IODINE 679 



Chemistry 



Iodine is soluble to the extent of 1.33 mM in water at 20°, which is much 

 less than the other halogens. There is interaction with the water, which 

 initially was written as a hydration: 



I, + H,0 ^ I, ■ H^O 



but evidence pointing to the highly polarized state of iodine in the complex 

 has suggested the following reaction: 



I, + H3O t:, 1+ . H,0 + I- 



The equilibrium constant for this reaction has been estimated as roughly 

 lO^^*'. Iodine may also undergo hydrolysis: 



I2 + H2O ^ I- + HOI + H+ 



the equilibrium constant being 3 X 10~^^. The hypoiodous acid formed has 

 a p^^ of 12.3 so is little ionized at physiological pH's. The hypoiodous acid 

 can also go to iodate, especially in alkaline solution: 



3 HOI -> IO3- + 2 I- + 3 H+ 



A third reaction of iodine is with the iodide ion: 



I, + I- ±^ I3- 



to form the triiodide, which is the principal reason for the greater solubility 

 of iodine in KI solutions. The equilibrium is given by: 



(I2) (I-) 



1.38 X 10-3 



(I3-) 



Thus iodine would be soluble in 50 mM KI solution to the extent of 46 mM, 

 an appreciable increase over the 1.33 mM in water. The production of nas- 

 cent oxygen by the reaction: 



I2 + H2O -> 2 H+ + 2 I- + O 



which has been believed to be involved in protein oxidations, does not occur 

 with iodine, although the other halogens react to some extent in this way. 

 In most biological work, iodine is dissolved in fairly strong KI or Nal so- 

 lution. This not only serves to increase the solubility, but limits the fraction 

 of the iodine in other forms (HOI, IO3", and I+'HaO); the principal form 

 here is presumably the Ig" anion. However, there will always be significant 

 concentrations of iodine present. The relative importance of the Ig and Ig" 



