The added volume of ferric chloride 0.025 M solution was varied; 0.25, 0.5, 1, 1.5, and 2 mL, while the other mixture constituents and test procedure remained unchanged, see paragraph 2.1. Figure shows a proportional increase of absorbance with increasing ferric chloride solution volume up to 1 mL, followed by a slight further increase up to 2 mL ferric chloride addition. Thus, oxidizing agent need to be present in reaction mixture in sufficient amount for satisfactorily performance.