Advanced LogD Input Form
Example Data Table
| Case | Type | logP | pH | pKa | Formula Path | Expected Meaning |
|---|---|---|---|---|---|---|
| A | Acid | 2.70 | 7.40 | 4.50 | Acid correction | Ionization lowers logD |
| B | Base | 3.10 | 7.40 | 8.80 | Base correction | Ionized base lowers logD |
| C | Neutral | 1.90 | 7.00 | N/A | logD equals logP | No ionization correction |
| D | Measured | N/A | 7.40 | N/A | Concentration ratio | Uses observed phase values |
Formula Used
General ratio: logD = log10(Corganic / Caqueous)
Monoprotic acid: logD = logP - log10(1 + 10^(pH - pKa))
Monoprotic base: logD = logP - log10(1 + 10^(pKa - pH))
Neutral compound: logD = logP
Ampholyte approximation: logD = logP - log10(1 + 10^(pH - acidic pKa) + 10^(basic pKa - pH))
How to Use This Calculator
- Select formula mode when logP, pH, and pKa are known.
- Select ratio mode when phase concentrations are measured.
- Choose the compound type that best matches the molecule.
- Enter acidic pKa, basic pKa, or both when required.
- Press the calculate button to show the result above the form.
- Use CSV or PDF buttons to save the result.
LogD Formula Calculator Article
What LogD Means
LogD is a practical measure of distribution between organic and aqueous phases. It is often used when a compound can ionize. LogP describes the neutral compound only. LogD changes with pH because ionized species usually prefer water. This makes logD helpful for screening, extraction planning, formulation work, and learning tasks.
Why pH Matters
A weak acid becomes more ionized as pH rises above its pKa. A weak base becomes more ionized as pH falls below its pKa. More ionization usually lowers organic partitioning. The calculator applies this idea with common acid and base equations. It also provides a simple ampholyte approximation for compounds with both acidic and basic centers.
Formula Method
Formula mode starts with logP. It then subtracts an ionization correction. For acids, the correction uses pH minus pKa. For bases, it uses pKa minus pH. Neutral compounds need no correction. This helps estimate the apparent distribution at a chosen pH. The ionization factor also shows how strongly the pH condition affects the result.
Ratio Method
Ratio mode uses measured concentrations. Enter organic and aqueous phase concentrations directly. You may also enter amounts and volumes. The tool then calculates each concentration before finding the ratio. This method is useful when experimental phase data is already available. It can support quick reporting without manual spreadsheet work.
Reading the Output
A negative logD suggests stronger aqueous preference. A moderate value can suggest balanced distribution. A high value suggests stronger organic phase preference. These are broad screening signals. Real systems may also depend on salt form, solvent system, temperature, binding, aggregation, and analytical method. Use the result as a calculation guide, not as final experimental proof.
Good Input Practice
Use consistent units in ratio mode. Both concentrations must share the same basis. If amounts and volumes are used, keep amount units consistent. Check pKa source quality before making decisions. Small pH or pKa changes can move logD strongly near the ionization point. Save results with the export buttons for records, comparisons, and study notes.
Frequently Asked Questions
What is logD?
LogD is the logarithm of a compound distribution ratio between organic and aqueous phases at a specified pH. It includes ionized and unionized forms.
How is logD different from logP?
LogP describes the neutral compound partition only. LogD includes pH effects and ionization, so it can change across different buffer conditions.
Which mode should I use?
Use formula mode when logP, pH, and pKa are known. Use ratio mode when you already measured organic and aqueous concentrations.
What formula is used for acids?
The acid formula is logD = logP - log10(1 + 10^(pH - pKa)). It estimates reduced distribution caused by ionization.
What formula is used for bases?
The base formula is logD = logP - log10(1 + 10^(pKa - pH)). It adjusts logP for ionized base presence.
Can this calculator handle neutral compounds?
Yes. For neutral compounds, the calculator uses logD = logP because there is no pH ionization correction in this simple model.
Can I export the result?
Yes. After calculation, use the CSV or PDF button. The export includes the main result, formula path, and interpretation.
Is the ampholyte option exact?
No. It is a practical approximation using acidic and basic pKa values. Complex molecules may need advanced speciation models.