complex is reduced to the ferrous (FeII) form, an intense blue colour develops with an absorption maximum at 593 nm.
To 3.3 ml of acetate buffer (pH 3.6) consisting of 18.5 ml 0.2 M CH3COOH and 1.5 ml 0.2 M CH3COONa, 0.33 ml 20 mM FeCl3 and 0.33 ml 10 mM TPTZ (2,4,6-tri(2-pyridyl)-s-triazine) in 40 mM HCl were added.