Partition coefficients are a fascinating and important concept in many fields, from chemistry and environmental science to medicine and pharmacology. They tell us about how a substance will distribute itself between two immiscible phases, like how a drug might move between your blood and tissues, or how a pollutant might spread through soil and water. A partition coefficient, denoted as P or log P, describes the ratio of the concentration of a compound in one phase (usually organic) to its concentration in another phase (often water) at equilibrium. Higher values of P indicate a greater preference for the organic phase, meaning the compound is more lipophilic (fat-loving). Lower values of P suggest a higher affinity for the aqueous phase, implying the compound is more hydrophilic (water-loving).

1. PHARMACEUTICS PRACTICAL
“DETERMINATION OF PARTITION
COEFFICIENT”
Presented & Written By: *Rahul Pal, Prachi Pandey.
M. Pharm (Pharmaceutics).
*Department of Pharmaceutics, NIMS Institute of Pharmacy, NIMS
University, Jaipur, Rajasthan, 303121, India.
*Rahul Pal, Prachi Pandey
Master’s of Pharmacy
“Pharmaceutics Practical”
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2. INTRODUCTION
› The movement of molecules from one phase to another phase is
called partitioning.
› The distribution or partition of a solute between immiscible
liquid is known as Nernst’s Distribution Law or simply
distribution law or partition law.
› “Partition coefficients is the measurement of lipophilicity and
hydrophilicity of any unknown sample”.

3. DEFINITION: PARTITION
COEFFICIENTS
› The partition coefficients is defined as “The ration of unionized
drug distributed between organic phase and aqueous phase
at equilibrium”.
OR
“Partition (P) or Distribution Coefficient (D) is the ratio of
concentration of a compound in the two phases of a mixture of two
immiscible solvents at equilibrium”.
Partition Coefficient (P) = X0 / Xw
Where; X0 = Fraction of drug in organic phase.
Xw = Fraction of drug in aqueous phase

4. Equilibrium Constant, K = Co/ Cw
Where; K = Partition coefficients/Distribution
coefficient/Distribution ratio.
Co= Equilibrium conc. of substance in organic phase.
Cw= Equilibrium Con. Of substance in aqueous phase.
General Feature:
- Partition coefficients have no units.
- If the partition coefficients of drug is more than one it is more
lipophilic.
- If the partition coefficients of drug is less than one it is
hydrophilic (less lipophilic).

5. MEASUREMENT OF PARTITION
COEFFICIENT
It can be measured by using the followings methods:
› *Shake flask (or tube) method.
› *HPLC method.
› Electrochemical method.
› Slow-stirring method.
› Estimation method based on individual solubilities.

6. SHAKE FLASK METHOD (SEPARATION
METHOD)
› In this method we take a separating funnel, in which we fill 50ml
oil and 50 ml of water.
› Now transfer the sample drug into the funnel and dissolve drug
by shaking process.
› When drug is dissolved completely, open the stopper which is
below and separate both of the solvents in different flask.
› Now we can determine the conc. of dissolved drug in both of the
solvents by various laboratory method (UV-Spectroscopy).

7. Separating Funnel

8. HPLC METHOD
The HPLC method for calculating partition coefficients offers a faster and more accurate
alternative to traditional shake-flask methods, especially for compounds with high
lipophilicity or instability.
1. Equipment and Materials:
› High-performance liquid chromatograph (HPLC) system with a biphasic column. These
columns contain a stationary phase coated with a lipophilic material like
octanol, allowing partitioning of analytes between the stationary and mobile phases.
› Standard solutions of your compound(s) at known concentrations.
› Mobile phase(s) compatible with the biphasic column and your compound(s). Usually, a
mixture of water and an organic solvent is used.
› Calibration standards for quantifying peak areas.
2. System Preparation:
› Equilibrate the HPLC system with the chosen mobile phase at the desired flow rate.
› Inject a standard solution onto the column and optimize chromatographic conditions
(e.g., retention times, peak shapes) for your compound(s).

9. Sample Analysis:
 Prepare your sample solution containing the compound(s) of interest.
 Inject the sample solution onto the column and record the chromatogram.
 Analyze the peak areas of your compound(s) and calibration standards using
appropriate software.
Calculations:
Calculate the retention factor (k') for your compound(s) using the formula:
k' = (t_R - t_M) / t_M
where:
•t_R is the retention time of your compound.
•t_M is the dead time of the mobile phase (measured by injecting un-retained
marker).
Calculate the distribution constant (Kd) using the formula:
Kd = k' + 1
•Finally, calculate the partition coefficient (log P) using the formula:
log P = log10(Kd)