Iodine Solubility with KI
Date: January 2009
Why is it that I2(iodine) which is otherwise insoluble in
water dissolves in water when a little amount of KI is added to the water?
Molecular Iodine (I) reacts with the iodide ion (I[-1]) forming the
triatomic ion (I)[-1] which is very water soluble. Iodine (I2) also
reacts with polyvinylpyrrolidone by forming a "sandwich" between adjacent
pyrrolidone rings. This substance is used in place of alcohol solutions of
iodine as an antiseptic, under the trade name "Providone".
Since I2 is a non-polar covalent molecule, it does not ionize in water, nor
can it be soluble in the polar water.
KI, which is a polar, ionic compound, will ionize and dissolve in water. When
KI dissolves in water, it ionizes to K(+) and I(-).
The I(-) will react with I2 to form I3(-). I3(-) being negatively charged will
dissolve in water.
Greg (Roberto Gregorius)
There is a stable I3- ion in water. It has an equilibrium: I2 + I- <--> I3- .
So the amount of iodine that will dissolve is proportional to the amount of KI added.
You will have to look around to learn what is its structure and why it stays bonded
A solution of I2 and KI with matched concentrations is considered KI3 solution.
Often called potassium tri-iodide.
When pretty concentrated and fully saturated with I2,
it is about the mildest chemical solution that will dissolve gold.
So it is useful for making gold conductive patterns on ceramic plates, used for microwave
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Update: June 2012