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Henderson–Hasselbalch Calculator (Buffer pH)

In short: Henderson–Hasselbalch calculator: find buffer pH, pKa, the base/acid ratio, or how much acid and conjugate base to use for a target pH, with presets for common buffers. It is free, works in your web browser and needs no download.

Pick a common buffer or type a pKa. Solve for pH, pKa or the [A⁻]/[HA] ratio, or let the calculator split a total concentration into acid and base amounts, in moles and grams. A weak-base mode handles pOH = pKb + log([BH⁺]/[B]).

Quick answer: pH = pKa + log([A⁻]/[HA]). 0.10 M acetic acid with 0.15 M sodium acetate (pKa 4.76) gives pH 4.94. Equal amounts give pH = pKa. A buffer works well within pKa ± 1.

Common buffer (sets pKa at 25 °C)

Target or measured pH pKa of the weak acid

[A⁻] conjugate base (M or mol) [HA] weak acid (M or mol)

Total buffer concentration [HA] + [A⁻] (M) Volume to make (mL)

Molar mass of acid form (g/mol, optional) Molar mass of base form (g/mol, optional)

pKb of the weak base

[B] weak base (M or mol) [BH⁺] conjugate acid (M or mol)

Henderson–Hasselbalch equation

pH = pKa + log₁₀([A⁻]/[HA]) [A⁻]/[HA] = 10^(pH − pKa) weak base buffer: pOH = pKb + log₁₀([BH⁺]/[B]) pH = 14.00 − pOH (25 °C) making a buffer of total C: [HA] = C / (1 + r) [A⁻] = C·r / (1 + r) r = 10^(pH − pKa)

Because both species are in the same volume, you can use moles instead of concentrations in the ratio. When [A⁻] = [HA], pH = pKa. For a base buffer the matching acid constant is pKa(BH⁺) = 14.00 − pKb, so NH₃/NH₄⁺ with pKb = 4.75 behaves like an acid buffer with pKa = 9.25.

Buffer capacity

A buffer works best within about pKa ± 1, where the base/acid ratio is between 0.1 and 10. Capacity is highest at pH = pKa and grows with the total concentration. The calculator reports β = 2.303·C·Ka[H⁺]/(Ka + [H⁺])², the moles of strong acid or base per litre needed to shift the pH by one unit (water's own contribution is ignored).

Common buffers and pKa values (25 °C)

Buffer	Acid / base pair	pKa	Useful pH range
Acetic acid / acetate	CH₃COOH / CH₃COO⁻	4.76	3.76 to 5.76
Formic acid / formate	HCOOH / HCOO⁻	3.75	2.75 to 4.75
Citric acid (pKa₁)	H₃Cit / H₂Cit⁻	3.13	2.13 to 4.13
Carbonic acid / bicarbonate	H₂CO₃ / HCO₃⁻	6.35	5.35 to 7.35
Phosphate (pKa₂)	H₂PO₄⁻ / HPO₄²⁻	7.21	6.21 to 8.21
HEPES	HEPES / HEPES⁻	7.5	6.5 to 8.5
Tris	TrisH⁺ / Tris	8.07	7.07 to 9.07
Ammonium / ammonia	NH₄⁺ / NH₃	9.25	8.25 to 10.25

pKa values are typical literature values at 25 °C and low ionic strength. Phosphate's other pKa values are about 2.15 and 12.3. Citric acid's are 3.13, 4.76 and 6.40. The carbonic acid value is the apparent pKa that includes dissolved CO₂. Tris changes by about −0.03 pH units per °C, so a Tris buffer made at 25 °C is more acidic at 37 °C.

Worked examples

Acetate buffer pH

0.10 M acetic acid and 0.15 M sodium acetate, pKa = 4.76.

pH = 4.76 + log(0.15/0.10) = 4.76 + 0.176

pH = 4.94

Adding strong acid (JEE/NEET style)

A buffer has 0.10 mol CH₃COOH and 0.10 mol CH₃COO⁻. 0.010 mol HCl is added.

H⁺ converts base to acid: base 0.09 mol, acid 0.11 mol. pH = 4.76 + log(0.09/0.11)

pH = 4.67 (only 0.09 units lower)

Ratio for blood-like phosphate

Target pH 7.40 with H₂PO₄⁻/HPO₄²⁻, pKa = 7.21.

[HPO₄²⁻]/[H₂PO₄⁻] = 10^(7.40 − 7.21) = 10^0.19

Ratio = 1.55 : 1

Make 1.0 L of 0.10 M acetate buffer, pH 5.00

r = 10^(5.00 − 4.76) = 1.738. [HA] = 0.10/2.738 = 0.0365 M, [A⁻] = 0.0635 M.

Sodium acetate: 0.0635 mol × 82.03 g/mol. Acetic acid: 0.0365 mol × 60.05 g/mol.

5.21 g CH₃COONa + 2.19 g CH₃COOH

Basic buffer from pKb

0.20 M NH₃ and 0.10 M NH₄Cl, pKb = 4.75.

pOH = 4.75 + log(0.10/0.20) = 4.45

pH = 14.00 − 4.45 = 9.55

Find pKa from a half-neutralisation

At half-neutralisation of a weak acid, the pH meter reads 3.75.

[A⁻] = [HA], so log(1) = 0.

pKa = 3.75 (formic acid)

Common mistakes

Ratio upside down. It is base over acid: log([A⁻]/[HA]). More base means a higher pH.
pKb in the acid equation. For NH₃/NH₄⁺ either use pOH = pKb + log([BH⁺]/[B]) or convert: pKa = 14 − pKb.
Forgetting the reaction first. When you add strong acid or base, do the stoichiometry (moles) before using the equation.
Using it outside its range. The equation assumes both species are much more concentrated than [H⁺] and [OH⁻]. It fails for very dilute buffers and for ratios far beyond 10 : 1.
Wrong pKa for polyprotic acids. Pick the pKa between the two species you actually have. H₂PO₄⁻/HPO₄²⁻ uses pKa₂ = 7.21, not 2.15.
Temperature. pKa values here are for 25 °C. Tris and amine buffers shift noticeably with temperature.

Drawing acid–base structures? Sketch buffer species, titration schemes and mechanisms in MolDraw, a free online chemical structure editor.

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Frequently asked questions

What is the Henderson–Hasselbalch equation?

pH = pKa + log₁₀([A⁻]/[HA]). It gives the pH of a buffer made from a weak acid HA and its conjugate base A⁻. It comes from rearranging the Ka expression and taking logs.

How do I calculate the ratio of base to acid for a target pH?

Use [A⁻]/[HA] = 10^(pH − pKa). For an acetate buffer at pH 5.00, the ratio is 10^(5.00 − 4.76) = 1.74.

Can I use moles instead of concentrations?

Yes. Both species share the same volume, so the volume cancels in the ratio. This is why you can work in moles after adding a strong acid or base.

What is the effective range of a buffer?

About pKa ± 1 pH unit, where the base/acid ratio is between 0.1 and 10. Choose a buffer whose pKa is close to the pH you need.

How do I use the equation for a basic buffer like NH₃/NH₄Cl?

Use pOH = pKb + log([BH⁺]/[B]) and then pH = 14 − pOH. Or use pKa = 14 − pKb = 9.25 with pH = pKa + log([NH₃]/[NH₄⁺]). Both give the same pH.

Why does my measured buffer pH differ from the calculated value?

The equation uses concentrations, not activities, so ionic strength shifts the real pH by up to about 0.1 to 0.2 units. Temperature also changes pKa. Adjust the final pH with a meter.

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Last updated: October 18, 2026