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.