Partition Coefficient Explained: Chemistry Basics, Calculations & Examples

In physical science, partition coefficient (P) or distribution coefficient (D) is a ratio of a compound’s concentrations in the mix of two immiscible solvents at the equilibrium. The ratio is the comparison of the solute’s solubilities in the two liquids. However, P is also the concentration ratio of the un-ionized species of compounds. On the contrary, D is the concentration ratio of all the species of compounds (both ionized and unionized).

Understanding How Partition is Different From Distribution Coefficient

So, what is the partition coefficient and how does it differ from the distribution coefficient? Partition coefficient happens to be the ratio of concentrations of the un-ionized species of any compound in the mixture of 2 immiscible stages. This phenomenon is generally denoted as the “P”. And the two phases must be in equilibrium with one another in order to determine the two-phase system’s partition coefficient. The ratio demonstrates the solubility’s measure of every un-ionized species in the mixture.

The aspect that distinguishes partition coefficient from distribution coefficient is that P refers to concentration of the un-ionized chemical species of the compound while distribution coefficient is the concentration of both the ionized as well as un-ionized chemical species of any compound.

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Outlining The Partition Coefficient Formula

The partition coefficient formula,

\[\left ( K_{D}\right ) = \frac{\left [ C \right ]_{s}}{\left [ C \right ]_{m}}\]

Here,

In a stationary phase, the overall concentration of a solute happens to be \[C_{s}\] .

In a mobile phase, the overall concentration of a solute happens to be \[C_{m}\] .

Solving the Different Partition Coefficient Problems

Explained problems are on the basis of the Partition Coefficient Formula. Learn more to get an in-depth insight into the answers.

1st Problem:

The solute’s concentration in a mobile phase happens to be 5.00M. It has 7.00M in its stationary phase. So, find out the partition coefficient \[K_{d}\] between hexane and water.

Answer:

As per what’s given:

\[C_{s}\] is 7.00M and \[C_{m}\] is 5.00M

So, here’s applying the numeric in a corresponding formula as per given below

\[\left ( K_{D}\right ) = \frac{\left [ C \right ]_{s}}{\left [ C \right ]_{m}}\]

Kd = \[\frac{7.00}{5.00}\]

\[K_{d}\] = \[\frac{7.00}{5.00}\]

\[K_{d}\]  = 1.4

2nd Problem:

Let’s think of the aqueous solution of water and chloroform, in which Kd happens to be 6.40. The solute’s concentration in the middle stage is around 0.415M. In the stationary phase, you need to compute the solute’s concentration.

Answer:

As per what’s given:

\[K_{d}\]  is 6.40 and \[C_{m}\]  is 0.415M

So, you need to substitute values in a corresponding formula.

\[\left ( K_{D}\right ) = \frac{\left [ C \right ]_{s}}{\left [ C \right ]_{m}}\]

6.40 = \[\frac{C_{s}}{0.415}\]

6.40 = \[\frac{C_{s}}{0.415}\]

Hence, \[C_{m}\] is 2.656

The Multiple Usages of Partition Coefficient & Distribution Coefficient

Both have an important role to serve via these uses in different industries:

1. Pharmacology

The distribution coefficient of the drug strongly affects its ease to reach the intended target in a body. It also depends on how strong the effect is upon reaching the target and how long it remains inside the body in the active state. Thus, the molecule’s log P is the criterion used in the decision-making by the chemists in a pre-clinical drug discovery.

2. Pharmacokinetics

Here, the distribution coefficient consists of a strong influence on the ADME properties. Thus, the compound’s hydrophobicity is a significant determinant. Additionally, for the drug to orally get absorbed, it must pass through the lipid bilayers in an intestinal epithelium. For an effective transport, it must be enough hydrophobic to partition into a lipid bilayer. But it must not be too hydrophobic. Hydrophobicity has a significant role to play in determining where the drugs get distributed within one’s body upon absorption. As a result, it ensures how they get metabolized & excreted.

3. Pharmacodynamics

The hydrophobic effect happens to be a driving force for binding drugs to the receptor targets. On the contrary, hydrophobic drugs are toxic as they are retained for a long period of time. They have a broader distribution within one’s body and are less selective in the binding to proteins. Finally, they often are extensively metabolized. In rare cases, metabolites might become chemically reactive. So, making the drug more hydrophilic while it retains the adequate binding affinity to therapeutic protein targets is advisable.

4. Agrochemical Surveys

The partition coefficient application and significance does not end in the medical domain. It also serves in the agrochemical science. Hydrophobic insecticides & herbicides are more active. As a matter of fact, hydrophobic agrochemicals have long half-lives. Thus, they display increased chances of adverse environmental impact.

5. Environmental Science

The compound’s hydrophobicity can deliver scientists the indication of how a compound may be taken up in the groundwater for polluting waterways, its toxicity to aquatic life and animals. Partition coefficient predicts a mobility of radionuclides in the groundwater.

6. Metallurgy

Partition coefficient is a significant factor to determine the different impurities and how they get distributed between solidified and molten metal. It’s a crucial parameter for the purification by using zone melting. It demonstrates how effectively the impurity can get removed by using directional solidification.