitrate, while tannin precipitates both. A dilute solution of potassium cyanide gives a red coloration which disappears on standing, but is restored by shaking with air. If to even a very dilute solution of gallic acid, sodic arsenate, or some other faintly alkaline salts be added, the mixture absorbs oxygen and becomes a deep green. Aqueous solution of picric acid to which excess of ammonia has previously been added gives a red coloration, changing to green. Tannic and pyrogallic acid produce no reaction with cyanide, and with ammonic picrate a reddish coloration only. Gallic acid reduces silver nitrate and gold chloride rapidly when hot, but not Fehling's solution, and decolorises acidified potassic permanganate. If tannin and other oxidisable bodies be removed from its solution it may be estimated quantitatively by titration with permanganate in presence of indigo (see p. 118). It may be separated from tannin by gelatin or hide raspings (see pp. 121, 124). Gallotannic and quercitannic acids may also be removed by precipitation with ammoniacal solution of cupric sulphate, or by cupric acetate, in presence of excess of ammonic carbonate (see also p. 125). Many other tannins, however, give precipitates with cupric salts which are soluble in ammonia and ammonic carbonate. In absence of such tannins it may be estimated gravimetrically by precipitation with cupric acetate. The precipitate is rapidly washed with water and digested with a solution of ammonic carbonate, in which it dissolves; any insoluble cupric tannate is filtered off, the solution is evaporated to dryness and the residue moistened with nitric acid and ignited. The weight of the remaining cupric oxide multiplied by 0·9 gives the weight of the gallic acid plus a little tannin dissolved by the ammonic solution. Gallic acid may also be separated from tannin by lead acetate strongly acidified with acetic acid, by which tannic acid is precipitated, while lead gallate is dissolved.