Distribution of iodine between two immiscible solvents

In this practical, students observe iodine crystals dissolving in equal volumes of two immiscible solvents, aqueous potassium iodide and cyclohexane, and familiarize themselves with the different colours of the two solutions. To each solution they add the other solvent, and observe the gradual extraction of the iodine from one layer to the other until an equilibrium distribution is reached.

If the quantity of iodine and the volume of each solvent are the same in both tubes, they should be able to observe that the same equilibrium position has been reached in both tubes. Using further volumes of the two solvents, they can demonstrate that it is possible to achieve almost complete extraction from one solvent to the other.

If the teacher is satisfied that the students concerned are able to use cyclohexane and iodine safely, this can be a class experiment taking about 30 minutes, followed up by a discussion of the observations.

Alternatively, it can be done as a demonstration, accompanied by discussion of the observations being made as they happen, which will probably take 30 minutes in all. However, the teacher will need to use larger quantities in larger tubes to ensure clear visibility for the class.

Equipment

Apparatus

• Eye protection
• Test tubes, x2
• Corks to fit test tubes, x2
• Test tube rack
• Dropping pipettes, x2
• Spatula

Chemicals

• Cyclohexane (HIGHLY FLAMMABLE, HARMFUL, DANGEROUS FOR THE ENVIRONMENT), 10 cm³ (see notes 3 and 4 below)
• Iodine crystals (HARMFUL, DANGEROUS FOR THE ENVIRONMENT), a few small crystals (see note 5 below)
• Potassium iodide solution, 0.5 M, 20 cm³

Health, safety and technical notes

• Read our standard health and safety guidance.
• Wear eye protection throughout.
• Cyclohexane, C₆H₁₂(l), (HIGHLY FLAMMABLE, HARMFUL, DANGEROUS FOR THE ENVIRONMENT) – see CLEAPSS Hazcard HC045b. As this is a small-scale qualitative experiment in test tubes, only small quantities are needed. There should be no naked flames anywhere in the laboratory during this practical work. A suitably labelled waste container should be provided to collect mixed aqueous/cyclohexane residues, which can then be separated after the lesson so that cyclohexane residues can be disposed of in an approved manner.
• CLEAPSS suggests Volasil 244 (an organosilicon solvent) as a safer alternative to organic solvents for use in iodine distribution quantitative experiments at post-16 level. However, this substance is expensive, and probably not justified given the small quantities of cyclohexane in use in this qualitative experiment. If you wish to consider this alternative further, you should consult CLEAPSS Hazcard HC106 and also contact CLEAPSS for more details.
• Iodine, I₂(s) (HARMFUL, DANGEROUS FOR THE ENVIRONMENT) – see CLEAPSS Hazcard HC054. Each working group needs a few very small crystals of approximately the same size, provided in a small closed sample tube, from which students can use a spatula to remove a single crystal at a time. It may be advisable to put an iodine crystal into each of the test tubes before the lesson, to avoid problems with spillage iodine of crystals.
• Potassium iodide solution, KI(aq) – see CLEAPSS Hazcard HC047b and CLEAPSS Receipe Book RB072.

Procedure

1. Select two small crystals of iodine of the same size (do NOT touch the crystals – iodine stains skin and clothes), and put one into each of two test tubes.
2. Pour a 2 cm depth of cyclohexane into one test tube and a 2 cm depth of potassium iodide solution into the other. Cork the test tubes and shake until the iodine dissolves. Observe the different colours formed in the two solvents.
3. Take the tube with the cyclohexane solution, and add an equal volume of potassium iodide solution, without shaking.
4. Take the tube with the potassium iodide solution, and add an equal volume of cyclohexane, without shaking.
5. Watch for colour developing in the newly added solvent in each tube.
6. Cork both tubes and shake them gently, watching for changes in colour intensity in each tube.
7. Finally shake both tubes vigorously for 15 seconds, allow the two layers to settle out, then compare the colours in each solvent between both tubes. The situation you should have achieved is called an equilibrium.
8. Use a dropping pipette to remove the bottom layer (coloured potassium iodide solution) from one test tube, then add the same volume of fresh potassium iodide solution to the cyclohexane layer and shake.
9. Repeat the removal of the coloured potassium iodide solution and addition of fresh potassium iodide solution two or three times. What happens to the amount of iodine left in the cyclohexane layer?
10. At the end of the experiment, pour the residues from both tubes into the waste container provided – do NOT pour down the sink.

Teaching notes

The varying colour of iodine in different solvents is an intriguing phenomenon, but for students for whom this experiment is suited, the reasons explained below may well be beyond their level of chemistry.

The solubility of iodine in organic solvents such as cyclohexane produces a purple solution, similar in colour to iodine vapour, in which non-polar iodine molecules have simply been separated from each other in the original crystal structure by non-polar solvent molecules.

Iodine is only slightly soluble in water. Its solubility in aqueous potassium iodide, forming a yellow-brown solution, is due to the formation of a stable tri-iodide ion, I₃⁻. The tri-iodide anion dissociates back to iodine molecules and iodide ions, resulting in the equilibrium:

I₂(aq) + I^–(aq) ⇌ I₃^–(aq)

This is a complication that teachers may well feel lies beyond what the students need to consider in this simple experiment.

The important point for students in this experiment is that they grasp the idea that iodine molecules can move between the two solvents, eventually producing an equilibrium which is the same no matter which direction it is approached from, as achieved in step 8 above.

In practice the small iodine crystals are unlikely to be identical in size, resulting in deeper colours in one tube than the other. The teacher needs to be aware of this, and may need to repeat up to stage 8 with more carefully selected crystals.