At pH lower than 3.5, the complex of Fe(II)-5-Br-PADAP cannot form readily although Fe3+ is reduced by ascorbic acid. A pH greater than 6 will result in the decrease in absorbance due to the hydrolysis of
Fe3+ or Fe2+. Therefore, pH 5.5 was found to be best for the extrac-tion of Fe(II)-5-Br-PADAP complex.