Redox-potentiometric/titrimetric analysis of aqueous iodine solutions

Redox-potential, titrimetric oxidation capacity and pH were used to analyze aqueous iodine solutions over a concentration range of five decades. By a single operation the equilibrium concentrations of I(-) I(2), I(3)(-), I(5)(-), I(6)(2-) HOI and OI(-) were assessed. Accuracy was most influenced by the redox-potential showing a repeatability of < +/- 0.1 mV in solutions containing appreciable iodide (Lugol's solution) which increased to +/-1.5 mV at a 1:100 000 dilution. Together with the uncertainty of calibration this equals a calculated total error which ranges for "free iodine" ([I(2)] + [HOI]), from +/- 0.5% to +/- 4.2% and for total iodide from +/- 0.7 to +/- 5.3%. Accuracy was assessed by a comparison of the evaluated equilibrium concentrations and parameters derived therefrom, such as total iodide and redox-potential, with measured resp. calculated values. The deviations at ionic strength < 0.1 M were below +/- 3% and revealed a satisfactory accuracy of the method. As a unique feature it was possible to monitor the disproportionation in a iodide-free iodine solution with an initial concentration of 16% hypoiodic acid (HOI) at pH 7.6. Showing a half-life time of only approximate to 75 s it was confirmed that stability largely depends on HOI concentration.