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Pyrogallol

acid, deg and gallic

PYROGALLOL (derived from gallic acid and heat). Formula, molecular weight, 126; synonyms, trihydroxybenzene, pyrogallic acid, pyrogallin, trihydroxybenzene derived from gallic acid by destructive distillation, carbon dioxide being given off as one of the products of decomposition, thus: It is prepared as follows: Gallic acid dried at ioo deg. C. is mixed with three times its weight of powdered pumice stone to equalize the heat, and distilled in a retort, through which a slow stream of CO, is passed, the retort being heated by an oil-bath between zio deg. and 220 deg. C. A solution of pyrogallol, for use as a developer for photographic purposes, can readily be pre pared by heating ten grammes of gallic acid with 3o c.c. of glycerine to 195 deg. C , so long as carbon dioxide is evolved. It is then made up with water to one litre. Of course, by this method the pyrogallol is obtained mixed with glycerine.

Pyrogallol crystallizes in fine needles felted together in light white tufts. It fuses at i r5°C., and boils at zio°C. It is very soluble in water (i in 24 parts) and in alcohol and ether, and is more or less poisonous. Metagallic acid is sometimes present in samples of pyro, which have

consequently a slightly brownish color, and leave a residue when dissolved in water.

If the solution be mixed with an alkali it absorbs oxygen from the air, turning brownish black, and forming carbonates, acetates, and other products, a small quantity of carbon dioxide being evolved. It is a most powerful deoxidizer and reducing agent. Its action upon the silver salts causes it to be most extensively used as a developing agent. The reducing power of an alkaline solution of pyrogallol is in tact so great that it cannot be used alone, as the whole plate would be reduced, and we should get what is termed " fog." For this reason, therefore, a restrainer is added. The best for this purpose is a soluble bromide such as that of potassium.

Owing to the rapid oxidizing properties of pyrogallol solution it is very difficult to preserve. All of the following substances have been recommended for the purpose:—Formic acid, glycerine and alcohol, potassium metabisulphite, sodium sulphite, and citric acid.