Half-Equivalence pH Calculator
At the half-equivalence point of a weak acid titration, half the acid has been converted to its conjugate base, so their concentrations are equal and the pH equals the pKa. This calculator uses the Henderson-Hasselbalch equation to return the pH for any buffer composition you enter. Provide the acid dissociation constant (Ka) and the concentrations of conjugate base and acid; the tool computes pKa and the resulting pH. Set the two concentrations equal to recover the classic half-equivalence identity, pH equals pKa.
Henderson-Hasselbalch formula
pKa = -log10(Ka)
pH = pKa + log10([A-] / [HA])
at half-equivalence [A-] = [HA]
so pH = pKa + log10(1) = pKa
For acetic acid, Ka is about 1.8e-5, giving pKa of about 4.74. At equal acid and conjugate base concentrations, the pH equals that pKa.
Buffer and titration facts
- The half-equivalence point is where exactly half the weak acid has been titrated.
- At that point [HA] equals [A minus], so pH equals pKa.
- A buffer resists pH change most strongly near its pKa.
- A smaller pKa indicates a stronger acid.
- The equation assumes ideal behaviour and comparable acid and base amounts.
Half-equivalence pH: frequently asked questions
Why does pH equal pKa at the half-equivalence point?
At the half-equivalence point of a weak acid titration, exactly half the acid has been neutralised, so the concentration of the acid (HA) equals the concentration of its conjugate base (A minus). The Henderson-Hasselbalch equation, pH = pKa + log10([A minus]/[HA]), then reduces to pH = pKa because log10(1) is 0.
What is the Henderson-Hasselbalch equation?
It relates the pH of a buffer to the acid dissociation constant and the ratio of conjugate base to acid: pH = pKa + log10([A minus] / [HA]). It is valid when both the acid and its conjugate base are present in appreciable, comparable amounts.
How is pKa related to Ka?
pKa is the negative base-ten logarithm of the acid dissociation constant: pKa = -log10(Ka). A smaller pKa means a stronger acid. This calculator can take either Ka or a known pKa via the ratio it computes from your inputs.
Does this work for any buffer ratio, not just half-equivalence?
Yes. Enter the conjugate base concentration and the acid concentration to compute the pH at any buffer composition. When you set them equal, you recover the half-equivalence result where pH equals pKa.
What are the assumptions and limits?
The Henderson-Hasselbalch equation assumes ideal behaviour, activity coefficients of 1, and that the approximation of using formal concentrations holds. It is least accurate for very dilute solutions or for very strong or very weak acids where the equilibrium assumptions break down.
Official sources
Reviewed by the CalculatorHub team, edited by James Graham, 16 June 2026. See our methodology.