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Relation between kd and distribution ratio derivation and expression
Accepted Answer
The distribution ratio (D) and the partition coefficient (Kd) are related, but not identical. Here's the breakdown:
Partition Coefficient (Kd): This represents the equilibrium constant for the distribution of a solute between two immiscible phases, typically an organic phase and an aqueous phase. It is an intrinsic property of the solute and the two phases. The expression for Kd is:
Kd = [Solute]_organic / [Solute]_aqueous
Distribution Ratio (D): This is a more general term that accounts for all forms of the solute in each phase. It considers not only the free solute but also any complexes or ions the solute may form in each phase. The expression for D is:
D = (Total [Solute]_organic) / (Total [Solute]_aqueous)
Relationship:
If the solute exists solely in its free form in both phases, then D and Kd are equal. This is often the case for simple, non-ionic compounds.
If the solute forms complexes or ions in either or both phases, then D and Kd will differ. D will be higher than Kd because it takes into account all forms of the solute in each phase.
Derivation:
The exact derivation depends on the specific chemical species involved and their interactions with the phases. However, you can generally relate D and Kd by considering the equilibrium constants for the formation of any complexes or ions. For example, if the solute forms a complex with a reagent in the aqueous phase, you would need to consider the formation constant of that complex in the calculation of D.
In summary, D is a more general concept than Kd. Kd represents the equilibrium distribution of the free solute, while D considers all forms of the solute in each phase. The relationship between them depends on the specific chemical species and the formation of complexes or ions.
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