Acidum Pyrogallicum. Pyrogallic Acid. Acidum Pyrogallicum Oxidatum.

Related entry: Gallic acid - Tannic acid

C₆H₆O₃ = 126.048.

Pyrogallic acid (Pyrogallol, U.S.P.), C₆H₃(OH)₃, is a trihydroxyphenol, obtained by the action of heat on gallic acid. It occurs in light, white, feathery or dense, hard crystals, which become discoloured on exposure to light and air. On careful heating it should sublime without residue. Melting-point, 131°. The solution in water should be clear, colourless, and neutral. A solution of pyrogallic acid gives a green colour with solution of ferrous sulphate, a red colour with solution of ferric chloride, and rapidly turns black in contact with air and alkalies. The acid should be kept in well-closed amber-coloured bottles and not exposed to light.

Soluble in water (1 in 2), alcohol, or ether.

Action and Uses.—Pyrogallic acid is a powerful antiseptic by reason of its affinity for oxygen. Taken internally it exerts a toxic action on the blood, methaemoglobin is formed, passes into the plasma, and leaves the red corpuscles as granular debris; jaundice and acute nephritis may follow, hence the drug is now very rarely given internally. It is used as an antiseptic and mildly irritant ointment in chronic skin diseases such as psoriasis, also to destroy lupus; it is of value as a parasiticide in ringworm. Mixed with four times its weight of starch, the acid is applied as a powder to phagedaenic chancres. It has the disadvantage of staining the skin and hair black, and must be used with caution on account of the danger of absorption. Stains upon the skin may be removed with ammonium persulphate. Pyrogallic acid is an ingredient of hair dyes, usually with silver nitrate; and is largely used as a reducing agent in photography. When administered internally, it is given in aqueous solution, or in pills, which may be massed with a little simple syrup, and must be kept in a dark place. In consequence of its toxic properties, several allied compounds have been used as substitutes in the various ointments, pastes, plasters, and varnishes of which it is an ingredient. The chief of these is the so-called pyrogallol oxide (see Acidum Pyrogallicum Oxidatum); other similar compounds are pyrogallol monoacetate (Eugallol), pyrogallol triacetate (Lenigallol), gallacetophenone (trioxyacetophenone), and pyrogallol disalicylate (Saligallol).

Dose.—3 to 10 centigrams (½ to 1 ½ grains).

PREPARATIONS.

Unguentum Acidi Pyrogallici, B.P.C.—PYROGALLIC ACID OINTMENT. Syn.—Pyrogallol Ointment. 1 in 8 (soft paraffin).

Used in psoriasis, ringworm and lupus. For use in psoriasis it is frequently diluted with 1 to 5 parts of lard or soft paraffin. It should be used with caution, and should not be applied to large areas. Jarisch's ointment is composed of 60 grains of pyrogallic acid mixed with 1 ounce of lard.

Unguentum Acidi Pyrogallici Compositum, B.P.C.—COMPOUND PYROGALLIC ACID OINTMENT. Syn.—Unna's Compound Pyrogallol Ointment.

Pyrogallic acid, 5; ammonium ichthosulphonate, 5; salicylic acid, 2; Soft paraffin, 88. Used chiefly in psoriasis and chronic eczema. It should net be applied over large surfaces, on account of the danger of absorption.

ACIDUM PYROGALLICUM OXIDATUM.
OXIDISED PYROGALLIC ACID.

Synonyms.—Pyrogallol Oxidatum; Pyrogallol Oxide; Pyraloxin.

Oxidised pyrogallic acid is prepared by the action of air and ammonia on pyrogallol. It occurs as a dark brown powder.

Slightly soluble in water, but insoluble in absolute alcohol or ether.

Action and Uses.—Oxidised pyrogallic acid is used in the treatment of leprosy and of such chronic skin diseases as eczema and psoriasis, generally in the form of an ointment. It has the advantage over pyrogallol that it does not blacken the skin, is less liable to set up inflammation, and does not give rise to toxic effects through absorption; it is also more permanent and therefore much better adapted for use in plasters and soaps. It has been used with success in the form of eye-drops (1 in 200 to 1000) in chronic cases of conjunctivitis.

PREPARATION.

Unguentum Acidi Pyrogallici Oxidati, B.P.C.—OXIDISED PYROGALLIC ACID OINTMENT. Syn.—Unguentum Pyrogallol Oxidati; Pyrogallol Oxide Ointment.

Oxidised pyrogallic acid, 5; salicylic acid, 5; hydrous wool fat, 90. Used, with reported satisfactory results, in cases of leprosy.

The British Pharmaceutical Codex, 1911, was published by direction of the Council of the Pharmaceutical Society of Great Britain.