Experimental Study of a Closed System in the Chlorine Dioxide-Iodine-Malonic Acid-Sulfuric Acid Oscillation Reaction by UV-vis Spectrophotometric Method

Abstract

The appearance of oscillations depends critically on the pH for the closed system ClO2-I2-malonic acid in the absence of sulfuric acid, and was investigated by determining the absorbance of I3− with reaction time at 280 nm. The pH should be 3.2–3.8. The initial concentrations of malonic acid, chlorine dioxide, iodine and sulfuric acid have great influence on the oscillation at 280 or 350 nm. The oscillation occurs as long as the reactants are mixed at 280 nm. However, at 350 nm the oscillation is preceded by a pre-oscillatory or induction period. The amplitude is small at the beginning stage but then increases with the reaction time. Finally, the oscillation ceases suddenly. The amplitude and the number of oscillations are associated with the initial reactant’s concentration. The higher is the initial concentration of malonic acid, iodine or sulfuric acid, the bigger is the amplitude. Also, the number of oscillations becomes small. An opposite influence exists for chlorine dioxide. The oscillation curve is more regular and smooth at 350 nm than that at 280 nm. The oscillation becomes more obvious by adding starch at 581 nm for I3−–starch complex (SI3−) than that observed without adding starch at 280 nm. The curve’s shape at 581 nm is very different from that at 280 or 350 nm. The equation for the triiodide ion reaction rate changing with reaction time and the effect of initial concentrations on the oscillation stage were obtained. Based upon the experimental data in this work and in the literature, a plausible reaction mechanism was proposed for the oscillation reaction.