Iodine-iodide-triiodide equilibrium

Foote and Vance suggested the addition of thiocyanate just before the end point. Cu(I) thiocyanate is less soluble than the iodide, and it is expected that at least the surface layers of the Cul will be transformed into CuSCN, which will have less tendency to adsorb iodine (as triiodide ion). Also, the last of the Cu(II) should produce the thiocyanate rather than the iodide of Cu(I), and the greater insolubility should bring about a more favorable equilibrium near the end point. Less interference from the formation of Cu(II) complexes should be experienced, owing to the more favorable equilibrium. The thiocyanate should not be added until most of the iodine has been reduced by thiosulfate, or appreciable reduction of iodine by thiocyanate may occur. ... 

KAT/GEB] Katzin, L. I., Gebert, E., The iodide-iodine-triiodide equilibrium and ion activity coefficient ratios, J. Am. Chem. Soc., 77, (1955), 5814-5819. Cited on page 441. 

If hydrogen peroxide and iodide are in excess concentrations in the buffered solution, then the iodine rate production is constant. The second step in triiodide formation is the equilibrium conversion of iodine to triiodide given by... 

The sulfur(II) compound peak height increases with the rise in the iodide ion concentration owing to the shift in the iodine-triiodide equilibrium that results in the growing concentration of triiodide ions. 

In the iodine solution, iodine, I2, and iodide, I, ions are in equilibrium with triiodide, as shown below. 

A spectrophotometric study of the reaction of 4-ethoxyphenyl tellurium iodide with iodine in toluene showed that the tellurium iodide is in equilibrium with a tellurium iodide iodine adduct and the aryl tellurium triiodide . 

The thermal effect of the Is forming reaction in an aqueous solution in the presence of alkali metal iodides was evaluated early 60th of the last Century by the colorimetric method [41]. The entropy and enthalpy of the I3 formation were calculated. By means of the Raman scattering the related equUibrium rates of forward and reverse reactions of I3 formation at the temperature of 298 K and ionic strength of 0.02 were evaluated. They amount to 6.2 X 10 M c , and 8.5 X 10 M c , respectively [42]. These data indicate that in the presence of alkali metal ions the equilibrium is shifted towards the formation of the triiodide ion, so that in comparison I3 , molecular iodine and I in aqueous solution could hardly be detected. This was confirmed by the results of quantum-chemical calculations. They revealed that in such a solution the equilibrium is shifted toward the formation of poliiodide anions [38]. 

When iodine in the aqueous layer reacts widi iodide in the aqueous solution, die triiodide ion is formed in equilibrium with die two reactants. If such an aqueous solution is equilibrated with the organic solvent used above, the constant D iqiplies to molecular iodine. Using the determined D and die results of equilibrium mixtures of iodine and KI in the aqueous layer and in CCI4, the equilibrium constant of the above reaction can be determined. 

---