Measurements of association constants of iodine with cyclodextrins and with iodide or bromide ion in aqueous medium, based on volatilization rate of iodine from aqueous into gaseous phase.

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Iodine molecules, which are volatile, are expelled from an aqueous medium into the gaseous phase by introducing an inert gas at a constant flow rate into the aqueous phase, and the amount of the released iodine is measured as a function of aeration time. In the presence of a nonvolatile solute which associates with molecular iodine, the releasing rate of the volatile solute is reduced with increasing degree of the association between both the solutes in the aqueous medium. The method was applied to the case of association of iodine with α-, β-, or γ-cyclodextrin; the formation constants of the three 1:1 complexes are (1.5±0.1)×104, (1.5±0.2)×102, and 24±3 dm3 mol−1, respectively, at 25.0 °C and low ionic strengths. The complexation of iodine with iodide or bromide ion was also studied; the I3− and I2Br− formation constants are (6.5±0.1)×102 and 16±1 dm3 mol−1, respectively, at 25.0 °C and I=0.1(NaClO4).
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