# Distribution Law

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