Distribution of Benzoic Acid between Benzene and Water:
In the aqueous phase, benzoic acid is partially ionized
HBz + H2O ↔ H3O+ + Bz -
In the benzene phase, partial dimerization of benzoic acid takes place
2 HBz ↔ (HBz)2
The three species, HBz, Bz¯, (HBz)2 will have their own KD values
K DHBZ = [HBz]o/[HBz]a
K DBz = [Bz-]o/[Bz - ]a
K D( HNz ) 2 = [(HBz) 2 ]o/ [(HBz) 2 ]a
Subscripts "o" and "a" represent the organic and aqueous phases. It is known that the benzoate ion remains almost completely in the aqueous phase and the benzoic acid dimer in the organic phase. As a chemist, you will usually be interested in knowing where is benzoic acid, whether a part of this acid is ionized or dimerized.
D = Total benzoic in the aqueous phase/Total benzoic in the aqueous phase
D = [HBz]o + [(HBz)2 ]o/ [HBz]a + [Bz-]a
It is evident that D will not remain constant over a range of experimental conditions. Suppose we increase the pH of the aqueous phase, D will be lowered. This is because benzoic acid will be converted to benzoate ion which does not extract in the organic phase.