To calculate LogP, you determine the partition coefficient (P) of a substance between an organic solvent and water, and then take the base-10 logarithm of that value.
Based on the provided definition:
LogP = log10 (Partition Coefficient)
And the Partition Coefficient is defined as:
Partition Coefficient, P = [organic]/[aqueous]
Here, [ ] indicates the concentration of the solute (the substance you are measuring) in the specified phase:
- [organic]: The concentration of the substance in the organic solvent.
- [aqueous]: The concentration of the substance in the aqueous phase (water).
Understanding the Partition Coefficient (P)
The Partition Coefficient (P) is a ratio that describes how a substance distributes itself between two immiscible or partially miscible solvents. By convention, LogP specifically refers to the partition between an organic solvent (typically octanol, due to its relevance as a model for biological membranes) and water.
A higher concentration in the organic phase compared to the aqueous phase results in a larger P value. Conversely, a higher concentration in the aqueous phase results in a smaller P value (between 0 and 1).
The Calculation Steps in Practice
Calculating LogP typically involves the following conceptual steps:
- Partitioning: Mix a known amount of the substance with a system of two immiscible solvents (e.g., octanol and water). Allow the mixture to equilibrate. The substance will distribute itself between the two phases based on its affinity for each solvent.
- Measurement: Measure the concentration of the substance in both the organic phase ([organic]) and the aqueous phase ([aqueous]).
- Calculate P: Divide the concentration in the organic phase by the concentration in the aqueous phase using the formula P = [organic]/[aqueous].
- Calculate LogP: Take the base-10 logarithm of the calculated P value: LogP = log10(P).
Formulas for LogP Calculation
The core formulas, derived directly from the definition, are:
| Parameter | Formula | Description |
|---|---|---|
| Partition Coeff. P | P = [organic] / [aqueous] | Ratio of concentrations in organic vs. aqueous phases. |
| LogP | LogP = log10(P) | Base-10 logarithm of the Partition Coefficient. |
Simple Calculation Example
Let's consider a hypothetical substance partitioned between octanol (organic) and water (aqueous).
- Suppose the concentration of the substance measured in the octanol phase ([organic]) is 10 units.
- Suppose the concentration of the substance measured in the aqueous phase ([aqueous]) is 1 unit.
-
Calculate P:
P = [organic] / [aqueous]
P = 10 / 1
P = 10 -
Calculate LogP:
LogP = log10(P)
LogP = log10(10)
LogP = 1
In this example, the substance has a LogP of 1, indicating it is 10 times more concentrated in the organic phase than the aqueous phase under these conditions. A higher LogP value suggests greater lipophilicity (fat-liking), while a lower or negative LogP value suggests greater hydrophilicity (water-liking).
LogP is a fundamental physicochemical property used in fields like medicinal chemistry, environmental science, and pharmacology to predict a substance's behavior in different environments and biological systems.
