Wednesday, December 07, 2011

Lab Notes: Elemental Iodine - wrap up

In my previous experiment I extracted elemental iodine from Tincture of Iodides. Now to explore how it happens. I knew that the mixture of hydrogen peroxide and hydrochloric acid caused elemental iodine to precipitate from the solution, but didn't understand the mechanism. Now, after a bit of exploring and research, I understand the reaction mechanism. Starting with what is known:

You can see the potassium is left as a positive ion, as the iodine is no longer combined with it. Also note the Iodine was in the -1 oxidation state, but is now in the 0 oxidation state. This means it has been oxidized, not reduced. So, an oxidizer is needed to move the state up one. Hydrogen peroxide is readily available, and at low concentrations is relatively safe to work with. No oxygen was evolved in the reaction between the hydrogen peroxide and potassium iodide, so what is hydrogen peroxide reduced to?

Water and an oxygen radical? Not quite. I'll get to why in a moment.Since no oxygen is evolved from this reaction, I know that oxygen is not generated, so the hydrogen peroxide must be reduced to water. In order for that to occur, there must be hydrogen ions available to balance the equation. So, this pair of half-reactions is most likely:

Adding an acid allows the hydrogen peroxide to decompose to 2 water molecules. After combining the 2 equations, you obtain this equation:

Which is the mechanism behind the reaction. It is a redox reaction, where the Iodine is oxidized, and the hydrogen peroxide is reduced. In my reaction I used hydrochloric acid, so putting everything together, this is the resulting reactants and products:

As you can see, the result is elemental iodine precipitating out of a solution of potassium chloride.

No comments: