What is the Difference Between Partition Coefficient and Distribution Coefficient?

The partition coefficient and distribution coefficient are related concepts in the physical sciences, but they have distinct differences:

• Partition Coefficient: This refers to the ratio of concentrations of un-ionized species of a compound in a mixture of two immiscible phases, usually water and a hydrophobic phase. It is typically defined at concentrations far from saturation and has no units (concentration/concentration). The partition coefficient is given by the formula:

$$P = \frac{[drug]{organic\ phase}}{[drug]{aqueous\ phase}}$$

where $$P$$ is the partition coefficient, and $$[drug]{organic\ phase}$$ and $$[drug]{aqueous\ phase}$$ represent the concentrations of the drug in the organic and aqueous phases, respectively.

• Distribution Coefficient: This refers to the ratio of concentrations of all species of a compound (ionized plus un-ionized) in a mixture of two immiscible phases. It takes into account the concentration of the drug in its molecular form and its ionized form in water. The distribution coefficient is given by the formula:

$$D = \frac{[drug\ molecule]{water} + [drug\ ion]{water}}{[drug\ molecule]{organic\ phase} + [drug\ ion]{organic\ phase}}$$

where $$D$$ is the distribution coefficient, and $$[drug\ molecule]{water}$$, $$[drug\ ion]{water}$$, $$[drug\ molecule]{organic\ phase}$$, and $$[drug\ ion]{organic\ phase}$$ represent the concentrations of the drug in its molecular form and ionized form in water and the organic phase, respectively.

In summary, the key difference between the partition coefficient and distribution coefficient is that the partition coefficient considers only the un-ionized species of a compound, while the distribution coefficient takes into account both ionized and un-ionized species.

Comparative Table: Partition Coefficient vs Distribution Coefficient

The partition coefficient and distribution coefficient are related concepts, but they have some differences depending on the chemical species being considered. Here is a summary of their differences in a table format:

Both partition and distribution coefficients are used in various industries, such as chemical and pharmaceutical sciences, where they are crucial for determining impurity removal in purification processes. Additionally, these coefficients are considered in consumer product development, such as make-up, topical ointments, dyes, and hair colors.