Question

How do i determine the distribution coefficent of benzoic acid? I used 0.050 grams of benzoic acid and performed extractions 3 times using 2.0 ml of methylene cloride and 2.0 mL of water and got around 0.035 grams after all the extractions done.

also is there a correlation between the values of the distribution coefficent and the polarity of the compound?

thank you

Answer 1

A solute distributes between the two immiscible liquids when a solute is soluble in each of two immiscible liquids. The distribution of solute between the two liquids takes place in such a way that the ratio of its concentration or solubility in the two liquid phases is equal to a constant known as the partition coefficient, Kpc or distribution coefficient, Kd.

Distribution coefficient, K_D = Solubility in organic solvent (g/xmL) Solubility in water (g/xmL)

Initial mass of benzoic acid = 0.050 g

Volume of methylene chloride = 2 mL 3 (extractions) = 6 mL

Volume of water = 2 mL 3 (extractions) = 6 mL

Mass of benzoic acid in methylene chloride = 0.035 g

Mass of benzoic acid in water = 0.050 g - 0.035 g = 0.015 g

Therefore,

K_D = 0.035 g 6 mL 0.015 g 6 mL = 2.3

Yes, there is a correlation between the values of the distribution coefficent and the polarity of the compound.

If K_D < 1, the compound is mostly in water

If K_D = 1, the compound is mostly in both

If K_D > 1, the compound is mostly in organic solvent

i.e. the most polar should be most soluble in water and the least polar should be the most soluble in methylene chloride.

Since K_D = Mass in organic layer Mass in water layer

The most polar should have the least K_D and the least polar should have the highest K_D.