Partition Coefficient (Log P) Calculator

The partition coefficient (P or K_D) describes how a compound distributes between two immiscible solvents at equilibrium. Log P is the most commonly used form in chemistry and pharmacy.

Definition:

P = [solute]_organic / [solute]_aqueous

log P = log₁₀(P)

For octanol-water system (standard in pharmacology):

• log P < 0: Compound is more hydrophilic (water-loving)
• log P 0–3: Moderate lipophilicity — good oral drug absorption range
• log P > 3: Highly lipophilic (fat-loving) — poor water solubility
• log P > 5: Very lipophilic — poor bioavailability, accumulates in fat

Lipinski’s Rule of Five (drug-likeness): For oral bioavailability, a drug should have:

• MW ≤ 500 g/mol
• log P ≤ 5
• H-bond donors ≤ 5
• H-bond acceptors ≤ 10

Liquid-liquid extraction efficiency:

For a single extraction with volumes V_org and V_aq:

Fraction extracted = P × V_org / (P × V_org + V_aq)

E (%) = 100 × P × V_org / (P × V_org + V_aq)

Multiple extractions: After n extractions, fraction remaining in aqueous phase:

f_remaining = (V_aq / (P × V_org + V_aq))^n

Why multiple small extractions beat one large extraction: By Craig’s theorem, n extractions with volume V/n each are more efficient than one extraction with volume V (for P > 1 systems).

Distribution coefficient (D) at specific pH:

For weak acids/bases, pH affects the ionization and thus the apparent partition:

D = P × f_unionized

For a weak acid (pKa known): D = P / (1 + 10^(pH - pKa))

For a weak base: D = P / (1 + 10^(pKa - pH))

Common log P values: