Question

How do you determine the partion coefficent experimentally for benzoic acid? Please provide an example.

Answer 1

The distribution of a solute in two solvents depending on it's solubility in solvents is called as partition coefficient. It is denoted as K.

The formula for finding K is:

K= concentration of solute in organic phase/ concentration of solute in aqueous phase.

For example let us consider the partition coefficient of benzoic acid in water and benzene.

For this take saturated solution of benzoic acid in a separating funnel and add about 25 mL of water and 25 mL of benzene and shake it vigorously so that it gets distributed in 2 solvents.

Allow it the mixture to settle till 2 layers can be seen clearly. The bottom layer is the aqueous layer and the top layer is the organic layer.

Collect both these layers separately in 2 different conical flasks.

Now using a pipette take 5mL of organic layer into another conical flask containing 10mL water and titrate it using 0.1N NaOH using phenolphthalein as indicator. The end point will be colourless to pink.

Similarly take 5mL of aqueous layer and titrate it against 0.1N NaOH.

By using the volume required to reach point and normality of NaOH, the concentration in organic layer and aqueous layer can be found out.

Thus by substituting this on equation for partition coefficient, K can be found.