# Relationship between partition coefficient and distribution Partition and distribution coefficients. Absorption of drugs. There are a number of ways that drugs are absorbed,1 but the most common route is passive transport. Partition coefficients are defined as the concentration ratio of a chemical between two . is measured by the partition coefficient or distribution coefficient which is the ratio of This ratio is therefore a measure of the difference in solubility of the . The partition coefficient is defined as the ratio of the concentration of a solute in the . distribution of a compound within a biosystem, its interaction with competing binding Relationship between blood−brain barrier uptake and log P for

Although extraction efficiency increases dramatically with the first few multiple extractions, the effect quickly diminishes as we increase the number of extractions Figure 7.

In most cases there is little improvement in extraction efficiency after five or six extractions.

### Liquid–Liquid Extractions - Chemistry LibreTexts

Plot of extraction efficiency versus the number of extractions for the liquid—liquid extraction in Example 7. One approach is to carry out the extraction on a solution containing a known amount of solute. After extracting the solution, we isolate the organic phase and allow it to evaporate, leaving behind the solute. In one such experiment, 1. After extracting with 5. As a result, the distribution ratio does not depend on the composition of the aqueous phase or the organic phase. Scheme for a liquid—liquid extraction of the weak acid, HA. Although the weak acid is soluble in both phases, its conjugate weak base, A—, is soluble only in the aqueous phase.

Distribution Law / Partition Law & Coefficient

The Ka reaction, which is called a secondary equilibrium reaction, affects the extraction efficiency because it controls the relative abundance of HA in solution. If the solute participates in an additional equilibrium reaction within a phase, then the distribution ratio and the partition coefficient may not be the same.

For example, Figure 7. Calculate the extraction efficiency if we extract a Repeat for pH levels of 5. Solution When the pH is 3. The same calculation at a pH of 5. The extraction efficiency in Example 7.

## 7.7: Liquid–Liquid Extractions

A graph of extraction efficiency versus pH is shown in Figure 7. The greatest change in extraction efficiency occurs at pHs where both HA and A— are predominate species. A ladder diagram for HA is superimposed along the x-axis, dividing the pH scale into regions where HA and A— are the predominate aqueous phase species. Click here to review your answer to this exercise.

Unfortunately, many ligands are not very soluble in water or undergo hydrolysis or oxidation in aqueous solutions. For these reasons the ligand is added to the organic solvent instead of the aqueous phase. The ligand initially is present in the organic phase as HL.

For efficient transport, the drug must be hydrophobic enough to partition into the lipid bilayer, but not so hydrophobic, that once it is in the bilayer, it will not partition out again.

Pharmacodynamics[ edit ] In the context of pharmacodynamics what a drug does to the bodythe hydrophobic effect is the major driving force for the binding of drugs to their receptor targets. In some cases the metabolites may be chemically reactive.

### Partition coefficient - Wikipedia

Hence it is advisable to make the drug as hydrophilic as possible while it still retains adequate binding affinity to the therapeutic protein target. Agrochemical research[ edit ] Hydrophobic insecticides and herbicides tend to be more active. Hydrophobic agrochemicals in general have longer half lives and therefore display increased risk of adverse environmental impact.

It is a critical parameter for purification using zone meltingand determines how effectively an impurity can be removed using directional solidificationdescribed by the Scheil equation. The log P of a solute can be determined by correlating its retention time with similar compounds with known log P values.

Moreover, since the value of log P is determined by linear regressionseveral compounds with similar structures must have known log P values, and extrapolation from one chemical class to another—applying a regression equation derived from one chemical class to a second one—may not be reliable, since each chemical classes will have its characteristic regression parameters. The method does, however, require the separate determination of the pKa value s of the substance. Electrochemical[ edit ] Polarized liquid interfaces have been used to examine the thermodynamics and kinetics of the transfer of charged species from one phase to another.

Two main methods exist.

• Partition coefficient
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Here a reaction at a triple interface between a conductive solid, droplets of a redox active liquid phase and an electrolyte solution have been used to determine the energy required to transfer a charged species across the interface. For example, tens of thousands of industrially manufactured chemicals are in common use, but only a small fraction have undergone rigorous toxicological evaluation.

Hence there is a need to prioritize the remainder for testing.