Extraction

Equipment

250 or 500-mL separatory funnel, four 100-mL beakers, overhead projector, ringstand with iron ring.

Reagents

About 500 mL of water saturated with iodine and about 300 mL of chloroform or cyclohexane.

Presentation

1. Half fill the separatory funnel with the water/iodine solution. Be certain the stopcock is closed first!
2. Dispense ~20 mL of the water/iodine solution into a 100 mL beaker.
3. Place the beaker on the overhead projector, so that it may be seen that some of the light is absorbed by the solution.
4. Add 75-100 mL of chloroform or cyclohexane to the separatory funnel. Stopper the separatory funnel.
5. Hold the stopper in place and invert the separatory funnel 2-3 times fairly quickly.
6. Right the separatory funnel and loosen the stopper to vent some of the pressure that has built up.
7. Replace the stopper, and hold it as you invert several more times. The organic layer should be distinctly purple.
8. Continue to invert until the color of the chloroform layer becomes constant.
9. Drain the chloroform layer into a container.

Repeat steps 2-9 for as many times as you care to. The amounts stated here are for a series of 3 extractions.

Hazards

Vapor of iodine is a severe irritant and lachrymator. Solutions of iodine irritate skin. Contacts may cause skin burns. Chloroform may irritate eyes and skin (with no serious damage). Chloroform is a suspected carcinogen and teratogen. Therefore, avoid contacts with iodine and chloroform; they should be handled with care. Excessive inhalation of cyclohexane is irritating to the upper respiratory tract. Repeated contact with the skin can cause dermatitis.

Discussion

Iodine is a non-polar molecule this has a weak interaction with the hydrogen bonded water molecules. The energy associated with the iodine/ water interactions is not enough to compensate for the lost energy of the water/water interactions. This ultimately means that not much iodine will dissolve in water. If a solvent with weaker solvent/solvent interactions than water were introduced to this system, iodine would find it easier to disrupt these interactions and insert itself (dissolve) between the solvent molecules, cyclohexane or chloroform are such solvents. They have no hydrogen bonding and are only very slightly polar. Energywise, it is more favorable for the iodine to dissolve in the non-polar solvent than in the water, so it exists preferentially (but not totally, an equilibrium will exist) in the non-polar solvent. The release of energy as the iodine switches solvents is partially responsible for the initial build up of pressure in the separatory funnel.

This same argument explains why the non-polar solvent does not dissolve in the water, so two layers are produced when the two solvents are added together.

The transfer of iodine from the water to the chloroform is an equilibrium process. This can be seen in the series of beakers of the iodine/water that were dispensed from the separatory funnel. After each extraction of iodine with chloroform, the amount of iodine remaining in the water became less and less. The series of beakers becomes less and less colored as the iodine responsible for the color is removed.

Why do the extraction in a series of small additions instead of one large addition?

Assume that the iodine distributes itself between the water and chloroform layer in a ratio of 1:9 respectively. This means that one extraction will remove 90% of the original iodine from the water, leaving 10% behind. A second extraction removes 90% of the remaining iodine from the water. In terms of the original amount of iodine that would be:

(0.10)(0.90) = 0.090 or 9%
This leaves 0.10 - 0.090 = 0.010 or 1% of the original iodine in the water and 0.990 or 99.0% of the original iodine is in the two chloroform extractions. This is certainly better than the 90% gained from one extraction.

This could go on and on depending upon your patience and how much chloroform you can lay your hands on. Three extractions would leave only 0.0010 or 0.1% of the original iodine in the water. The process of extractions reaches its ultimate practical limits in a technique known as chromatography. This a technique where the number of extractions has grown huge (100’s of thousands in some cases) and the size of the solvent fractions has shrunk to essentially a shell of a few solvent molecules surrounding a solute molecule (iodine in this case).

References

1. Alyea and Dutton, p. 223.
2. Tutorial Video Tape IX found in Learning Resources Centers in St. Paul Library. Tutorial Video Tape XIII also found in the Learning Resources Centers.