Buffer pH Calculator — Henderson–Hasselbalch Equation
Calculate the pH of any weak acid/conjugate base buffer from pKa and the ratio of component concentrations using the Henderson–Hasselbalch equation. Ideal for preparing lab buffers at a target pH.
mol/L
mol/L
pH = pKa + log₁₀([A⁻] / [HA]) — Henderson–Hasselbalch equation
- 1
[A⁻] / [HA] ratio
0.1 ÷ 0.1 = 1The ratio of conjugate base to weak acid concentration drives the pH shift from pKa. - 2
log₁₀(ratio)
log₁₀(1) = 0 - 3
Buffer pH
4.75 + (0) = 4.75
How does this calculator work?
Henderson–Hasselbalch: pH = pKa + log₁₀([A⁻]/[HA]). When [A⁻]=[HA], pH = pKa. Buffer is effective within ±1 pH unit of pKa. To hit a target pH, set the ratio [A⁻]/[HA] = 10^(pH − pKa). Choose a weak acid whose pKa is close to the desired pH for maximum buffer capacity.
Formula
How this is calculated
The Henderson–Hasselbalch equation pH = pKa + log₁₀([A⁻]/[HA]) relates the pH of a buffer solution to the acid dissociation constant of the weak acid and the ratio of conjugate base [A⁻] to undissociated acid [HA]. When the two concentrations are equal the log term is zero and pH = pKa exactly. Increasing the base-to-acid ratio shifts pH above pKa; a ratio below 1:1 shifts it below.
The equation is derived by taking the negative logarithm of the Ka expression Ka = [A⁻][H⁺]/[HA], rearranging for [H⁺], and applying the definition pH = −log[H⁺]. It assumes that the concentrations of HA and A⁻ are much larger than the change caused by ionisation — the "buffer approximation" — which holds well when concentrations are at least 0.01 mol/L and pH stays within roughly ±1 of pKa.
The percent-ionized figure ([A⁻]/([A⁻] + [HA]) × 100) shows how much of the total buffer is in the deprotonated (base) form at the calculated pH. For precise work at low ionic strength or extreme dilutions, use the full Ka expression accounting for the autoionisation of water.
Frequently asked questions
Pick a weak acid whose pKa is within ±1 of your target pH. Then use the Henderson–Hasselbalch equation rearranged to give the ratio [A⁻]/[HA] = 10^(pH − pKa). Weigh out both components in that molar ratio at the total concentration you need. For example, to make pH 5.0 acetate buffer (pKa 4.75), ratio = 10^0.25 ≈ 1.78, so mix 1.78 parts sodium acetate to 1 part acetic acid.
At pH far from pKa, one component is nearly completely consumed — either [HA] → 0 or [A⁻] → 0 — and the buffer approximation fails. The solution is no longer effectively buffered because there is nothing to neutralise added base or acid. Practically, Henderson–Hasselbalch is reliable when the ratio [A⁻]/[HA] is between 0.1 and 10 (i.e., pH within ±1 of pKa).
pKa is a property of the weak acid itself — it is fixed by the acid's chemistry (−log Ka). pH is a property of the solution — it depends on the concentrations of all species present. A buffer at pH = pKa has equal concentrations of acid and conjugate base, but pH can be tuned above or below pKa by adjusting the ratio.
Also known as
TG we-Calculate Editorial Team. (2026). Buffer pH Calculator — Henderson–Hasselbalch Equation [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/buffer-ph-calculator
TG we-Calculate Editorial Team. "Buffer pH Calculator — Henderson–Hasselbalch Equation." TG we-Calculate. 2026. https://we-calculate.com/calculator/buffer-ph-calculator.
TG we-Calculate Editorial Team, "Buffer pH Calculator — Henderson–Hasselbalch Equation," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/buffer-ph-calculator
@misc{wecalculate_buffer_ph_calculator, title = {Buffer pH Calculator — Henderson–Hasselbalch Equation}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/buffer-ph-calculator}}, year = {2026}, note = {TG we-Calculate} }
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