Calculator Input
Use ascending pKa values. For richer curves, try smaller pH steps like 0.25.
Example Data Table
| System | Model | Total concentration (M) | pKa1 | pKa2 | pKa3 | Target pH |
|---|---|---|---|---|---|---|
| Carbonic acid | Diprotic | 0.0100 | 6.35 | 10.33 | — | 7.40 |
| Phosphoric acid | Triprotic | 0.0200 | 2.15 | 7.20 | 12.35 | 6.80 |
| Acetic acid | Monoprotic | 0.0500 | 4.76 | — | — | 5.50 |
Formula Used
General idea: each species fraction is an alpha term, αi, derived from acid dissociation equilibria and normalized by a shared denominator.
Monoprotic system: D = [H+] + Ka1
α0 = [H+]/D, α1 = Ka1/D
Diprotic system: D = [H+]2 + Ka1[H+] + Ka1Ka2
α0 = [H+]2/D, α1 = Ka1[H+]/D, α2 = Ka1Ka2/D
Triprotic system: D = [H+]3 + Ka1[H+]2 + Ka1Ka2[H+] + Ka1Ka2Ka3
Species concentration = αi × CT. Average charge = Σ(charge × αi). The dominant species has the highest α value.
How to Use This Calculator
- Enter a system name so your exports stay clear.
- Select monoprotic, diprotic, or triprotic behavior.
- Type the total analytical concentration in molarity.
- Provide the target pH where you want speciation results.
- Enter pKa values in ascending order.
- Set the pH scan start, end, and step size.
- Press Calculate Speciation to see the result block above the form.
- Use the CSV or PDF buttons to save the distribution table.
FAQs
1. What does this calculator estimate?
It estimates how a weak acid system distributes among protonation states at a chosen pH and across a pH sweep.
2. What is an alpha fraction?
An alpha fraction is the portion of total analyte present as one specific chemical form. All species fractions add to one.
3. Why must pKa values be ascending?
Successive dissociation constants describe later proton losses. Entering them in ascending pKa order preserves the proper equilibrium sequence.
4. Can I use it for phosphoric acid?
Yes. Choose the triprotic option and enter the three pKa values. The tool will return each protonation state across pH.
5. Does ionic strength correction appear here?
No. This version uses thermodynamic-style pKa inputs directly and does not correct for ionic strength or activity coefficients.
6. What does average charge mean?
Average charge is the weighted mean charge of all species at the chosen pH. It helps summarize net protonation behavior.
7. Why does one species dominate near each pKa region?
As pH crosses each pKa, equilibrium shifts toward the next deprotonated form. The dominant species changes accordingly.
8. What step size should I use?
Use 1.00 for quick overviews. Use 0.10 to 0.25 when you need smoother curves or more detailed transition regions.