Calculate logP and the partition coefficient P from organic- and aqueous-phase concentrations, convert logP to P, or find either phase concentration.
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Understanding LogP and the Partition Coefficient
The partition coefficient here compares neutral-solute equilibrium concentrations in an organic phase and water. Conventional logP uses octanol/water; another solvent pair yields that pair’s log partition ratio. The ratio depends on solvent pair, temperature and chemical species. Larger ratios mean higher organic-phase concentration, not necessarily greater total extracted amount or solubility.
| Quantity | What it means | How to read it |
|---|---|---|
| P | Ratio of concentration in the organic phase to concentration in the aqueous phase | P > 1: favors organic phase P = 1: equal concentrations P < 1: favors aqueous phase |
| logP | Base-10 logarithm of P | Positive: more lipophilic Zero: equal phase concentrations Negative: more hydrophilic |
Quick reference values
| logP | P | Interpretation |
|---|---|---|
| -1 | 0.1 | About 10x more concentrated in the aqueous phase |
| 0 | 1 | Equal concentration in both phases |
| 0.5 | 3.16 | Moderate preference for the organic phase |
| 1 | 10 | About 10x more concentrated in the organic phase |
| 2 | 100 | Strong organic-phase preference |
| 3 | 1000 | Very strong organic-phase preference |
How to use this calculator correctly
- Measure the compound concentration in the organic phase after equilibrium.
- Measure the compound concentration in the aqueous phase after equilibrium.
- Enter both values using the same unit system (mg/L with mg/L, g/L with g/L, or mol/L with mol/L).
- The calculator returns both P and logP.
Important: The ratio is only meaningful when both concentrations are expressed on the same basis. If the units do not match, the result is not valid.
Why logP is useful
| Application | Why it matters |
|---|---|
| Drug discovery | Helps assess lipophilicity, membrane permeability, and formulation tradeoffs |
| Liquid-liquid extraction | Shows which phase is likely to retain more of the solute |
| Environmental chemistry | Helps estimate whether a substance prefers water or more hydrophobic environments |
| Analytical chemistry | Useful for designing separations and solvent systems |
Common mistakes to avoid
| Mistake | Why it causes problems |
|---|---|
| Using different units in each phase | The ratio no longer represents a true partition coefficient |
| Using concentrations before equilibrium | P is defined from equilibrium concentrations |
| Entering zero or negative values | logP requires a positive ratio |
| Comparing values from different solvent systems | Partition behavior depends on the specific organic/aqueous pair used |
| Confusing logP with logD | For ionizable compounds, pH can change observed distribution |
LogP vs. logD
| Term | Best used for | Key point |
|---|---|---|
| logP | Neutral-compound partitioning | Represents intrinsic preference between organic and aqueous phases |
| logD | Ionizable compounds at a specific pH | Changes with pH because ionized and neutral forms distribute differently |
If the compound can ionize in water, logD at a stated pH is often more informative than logP alone.
Example calculation
If the concentration in the organic phase is 65 mg/L and the concentration in the aqueous phase is 30 mg/L:
This result indicates the compound is somewhat more concentrated in the organic phase than in the aqueous phase.
Practical interpretation guide
| General range | Typical behavior |
|---|---|
| logP < 0 | Higher concentration in water; the degree depends on the magnitude |
| 0 to 1 | Low to mild lipophilicity |
| 1 to 3 | Moderate lipophilicity |
| 3 to 5 | For octanol/water, a high concentration ratio; this alone does not determine solubility |
| > 5 | Very hydrophobic behavior; interpretation should be made carefully in context |
For best results, use measured equilibrium concentrations, keep the unit basis consistent, and interpret the number alongside solvent choice, pH, and the chemical properties of the compound being tested.
