Calculator Inputs
Tip: pKa mode is convenient for common acids. Ka mode is useful when textbook tables already provide Ka1 and Ka2.
Formula Used
For a diprotic acid H2A, the two dissociation steps are:
HA- ⇌ H+ + A2-
The calculator uses the exact equilibrium framework instead of a single shortcut approximation:
Ka2 = [H+][A2-] / [HA-]
Species fractions are calculated with:
α0 = [H+]2 / D
α1 = Ka1[H+] / D
α2 = Ka1Ka2 / D
The exact pH comes from solving the charge-balance equation numerically:
How to Use This Calculator
- Enter the total diprotic acid concentration.
- Select the working unit: M, mM, or µM.
- Choose whether you want to enter Ka values or pKa values.
- Fill in the first and second dissociation constants.
- Leave Kw at 1×10-14 for typical 25°C calculations, or change it if needed.
- Choose the number of significant figures for the output.
- Click Calculate pH to display the result block above the form.
- Use the CSV or PDF buttons to save the calculated summary.
Example Data Table
| Acid Example | Total Concentration | pKa1 | pKa2 | Approximate pH | Observation |
|---|---|---|---|---|---|
| Oxalic acid | 0.10 M | 1.25 | 4.27 | 1.284 | Strong first dissociation keeps the solution highly acidic. |
| Malonic acid | 0.10 M | 2.83 | 5.69 | 1.941 | Moderate acidity with a much weaker second step. |
| Carbonic acid | 0.01 M | 6.35 | 10.33 | 4.176 | Weak diprotic behavior produces a noticeably higher pH. |
Frequently Asked Questions
1) Why is a diprotic acid different from a monoprotic acid?
A diprotic acid can donate two protons in separate equilibrium steps. That means its pH depends on both Ka1 and Ka2, not just one dissociation constant.
2) Why must Ka1 usually be larger than Ka2?
Removing the first proton is normally easier than removing the second. After one proton leaves, the remaining species is negatively charged and holds the second proton more strongly.
3) Should I enter Ka values or pKa values?
Use whichever form your source provides. The calculator converts pKa to Ka automatically, so both modes produce the same result when the numbers are equivalent.
4) Does this calculator use an approximation?
It uses an exact numerical charge-balance solution for the chosen constants and concentration. That makes it more reliable than a shortcut formula, especially when both dissociation steps matter.
5) What does the species distribution graph show?
It shows the fractions of H2A, HA-, and A2- across the pH scale. The vertical line indicates your calculated pH so you can identify the dominant form immediately.
6) Can I change Kw?
Yes. The default value suits standard room-temperature calculations, but you can adjust it when your class, lab, or reference uses a different ionic product of water.
7) Why are [H2A], [HA-], and [A2-] all reported?
These concentrations reveal how the acid distributes among its three forms. They are useful for equilibrium analysis, titration interpretation, and understanding which species dominates.
8) When is this calculator most useful?
It is useful for chemistry homework, laboratory preparation, revision, and checking manual calculations involving sulfuric, carbonic, oxalic, malonic, or other diprotic acid systems.