The FRAP assay (ferric reducing ability of plasma) was originally designed to test the antioxidant capacity of plasma samples but has been modified for use with plant extracts, fruit juices, and other types of water-soluble compounds.  The assay is based on the reduction of ferric tripyridyltriazine to ferrous tripyridyltriazine (TPZ), the latter forming a blue chromagen with an absorbance maximum at 593 nm.  This reaction is done at an acidic pH (3.6), which is necessary for the colored chromagen to form.