Kinetic spectrophotometric determination of trace zinc (II) by its inhibitory effect on the oxidation of arsenazo I with hydrogen peroxide

A spectrophotometric method for the determination of zinc(Ⅱ) is described,based on its inhibitory effect on the oxidation reaction of acid arsenazo I with hydrogen peroxide in pH =10.3 NH3-NH4Cl buffer medium at 100 ℃.The reaction rate is monitored spectrophotometrically by measuring the increase in the absorbance of arsenazo I at 515 nm after a fixed heating time(4 min,10 s).The calibration curve for the recommended method is linear in the concentration range over 0.0～2.0 μg/25 mL zinc(Ⅱ) and the detection limit of the method is 1.72 μg/L.The influence of the factors such as acidity,concentration of reactants,reaction time,temperature and co-existing ions on the reaction is discussed.The optimum conditions of reaction are established and some kinetic parameters are determined.The apparent activation energy of catalytic reaction is 84.00 kJ/mol.The method has been successfully applied for the determination of trace zinc(Ⅱ) in some milk powders with the relative standard deviations(RSD) over 2.4%～3.1% and the recovery over 98.2%～103.2%.