Calculator Input
Formula Used
This page uses a descriptor-based educational estimate, then converts the estimated pKa into pH-dependent species fractions.
| Step | Expression | Purpose |
|---|---|---|
| Descriptor estimate | pKa_pred = pKa_ref - ρΣσ + Δres + Δind + ΔHB + Δsolv + cI + α(T - 25) |
Builds a screening estimate from scaffold baseline and user-entered adjustments. |
| Weak acid ionization | Fraction ionized = 1 / (1 + 10^(pKa - pH)) |
Returns the deprotonated fraction for an acidic species. |
| Weak base protonation | Fraction ionized = 1 / (1 + 10^(pH - pKa)) |
Returns the protonated fraction for a basic species. |
| Ratio form | 10^(pH - pKa) |
Provides the Henderson–Hasselbalch species ratio at the selected pH. |
This is useful for screening and teaching. It is not a substitute for validated vendor models, measured literature data, or laboratory titration.
How to Use This Calculator
- Enter a compound name for your report.
- Choose whether the ionizable center behaves as a weak acid or weak base.
- Provide a reference pKa for the parent scaffold or a close analog.
- Add your net substituent sigma sum and sensitivity rho value.
- Apply optional resonance, inductive, hydrogen-bond, solvent, ionic-strength, and temperature adjustments.
- Enter the target pH and an uncertainty spread.
- Submit the form to view the estimate above the form, inspect species fractions, and review the pH curve.
- Download CSV or PDF for documentation and comparison work.
Example Data Table
| Compound | Type | Reference pKa | Σσ | ρ | Δsolv | Target pH | Illustrative Predicted pKa |
|---|---|---|---|---|---|---|---|
| Weak Acid A | Acid | 4.20 | 0.15 | 1.10 | 0.10 | 7.40 | 4.09 |
| Weak Base B | Base | 5.60 | -0.22 | 0.95 | 0.05 | 6.80 | 5.94 |
| Heteroaromatic C | Base | 3.80 | 0.30 | 1.25 | -0.12 | 7.40 | 3.27 |
Frequently Asked Questions
1) What does this tool calculate?
It estimates a screening-level pKa, then shows species fractions, neutral versus ionized percentages, and a pH-dependent curve for the chosen acid or base model.
2) Is this the official Chemicalize engine?
No. This page is an educational standalone estimator inspired by descriptor-based pKa workflows. Use vendor software or laboratory measurements when you need validated production results.
3) Why do I need a reference pKa?
The reference value anchors the model to a known scaffold. Substituent, solvent, ionic strength, and temperature adjustments then shift that baseline estimate.
4) How do Hammett sigma values affect prediction?
Positive sigma values usually represent electron-withdrawing effects and often lower pKa. Negative values represent donating effects and often raise pKa, depending on scaffold sensitivity.
5) Why does ionic strength matter?
Apparent pKa can shift with solution conditions. This calculator lets you test screening scenarios by applying a simple ionic-strength coefficient to the estimate.
6) Can I use this for polyprotic molecules?
Only as a rough first pass. Treat each ionizable center separately or use specialized software for coupled microstates, tautomerism, and multiple experimental pKa values.
7) Why does ionization change with pH?
pH controls the balance between protonated and deprotonated forms. The Henderson–Hasselbalch relationship converts the predicted pKa into species fractions across the pH range.
8) When should I trust experiments instead?
Prefer measured data for formulation, regulatory work, salt selection, complex solvents, metal binding, or any high-stakes decision where small pKa differences materially change outcomes.
Notes
Use this page to compare assumptions quickly, inspect how pH changes ionization, and export a small report for screening discussions.