Distribution of iodine between two immiscible solvents

Read our standard health & safety guidance

Lesson organisation

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.

Apparatus and chemicals

For a class practical, each working group will need:

Eye protection

Test-tubes, 2
Corks to fit test-tubes, 2
Test-tube rack
Dropping pipettes, 2
Spatula

Cyclohexane (Highly flammable, Harmful, Dangerous for Environment), 10 cm³ (see note 1)
Iodine crystals (Harmful, Dangerous for Environment), a few small crystals (see note 2)
Potassium iodide, 0.5 mol dm^-3 (Low hazard at this concentration), 20 cm³

Technical notes

Cyclohexane (Highly flammable, Harmful, Dangerous for Environment) Refer to CLEAPSS Hazcard 45B
Iodine (Harmful, Dangerous for Environment) Refer to CLEAPSS Hazcard 54A
Potassium iodide (Low hazard) Refer to CLEAPSS Hazcard 47B and Recipe Card 55

1 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 (see CLEAPSS Hazcard W).

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. However, anyone wishing to consider this alternative further should consult CLEAPSS Hazcard 106 and also contact CLEAPSS for more details.

2 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.

Procedure

HEALTH & SAFETY: Eye protection

a 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.

b 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.

c Take the tube with the cyclohexane solution, and add an equal volume of potassium iodide solution, without shaking.

d Take the tube with the potassium iodide solution, and add an equal volume of cyclohexane, without shaking.

e Watch for colour developing in the newly added solvent in each tube.

f Cork both tubes and shake them gently, watching for changes in colour intensity in each tube.

g 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.

h 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.

i 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?

j 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, to form 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 h 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 h with more carefully selected crystals.

Health & Safety checked, August 2008