Calculator Inputs
Formula Used
General relationship: LogP = LogD - log10(neutral fraction)
Acid: LogP = LogD + log10(1 + 10^(pH - pKa))
Base: LogP = LogD + log10(1 + 10^(pKa - pH))
Neutral: LogP = LogD
The method assumes one dominant ionizable group. It estimates intrinsic neutral partitioning from an observed distribution value.
How To Use This Calculator
- Enter the measured LogD value from your experiment or source.
- Add the pH used during the LogD measurement.
- Enter the relevant pKa for acidic or basic compounds.
- Select acid, base, or neutral as the ionization type.
- Choose decimal places for the displayed result.
- Press calculate and review the result below the header.
- Use CSV or PDF download for reporting.
Example Data Table
| Compound Type | LogD | pH | pKa | Correction | Estimated LogP |
|---|---|---|---|---|---|
| Acid | 1.20 | 7.40 | 4.20 | 3.20 | 4.40 |
| Base | 0.90 | 7.40 | 8.80 | 1.42 | 2.32 |
| Neutral | 2.10 | 7.40 | Not used | 0.00 | 2.10 |
Understanding LogP From LogD
LogP describes the partition of a neutral compound between octanol and water. LogD describes distribution at a chosen pH. It includes neutral and ionized forms. Because ionized molecules usually prefer water, LogD is often lower than LogP. This calculator estimates the neutral partition value by applying an ionization correction.
Why The Correction Matters
A single LogD value can mislead screening work. Two compounds may show the same LogD at pH 7.4, yet have different intrinsic lipophilicity. The difference often comes from pKa and charge type. Acids ionize more as pH rises above pKa. Bases ionize more as pH falls below pKa. Neutral compounds need no correction, so LogP equals LogD.
Inputs Used By The Tool
Enter the measured LogD, the solution pH, and the relevant pKa. Then select acid, base, or neutral. For an acid, the tool uses pH minus pKa. For a base, it uses pKa minus pH. The correction is the logarithm of one plus that power term. The estimated LogP is LogD plus the correction. The neutral fraction is also shown, because it explains how much of the compound was partitioning as neutral material.
Practical Use Cases
Use this calculator during early compound review, formulation checks, environmental screening, or teaching. It is helpful when measured LogD values were produced at different pH conditions. It also supports quick sensitivity checks. Change pH or pKa and watch the correction move.
Interpreting Results Carefully
The estimate assumes a simple monoprotic acid or base. Real molecules may have several pKa values, zwitterionic states, salts, aggregation, or experimental errors. Those cases need deeper models. Still, this method gives a useful first approximation. Use the exported report with notes, units, and example rows to document assumptions clearly.
Model Limits And Assumptions
The calculation works best when the LogD experiment reached equilibrium. The octanol and water phases should be well separated. Temperature, buffer strength, cosolvents, and assay method can change measured values. Always compare results from similar methods. For multiply ionizable compounds, repeat the estimate for the dominant group only as a rough screen. If high accuracy is required, use measured LogP, full species distribution, or validated cheminformatics software. Keep records consistent so exported data remains easy to audit later.
FAQs
What is LogP?
LogP is the logarithm of the partition coefficient for the neutral form of a compound between octanol and water.
What is LogD?
LogD is the distribution coefficient at a specific pH. It includes neutral and ionized forms present in solution.
Why is pKa needed?
pKa helps estimate how much of a compound is neutral or ionized at the selected pH.
Can neutral compounds use this calculator?
Yes. Select neutral as the ionization type. The calculator will return LogP equal to LogD.
Does the acid formula differ from the base formula?
Yes. Acids use pH minus pKa. Bases use pKa minus pH. This reflects opposite ionization behavior.
Is this suitable for several pKa values?
It is best for one dominant ionizable group. Multiple ionization states need a more detailed species model.
Why is neutral fraction shown?
Neutral fraction shows the estimated share of compound present as neutral material. It explains the correction size.
Can I export my calculation?
Yes. After calculation, use the CSV or PDF button to download the result for documentation.