The partition coefficient is the ratio of the concentration of a substance in one phase to the concentration of a substance in the second phase. It is represented by K_pc.

K_pc=C_A/C_B

Where

K_pc= partition coefficient constant

A= one phase

B=Second phase

Some common terms

• Solute

The component which is present in less quantity in solution is called the solute.

• Solvent

The component which is present in large quantities in the solution is called a solvent.

• Solution

A mixture of solute and solvent is called a solution.

Example

Mixing salt in water, salt is solute, water is solvent and it is a solution of salt in water.

The physical significance of partition coefficient

Usually, the water is used as a universal solvent. So the partition coefficient measures how hydrophobic (water-hating) or how hydrophilic (water-loving) the substance is. It depends on the solubility of solute.

Nernst distribution law or Partition Law

This law states that a solute will always be distributed in the same proportions between two immiscible solvents, at constant temperature and pressure.

K_D=/

Where denotes the concentration of solute.

Applications of Nernst Distribution law

Solvent extraction

It is a procedure based on the extraction of non-polar, uncharged species from an aqueous solution into immiscible organic solvent or extraction of polar or ionized species into an aqueous solution from an organic solvent.

A method to separate solute on the basis of its relative solubilities in two different immiscible solvents usually water and organic solvent.

Figure

Consider two immiscible liquids A and B. Mix together in a separating funnel. When solute C is added to them and shaken together, two layers are formed. The solute distributes itself between two layers.

Under equilibrium conditions, the distribution of solute over two phases is determined by distribution law. After extraction, the two phases can be separated because of their immiscibility. A solute is then separated from the extract phase by techniques such as distillation and solvent is regenerated.

The advantages of this type of extraction are; it is simple and convenient, it is rapid, efficient, and applicable to macro and trace levels.

Distribution indicator

Suppose we have two liquids: water and benzene. We have solute iodine which is present in both solvents. Iodine is least soluble in water . if it is present in water then its quantity and existence cannot be easily determined. We had a few drops of organic solvent e.g carbon tetrachloride in the sample of water having iodine. The more quantity of iodine goes into the organic layer and produces intense violet color.

Partition chromatography

Partition chromatography is a modern technique to separate organic materials from each other. For this purpose a paste is prepared. This paste is applied at the top of the column of silica and soaked in water. Organic solvent which is immiscible is allowed to flow to the column. There are two phases i.e stationary(water) and mobile phase(organic solvent).

Each component of the solution distributes itself between these two phases according to their partition coefficient. The component which has more value of distribution coefficient is first to move down as the mobile phase moves. The component which has a lower value of distribution coefficient takes more time to move down. Both these components can be collected separately.

Partition coefficient in column chromatography is

K_pc=C_s/C_m

Where C_s is concentration in the stationary phase and C_m is the concentration in the mobile phase.

Desilverization of lead

When zinc is added to molten lead containing silver, the silver distributes itself between two layers. The large quantity of silver goes to the zinc layer. On the cooling zinc layer, we get silver-zinc alloy. The alloy is distilled in retort and silver is obtained.

Limitations of Distribution Law

The Nernst distribution law is applicable only when

• The temperature remains constant.
• Solutions are dilute.
• The solvents should be non miscible or slightly miscible.
• The molecular state of solute does not change i.e solute does not associate or dissociate.

In the case of association of solute in a solvent where its concentration is C₂ the law is modified as

K_D=C₁/nC₂^1/2

Where n is the number of molecules associated to give a suitable large molecule.

In the case, the solute dissociates then the law is modified as:

K_D=C₁/C₂(1-X)

Where X represents the degree of dissociation of the solute.

Distribution ratio or Partition ratio

The solute exists in different chemical forms due to dissociation (ionization), protonation, complexation, or polymerization, so a more practically useful expression that defines a distribution ratio or partition ratio, D, is

D= (C_s)_organic /(C_s)_aqueous

Where C_s represents the total concentration of all forms of distributing solute in each phase. If no interactions involving solute occur in either phase, then D and K_D would be identical.

Difference between Partition Coefficient and distribution coefficient

The partition coefficient refers to the concentration ratio of un-ionized species of a compound whereas the distribution coefficient refers to the concentration ratio of all species of ionized and unionized.

Concepts Berg

1. How do I calculate the partition coefficient?

Partition coefficient is the ratio of concentration of substance in one phase to the concentration of substance in second phase.

K_pc=C_A/C_B

1. What is partitioning and partition coefficient in chemistry?

When two immiscible liquids are mixed together two layers are formed. When a solute is added it distributes itself between two layers. So partitioning of solute between two liquids occurs.

The partition can be determined by the distribution coefficient or partition coefficient which states the ratio of concentration of solute in one phase to the concentration in the second phase.

1. What is the relation between leaf water potential and cuticle/water partition coefficient?
2. Can you accurately measure the partition coeff of a mixture?
3. What does it mean if the partition coefficient is less than 1?

As partition coefficient is

K_pc= C_organic/C_aqueous

If the partition coefficient is less than one it means the concentration of solute in the aqueous layer is greater than in the organic layer.

1. What is the partition coefficient in extraction?

In solvent extraction, the partition coefficient is the ratio of concentration of solute in the organic phase to the concentration in the aqueous phase.

K_pc= C_organic/C_aqueous

1. Why is the partition coefficient important?

Partition coefficient is the measure of lipophilic character of a drug.

It measures the relative solubility of solute.

1. What is the difference between solubility and partition coefficient?

Solubility is the amount of solute dissolved in a solvent. Solubility measures the amount of solute dissolved in one solvent. Where partition coefficient measures the amount of solute in two immiscible solvents.

1. Is the partition coefficient the same as the distribution coefficient?

The partition coefficient refers to the concentration ratio of un-ionized species of a compound whereas the distribution coefficient refers to the concentration ratio of all species of ionized and unionized.

1. What is the partition coefficient in chromatography?

Partition coefficient in chromatography is

K_pc=C_s/C_m

Where C_s is concentration in the stationary phase and C_m is the concentration in the mobile phase.

1. What is the lipid water partition coefficient of a drug?

It is the ratio of concentration of drug in two immiscible phases i.e organic and aqueous phase.

1. What’s the difference between log_P and log_D?

Log_p is the partition coefficient when a component is present between aqueous and lipophilic(ability to dissolve in lipids) phase.

Log_D is the partition coefficient when a component is present between lipid and aqueous phase.

1. What do we mean by liquid-liquid extraction?

Liquid- liquid extraction or solvent extraction is the process of separating a solute from a solution by adding another solvent in which solute is more soluble.In this way two layers are formed. Solute distributes itself in two solvents on the basis of its solubility in both solvents.

1. What are the available methods to find the coefficient of discharge?

Coefficient of discharge is the ratio of amount of water discharge to the amount theoretically possible at same pressure and same area of discharge. The available methods to find the coefficient of discharge are

• Venturi Meter
• Orificeplate
• Pitot tube
• Rotameter

1. How to determine the solubility of PbI₂?

Solubility of PbI₂ is determined by solubility product. In this case

PbI₂⇋Pb²⁺+2I^–

K_sp=

Where K_sp is a solubility product.

1. How is the distribution coefficient of benzoic acid determined?

Distribution coefficient of benzoic acid in aqueous phase(water) and organic phase(benzene) is determined by formula

K_D=Aqueous/C_organic

Distribution coefficient is the ratio of concentration of benzoic acid in aqueous phase and concentration of benzoic acid in organic phase.

1. What is Octanol-water partition coefficient?

Octanol- water partition coefficient is the ratio of concentration of chemical in octanol phase to its concentration in water phase. If the value of the partition coefficient is greater than 1 it means the chemical is more soluble in octanol than in water.