Difference Between True and Apparent Partition Coefficient

The key difference between true and apparent partition coefficient is that true partition coefficient is defined for a unionized system whereas apparent partition coefficient is defined for an ionized system.

The two terms true and apparent partition coefficient are used mainly in pharmaceutical chemistry, regarding drug production. In the case of drug ionization, true partition coefficient gives the distribution of the drug in its unionized state while apparent partition coefficient gives the distribution of the drug in its ionized state.

CONTENTS

1. Overview and Key Difference
2. What is True Partition Coefficient 
3. What is Apparent Partition Coefficient 
4. Side by Side Comparison – True vs Apparent Partition Coefficient in Tabular Form
5. Summary

What is True Partition Coefficient?

True partition coefficient is the ratio of concentrations of un-ionized species of a compound in a mixture of two immiscible phases. Generally, we can denote this phenomenon as “P”. The two different phases should be in equilibrium with each other to determine the partition coefficient of that two-phase system. This ratio represents the measure of the solubility of each un-ionized species in this mixture.

Figure 01: Equilibrium Between Organic Phase and Aqueous (Water) Phase

Normally, the two immiscible phases we consider here are solvents. Most of the times, it is a water-organic solvent system. Therefore, we often consider hydrophilic–hydrophobic systems when determining the partition coefficient. In this determination, the partition coefficient is a measure of lipophilicity or hydrophobicity of the solute that we are interested in. This phenomenon is very important in determining drug distribution throughout our body.

What is Apparent Partition Coefficient?

Apparent partition coefficient is the ratio of concentrations of ionized and unionized species of a compound in a mixture of two immiscible phases. We can denote this as “Papp”. It is dependent on the proportion of substance present in the solution (the amount of substance present in the solution is dependent on the pH of the solution). We can use a correction factor to express the apparent partition coefficient in terms of the true partition coefficient as follows;

Papp = P x funionized

Therefore, if the drug is ionized, then the value of funionized becomes 1 and Papp = P. Let us consider an example in order to understand the concept of apparent partition coefficient. If 100 mg of a drug is added to an immiscible mixture of water and organic solvent that is found to be 40 mg in the organic phase and remaining drug amount (66.7%) in water phase, the mass of the drug in water phase is (100-40) = 60 mg. The mass of the ionized drug in water is (60 x 0.667) = 40 mg. The mass of the unionized drug in water is (60 x 0.33) = 20 mg. Therefore,

• The concentration of the drug in the organic phase (in 50.0 mL of organic solvent) is (40/50) = 0.8 mg/L.
• The concentration of unionized drug in water is (20/50) = 0.4 mg/L.
• Concentration of the total drug in water is (60/50) = 1.2 mg/L.
• The percentage of the drug extracted into the organic phase is (40 mg/ 100 mg) x 100 = 40%.
• The true partition coefficient of the unionized drug is (drug in organic phase / unionized drug in water) = 0.8/0.4 = 2.
• The apparent partition coefficient is (drug in organic phase / total drug in water phase) = (0.8/1.2) = 0.67.

What is the Difference Between True and Apparent Partition Coefficient?

True and apparent partition coefficients describe the distribution of a drug through the system. The key difference between true and apparent partition coefficient is that true partition coefficient is defined for a unionized system whereas the apparent partition coefficient is defined for an ionized system.

Below infographic summarizes the differences between true and apparent partition coefficient.

Summary – True vs Apparent Partition Coefficient

The two terms true and apparent partition coefficient are used mainly in pharmaceutical chemistry, regarding drug production. True and apparent partition coefficients describe the distribution of a drug through the system. The key difference between true and apparent partition coefficient is that true partition coefficient is defined for a unionized system whereas the apparent partition coefficient is defined for an ionized system.

Reference:

1. “Lecture 4 – Physicochemical Properties of Drugs.” Slideshare, 17 July 2010. Available here.

Image Courtesy:

1. “Separatory funnel with oil and colored water” By PRHaney – Own work (CC BY-SA 3.0) via Commons Wikimedia

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