Weak-acid and strong-base titration curves
A titration curve plots solution pH against added titrant volume. For a weak monoprotic acid titrated with strong base, different regions require acid equilibrium, buffer, conjugate-base hydrolysis, or excess-base calculations.
At half equivalence, the acid and conjugate-base concentrations are equal and pH equals pKa in the ideal Henderson–Hasselbalch model. At equivalence, conjugate-base hydrolysis makes pH greater than 7 at 25 °C.
How to use the titration curve calculator
- Enter acid data: Provide positive weak-acid concentration, volume, and Ka.
- Enter titrant concentration: Use the strong-base molarity on the same amount basis.
- Choose a volume: Enter a nonnegative titrant volume whose pH you want to evaluate.
- Generate the curve: Review the selected pH, equivalence landmarks, model scope, and curve.
Formula and variables
The full curve uses a region-appropriate concentration-equilibrium approximation before, at, and after equivalence.
Veq = CaVa/Cb; pHhalf = pKa; pKa = −log₁₀Ka- Ca — Acid concentration
- Initial weak-acid molarity (mol/L)
- Va — Acid volume
- Initial weak-acid volume (mL)
- Ka — Acid dissociation constant
- Equilibrium constant for the weak acid
- Cb — Base concentration
- Strong-base titrant molarity (mol/L)
Acetic-acid model
Titrate 50 mL of 0.1 M weak acid with Ka = 1.74 × 10⁻⁵ using 0.1 M strong base.
- Acid
- 0.1 M, 50 mL
- Base
- 0.1 M
- Ka
- 1.74 × 10⁻⁵
- Veq = 0.1 × 50/0.1 = 50 mL
- At 25 mL, pH = pKa
Result: Equivalence volume is 50 mL and half-equivalence pH is approximately 4.76.
The equivalence pH is above 7 because the conjugate base hydrolyzes water.
Understanding your results
Each curve region uses different chemistry
The buffer equation is not valid at the initial point, equivalence point, or after equivalence.
- Initial pH comes from weak-acid equilibrium.
- Before equivalence, acid and conjugate base form a buffer.
- At equivalence, conjugate-base hydrolysis controls pH.
- After equivalence, excess strong base controls pH.
Assumptions
- The acid is monoprotic and weak; the titrant is a fully dissociated strong base.
- Volumes are additive and temperature is 25 °C with pKw = 14.
- Concentrations approximate activities and the reaction reaches equilibrium.
Limitations
- Does not model polyprotic acids, weak bases, strong-acid curves, mixed solvents, precipitation, activity coefficients, or temperature-dependent pKw.
- Does not select an indicator or calculate measurement uncertainty.
- Approximation error can matter for extremely dilute or unusual systems.
Common mistakes
- Using Ka where pKa is required.
- Assuming every equivalence point has pH 7.
- Applying Henderson–Hasselbalch exactly at equivalence.
- Ignoring dilution by added titrant.
Practical use cases
Chemistry coursework
Visualize and check a standard weak-acid/strong-base titration.
Laboratory planning
Estimate curve landmarks before applying an experimental procedure.
Frequently asked questions
Why is pH equal to pKa at half equivalence?
The ideal acid and conjugate-base amounts are equal, so their ratio in Henderson–Hasselbalch is 1.
Why is equivalence pH above 7?
The conjugate base of the weak acid reacts with water to produce hydroxide.
Sources and review
- Acid-Base Titrations — OpenStax Chemistry: Atoms First 2e. Accessed 2026-07-14.
Reviewed 2026-07-14.