This assay measures the ferric reducing ability of plasma (FRAP). At low pH, when a ferric complex is reduced to the ferrous form (Fe2+), intense blue color with an absorption maximum at 593 nm develops. This reaction is nonspecific and any half-reaction which has a less-positive redox potential, under reaction conditions, then the Fe3+/Fe2+ complex half-reaction will drive Fe3+ complex reduction. Acidic conditions favor the reduction of the complex and, thereby, color development, showing that an antioxidant is present.
FRAP assay kit is recommended for total antioxidant activity of single antioxidants in an aqueous solution and added to the plasma.