The Detemination of the Formation Constants of the Triiodide Ion in Water–Alcohol Mixed Solvents at Various Temperatures

The formation constants of the triiodide ion were determined in water-alcohol mixed solvents by a spectro-photometric method at various temperatures. The formation reaction of the triiodide ion from an iodide ion and an iodine molecule is accompanied by a negative enthalpy change and a positive entropy change at various alcohol contents: ΔH°/kJ mol−1, −15.4 (in water) to −14.3 (in methanol) in the water–methanol system, −15.4 (in water) to −21.4 (in ethanol) in the water–ethanol system; ΔSx°/J K−1 mol−1, 36 (in water) to 57 (in methanol) in the water–methanol system, and about 36 at various ethanol contents in the water-ethanol system. The enthalpy term is the dominant factor in the water–ethanol mixtures, whereas the entropy term is the dominant factor in the water–methanol mixtures. In order to explain the thermodynamic behavior, the changes in the solvation properties as well as the activity coefficients of solutes must be considered in both the mixture systems.