Distribution Ratio
Distribution ratio is denoted by capital letter D.
• It is the ratio of total concentrations of species in organic phase ( phase 1) to the total concentrations of species in aquous phase.(phase 2) .
• Let us consider an example of benzoic acid distributed between two immiscible solvents ether and water
We know that, Here benzoic acid is a solute and ether is organic solvent and water is aquous solvent.
• kD= [HBz-]e /[ HBz-]a
Where,
• [HBz-]e stands for Concentrations of benzoic acid in the ether layer.
• [ HBz-]a stands for Concentrations of benzoic acid in the water layer.
As we know, In aquous medium Benzoic acid undergo partial ionization.
• HBz ionises as H+ + Bz-
• By applying law of mass actionaction, the ionization constant (Ka) of benzoic acid in aquous medium is written as,
• Ka = [H+]a [ Bz- ]a/ [ HBz]a
Where suffix a - on the right hand side stands for aquous.
• The distribution ratio in the two immiscible solvents
Ether and water is written as,
• D= [ HBz ] e/ [HBz]a+ [Bz-]e
• Where suffix a and e represent
ether and aquous layer respectively.
•Benzoic acid is present inin
Undissociated form -[ HBz]-
and
Dissociated form- [ Bz-] .
• So we can write , total concentration of benzoic acid in all the forms in aquous medium in denominator.
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