Triiodide ion formation equilibrium and activity coefficients in aqueous solution

The equilibrium quotient for the formation of triiodide was studied as a function of temperature, 3.8–209.0°C, and ionic strength, 0.02–6.61. The best-fit value for the molal equilibrium constant at 25°C is 698±10 and the corresponding partial molal enthalphy, entropy, and heat capacity of formation are: ΔHo=−17.0±0.6 kJ-mol−1, ΔSo=−0.6±0.3 J-K−1-mol−1, and ΔCpo=−21±8 J-K−1-mol−1. Activity coefficients of iodine were determined as a function of ionic strength (NaClO4) at 25°C and conclusions are drawn as to the corresponding ionic strength dependence of the triiodide anion.

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