Would water bond with Lugol's solution and travel as a new molecule across membrane?

Question

We did a lab in Biology and our results are supposed to show the diffusion of water into a bag of dialysis tubing placed in a solution of water and 4 mL of Iodine Potassium Iodide, or Lugol's solution. I can tell that the IKI diffused, but how do I tell if the water diffused if we didn't measure it? So maybe the IKI had to bond with water in order to diffuse across the membrane? Or is that wrong? Its the only way I can think of proving that water moved across the membrane.

Answer 1

Iodine is a non-polar molecule and therefore it is not vey soluble in water. In Lugol's solution I(-) (from KI) is also present and reacts with I2 giving the complex I3(-). The latter is negatively charged and thus is soluble in water.

I don't know exactly your experimental set-up but I guess you used I3(-) because of the colour to see that there are molecules travelling through the walls of the dialysis bag. Dialysis itself is based on osmosis so the membrane that makes the bag is permeable to water (and also to small ions, otherwise dialysis could not occur).

Think of it this way: a water molecule is much smaller than an I3(-) and they are both polar. So if I3(-) can move through the pores of the bag water can also do it perfectly well on its own.

However the direction of the movement of water doesn't have to be the same as that of I3(-). Movement of molecules is from solutions of higher to lower concentration, so actually the net movement of water (If you had distilled water in the bag and Lugol's solution on the outside) would be opposite to that of I3(-).

If you don't understand it e-mail me, but also describe the experiment in more detail so that I can be more specific.