Partition coefficients
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Define the term 'partition coefficient' (Kpc).
The partition coefficient (Kpc) is the ratio of the concentration of a solute in two immiscible solvents at equilibrium. It indicates how a solute distributes itself between the two solvents. Kpc = [solute in solvent 1] / [solute in solvent 2]
Write the expression for the partition coefficient (Kpc) of compound X between water and ether.
Kpc = [X]ether / [X]water. Note that the solvent listed first in the question is conventionally placed in the numerator.
A compound Y has a Kpc of 4 between water and chloroform, with chloroform in the numerator. If 10g of Y is dissolved in 100cm³ of water and shaken with 100cm³ of chloroform, what mass of Y will be in the chloroform layer?
Let 'x' be the mass in chloroform. Then (10-x)/0.1 is the concentration in water, and x/0.1 is the concentration in chloroform. Kpc = (x/0.1)/((10-x)/0.1) = 4. Solving for x gives x=8g.
Explain how the polarities of the solute and solvents affect the partition coefficient.
The 'like dissolves like' principle applies. A polar solute will have a higher Kpc value in a polar solvent, while a non-polar solute will have a higher Kpc value in a non-polar solvent, due to stronger intermolecular forces of attraction.
What effect does increasing the polarity of solvent 1 have on the partition coefficient (Kpc) if a polar solute is used?
If solvent 1 becomes more polar, then the solute will likely dissolve more favourably in solvent 1. This will lead to a larger [solute in solvent 1] and therefore a larger Kpc.
State two assumptions that are made when calculating partition coefficients.
1. The solute is in the same physical state (e.g. not associated or dissociated) in both solvents. 2. The two solvents are immiscible/do not mix.
Suggest how you can improve the extraction of a compound from an aqueous solution into an organic solvent using the partition coefficient principle.
Perform multiple extractions using smaller volumes of the organic solvent. This ensures a larger overall proportion of the solute transfers to the organic phase than with a single extraction using the entire volume.
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