Distribution Law or Partition Law

Partition Law or Distribution Law

This Law was given by Nernst. This law gives the relationship between the concentration of a given substance in two different phases in equilibrium with each other.

Suppose, if a small quantity of solute soluble in both the liquids is added then solute distributes it self in two liquid & equilibrium is set up. Both the liquids must be immiscible with each other If the concentration of the solute in two liquids are C1 & C2. According to this law,

C1/C2=K (constt.)

k= distribution or partition coefficient

Factors affecting Distribution Coefficient-

1. The value of K depends upon the temperature , nature of solute , nature of two solvents .
2. It does not depend upon the amount of solute or solvents taken.

If the solubilities of given solute in the two solvents at the given temperature are S1 & S2 respectively, then

C1/C2 = S1/S2 =K

Ex.      Water & ether are immiscible with each other. Solute succinic acid is soluble in both of these solvents. If small amount of succinic acid is added in a mixture . Then,

Concentration of succinic acid in water = Cwater

Concentration of succinic acid in ether = Cether

K =  Cwater /  Cether

water  is more than  Cether so C water is taken in the numerator &  C ether in the denominator.

Experimental Verification

For experimental verification of distribution law, different quantity of the solute are added in different  quantities of two solvents in many experiments . If  the value of distribution coefficient for each experiment remains the same. This shows that the data follows distribution law.

Conditions for Distribution Law

1. Both the solvents must be immiscible with each other
2. It is applicable only for dilute solution
3. Temperature remains constant throughout the experiment.
4. The molecular state of the solute in the two solvents should be the same.

Applications of Distribution Law

1. Determination of Solubility The values of K is equal to the ratio of solubilities of the solute in the two solvents.

K =C1/ C2 = S1/ S2
If value of K & solubility of the solute in one solvent is known then we can calculate the solubility of solute in other solvent.

1. Solvent Extraction. Extraction of one substance from a solution containing various substances by using a suitable solvent is known as solvent extraction.

“The amount of extracted substance is more if smaller amount of solvent are used many times rather than using larger amount of solvent a fewer times”.