At low pH, reduction of ferric tripyridyl triazine (Fe(III)(TPTZ)2) complex to ferrous form (which has an intense blue color) can be monitored by measuring the change in absorption at 593 nm. The reaction is nonspecific, in that any half reaction that has lower redox potential, under reaction conditions, than that of ferric ferrous half reaction, will drive the ferrous (Fe III to Fe II) ion formation. The change in absorbance is therefore directly related to the combined or total reducing power of the electron donating antioxidants present in the reaction mixture. But this method does not come without any shortcomings. One is that the reaction is carried out on a rather acidic environment. The other is that the main reagent Fe(III)(TPTZ)2 is not the only oxidant, many metal chelators in food extract could bind Fe(III) and form complexes that are also capable of reacting with antioxidants.
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