Chemical Properties
Also know nas pyrogallol and 1,2,3-trihydroxy-benzene,C6H3(OH)3 is white,lustrous crystals that turn gray on exposure to light. It is soluble in water,alcohol,and ether. A solution of pyrogallol acquires a brown color on exposure to air. This absorption of oxygen and change of color take place rapidly when the solution is made alkaline. Derivation is by heating gallic acid in an autoclave with three times its weight of water.
Uses are as a protective colloid in preparation of metallic colloidal solutions, photography, dyes, intermediates, synthetic drugs, medicine (antibacterial), process engraving, laboratory reagent, gas analysis (an oxygen absorber), reducing agent, and antioxidant in lubricating oils.
Uses
The white powdery crystal was made by the superheating of
gallic acid. Pyrogallic acid, popularly known as pyro, is one of
the earliest developers and its use has spanned three centuries.
It was first used in an acid state to physically develop wet
collodion negatives. When silver bromide gelatin plates were
adopted pyro was used again but in an alkaline state for chemical
development. It was used throughout the 20th century
and often chosen for the peculiar yellow stain it imparted to
gelatin negatives.
Purification Methods
It is recrystallised from aqueous alkaline solution (Na2CO3 or NaOH) by precipitation on acidification, filter it off and dry in a vacuum. [Suk Collect Czech Chem Commun 31 3127 1966, Beilstein 19 H 407, 19 II 417, 19 III/IV 599, 19/10 V 226.]
