Spectrophotometric determination of the formation constant of triiodide ions in aqueous-organic solvent or polymer mixed media both in absence and presence of a surfactant.

The formation constant of triiodide ions from iodine-iodide equilibrium in aqueous-organic solvent or polymer mixed media have been determined spectrophotometrically at three different temperatures 20, 30 and 40°C. The organic solvents chosen for the study are ethylene glycol, 2-methoxy ethanol, and 2-ethoxy ethanol while the polymers include polyethylene glycol (PEG), hydroxypropyl cellulose (HPC) or polyethylene oxide (PEO). Effect of a surfactant on the formation of triiodide ions in the mixed media has also been investigated. Though presence of the organic solvents led to an increase in the triiodide formation except for 2-ethoxy ethanol mixed media at low percentage, the increase in presence of PEG or the other polymer mixed media even at very low percentage was much higher as compared to those in ethylene glycol or its homologues. The increase in the formation constant has been discussed in terms of changes in its solvation properties and the hydrophobic character of the mixed media besides the solvent dielectric effects. The sharp increase in triiodide formation in presence of the polymer under study suggests the possibility of iodine being present as triiodide or other higher polyiodides in the inclusion complexes of iodine with other polymers like starch or PVA. The decrease in the triiodide formation in presence of surfactant micelles may, however, be attributed to solubilization of iodine by the micelles. Iodine was found to be better solubilized in TX-100 micelles as compared to SDS micelles.