The partition law

You may have used the immiscibility of cyclohexane (or some other organic liquid) with water as way of testing for the presence of iodine or bromine. The method is to shake a little cyclohexane with an aqueous solution in which iodine is thought to be present. (An aqueous solution is one that is made of a substance dissolved in water.) If iodine is there, the colour of the cyclohexane layer changes from clear to purple (figure 3.7). Iodine molecules happen to be much more soluble in cyclohexane than in water, so they collect in the cyclohexane, giving the characteristic colour change.

However, not all the iodine in the lower aqueous layer will dissolve in the cyclohexane. Provided the two layers are left in contact with each other for sufficiently long, and the temperature is kept constant, an equilibrium is set up between iodine molecules entering and leaving the two layers. At equilibrium

rate of iodine molecules = rate of iodine molecules

leaving the aqueous layer = leaving the cyclohexane layer

and then the concentrations of iodine in each layer are constant, and the ratio of the concentrations is constant. Whenever there is an equilibrium in chemistry, there is an associated equilibrium constant. In this case it is called the partition coefficient. We shall give it the symbol, Kpc. In this case we have

To answer this, we need to know that the partition coefficient between tetrachloromethane and water is 85 at 25°C:

Let us say that x grams of iodine go into the tetrachloromethane. This will leave (1 -x) grams of iodine in the aqueous layer. Then we have concentration of iodine in tetrachloromethane

= x/20gcm^-3

concentration of iodine in aqueous solution

= (1 -x)/40gcm^-3

Therefore,

This tells us that we should have 0.9770 g of iodine in the tetrachloromethane, with 0.0230 g of iodine left in the aqueous layer. If we wanted to collect the iodine, we would have to evaporate the tetrachloromethane carefully. [Safety note: This organic liquid is carcinogenic, so don't try thisyourself!]

Partition experiments are extremely important in the pharmaceutical industry and in studies of the environment. The solvent that is widely used is another organic Liquid called octan-1-ol (or sometimes 1-octanol, or just octanol), C₈H₁₇OH. The partition coefficients for a very large number of organic compounds dissolving in it in contact with water have been measured. For example, thousands of tonnes of the compound dichloromethane CH₂Cl₂ (known in industry as methylene chloride) are produced every year. The compound has many uses, one of which is as a paint stripper; unfortunately, it is also a highly persistent and dangerous pollutant - it can cause cancer. One way to determine its concentration in a sample of water is to do a partition experiment with octan-1-ol.

Octan-1-ol is also used to extract molecules from naturally occurring liquids. For example, the milk of nursing mothers hasbeen analyzed to show that breast milk can be contaminate s a result of the mother breathing in fumes of organic liquids such as dichloromethane.

Figure A separating funnel can be used for solvent extraction. The steps are as follows:

1 Add the aqueous and organic liquids to the separating funnel.

2 With the stopper in, shake the two liquids together.

3 Occasionally turn the funnel upside down, making sure that the stopper remains in place.
Open the tap to allow excess vapour to be released. Close the tap.

4 Turn the separating funnel the 'right' way up. Allow the liquids to separate, and run off the lower, aqueous, layer.

5 Keep the organic layer for further treatment.

Note: It is important not to shake the liquids too vigorously, because, if the droplets become too small, they take a long time to separate into layers again.

Safety note: Always wear safety glasses if you do this experiment, and never point the funnel at yourself or another student.

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