Distribution Law
Contents of
this chapter
• Definition of distribution law
• Solubility and distribution law
• Explanation of distribution law
• Limitations of distribution law
• Henry’s Law
• Applications of distribution law in pharmaceuticals
Learning
Objectives
• At the end of this lecture, student will be able to
– Define distribution law
– Explain the concept of distribution law
– Compare solubility and distribution law
– Discuss the limitations of distribution law
– Define Henry’s law
– Explain the various applications of distribution law
STATEMENT
OF NERNST’S DISTRIBUTION LAW
Nernst (1891) studied the distribution of several solutes
between different appropriate pairs of solvents. He gave a generalization which governs the
distribution of a solute between two non-miscible solvents. This is called
Nernst’s Distribution law (or Nernst’s Partition law) or simply Distribution
law or Partition law.
Definition:
If two immiscible solvents A and B are taken in a beaker,
they form separate layers. When a solute X which is soluble in both solvents is
added, it gets distributed or partitioned between them. Molecules of X pass
from solvent A to B and from solvent B to A. Finally a dynamic equilibrium is
set up. At equilibrium, the rate, at which molecules of X pass from one solvent
to the other is balanced
Distribution of
solute X between solvent A and B.
Concentration of X in A
= constant
Concentration of X in
B