Abstract
Tannic acid in aqueous solution is bound to mucins in formalin-fixed and formalin-free fixed tissues and its presence can be detected with ferric chloride as a dark gray, blue-black to black complex. This colored compound is readily extracted by acids and some chelating and bleaching agents and is changed to a reddish brown by alkalis. Hydrolysis in 1.2 N hydrochloric acid at 60°C for 4 hr or hot trichloroacetic acid prevents the tannin-iron reaction. Acetyl chloride or bromide is able to inhibit the binding of tannic acid to the mucosubstances and saponification restores the characteristic reaction. Several mechanisms for the attachment of tannic acid to the mucins appear possible.