Formula used
Distribution coefficient between an organic and aqueous phase is commonly defined as:
Kd = Corg / Caq
Where Corg is the equilibrium concentration in the organic phase and Caq is the equilibrium concentration in the aqueous phase.
Phase fractions (for a single equilibrium split with volumes) are estimated as:
forg = (Kd·Vorg) / (Kd·Vorg + Vaq) and faq = 1 − forg
pH-aware distribution (logD style) using a neutral partition coefficient P:
- Weak acid:
D = P / (1 + 10^(pH − pKa)) - Weak base:
D = P / (1 + 10^(pKa − pH))
Log form: log10(Kd) or log10(D).
The uncertainty range (measured mode) uses a simple ratio propagation: σ(Kd)/Kd ≈ √(σCorg² + σCaq²) with percent inputs treated as relative errors.
How to use this calculator
- Choose Measured concentrations if you have equilibrium values for both phases.
- Or choose pH-aware estimate if you know
P,pH, andpKa. - Enter phase volumes to estimate the split fractions and extraction percentage.
- Optionally provide an initial amount to compute amounts in each phase.
- Press Calculate. Results appear above the form under the header.
- Use Download CSV or Download PDF to save outputs.
Example data table
| Solute | Corg | Caq | Vorg | Vaq | Kd | Organic % |
|---|---|---|---|---|---|---|
| Acetophenone | 0.18 mol/L | 0.06 mol/L | 25 mL | 25 mL | 3.0 | 75.0% |
| Benzoic acid | 0.10 mol/L | 0.05 mol/L | 20 mL | 40 mL | 2.0 | 50.0% |
| Caffeine | 0.02 mol/L | 0.08 mol/L | 30 mL | 30 mL | 0.25 | 20.0% |
Kd = 0.18 / 0.06 = 3, and with equal volumes, forg = 3/(3+1) = 0.75.
Why distribution coefficients matter in separations
Distribution behavior governs how quickly a compound transfers between immiscible phases during solvent extraction and how it retains on chromatographic systems. A higher coefficient indicates stronger preference for the organic phase, which can improve recovery when extracting nonpolar solutes. In practice, the same solute may show different values as ionic strength, co-solvents, and temperature change. Tracking these shifts supports robust method design and repeatable cleanup performance.
Interpreting Kd, D, and log values for reporting
This calculator reports a ratio-based coefficient and its logarithm to support common laboratory notation. Measured mode uses equilibrium concentrations directly, while the pH-aware mode estimates a distribution ratio that accounts for ionization. Log values simplify comparison across orders of magnitude and are frequently used in method development and regulatory documentation. When reporting, include phase identities, temperature, and mixing conditions to avoid ambiguous comparisons.
Volume ratio effects on extraction yield
Recovery depends on both the coefficient and the phase volumes. Even with a favorable coefficient, a small organic volume can limit the fraction transferred. The fraction equations in the results block show the expected split after a single equilibrium contact, which helps plan solvent volumes, number of stages, and sample loading. For scale-up, keep the same volume ratio to maintain similar partition outcomes.
pH, pKa, and ionization behavior in aqueous systems
Weak acids and bases distribute differently because ionized species usually remain in water. Using pH and pKa, the calculator reduces the neutral partition coefficient to a practical distribution ratio under the stated conditions. This is useful for buffers, biological media, and cleanup workflows where pH control is a key method parameter. Small pH adjustments near pKa can cause large changes in distribution, so control and measure pH carefully.
Good measurement practice and uncertainty checks
Reliable coefficients require true equilibrium, consistent mixing, clean phase separation, and concentration measurements within the linear response range of the analytical method. Optional percent errors provide an approximate coefficient range to flag sensitivity to measurement noise. For critical work, repeat experiments, verify mass balance, and document temperature, shaking time, and phase composition. If the compound adsorbs to glassware or forms complexes, consider control blanks and recovery checks.
FAQs
1) What is the difference between Kd and logD?
Kd is a concentration ratio between phases at equilibrium. logD is the log10 of a distribution ratio that often includes ionization effects, especially when calculated from pH and pKa.
2) Do I need the same concentration units in both phases?
Yes. Any unit is acceptable, but both phases must use the same unit so the ratio is dimensionless. Volume units can be different from concentration units, as long as organic and aqueous volumes match.
3) Why does pH-aware mode require P, pH, and pKa?
P represents partitioning of the neutral form only. pH and pKa estimate the ionized fraction in water, which typically lowers the observed distribution into organic solvent.
4) Can I use this for chromatography retention estimates?
It supports quick screening because distribution tendencies correlate with retention for many systems. However, stationary phase chemistry and mobile phase composition can shift behavior, so confirm with experimental retention data.
5) How should I choose organic and aqueous volumes?
Use the predicted organic fraction to meet your target recovery. If recovery is low, increase organic volume or perform multiple extraction stages while keeping total solvent use and downstream concentration limits in mind.
6) Why do my experimental results differ from predictions?
Differences can come from incomplete equilibrium, emulsions, ion pairing, co-solvents, temperature shifts, or non-ideal activity coefficients. Confirm phase purity, replicate runs, and record buffer composition and mixing time.