× which was further purified by a Millipore Milli–Q plus water purification system. The standard stock solution of cerium(IV) (1000 mg mL-1) was prepared by dissolving 4.511 g of (NH4)4Ce(SO4)4.2H 2O in water containing 2 mL H2SO4 in a 1000 mL volumetric flask and standardized spectrophotometrically.38 The buffer solutions in the pH range 3-10 were prepared from 0.2 mol L-1 ethanoic acid/0.2 mol L-1 sodium ethanoate or triethanolamine buffer (mix 200 mL of 15% v/v triethanolamine