BYTETOOLS

Henderson-Hasselbalch Calculator

Work out buffer pH, pKa or the base-to-acid ratio you need, with a built-in pKa table and a warning when the buffer is working outside its useful range.

4.76
pH of the buffer
1
[A⁻] / [HA]
50%
Conjugate base fraction

pH = pKa + log₁₀([A⁻]/[HA])

  • pH 4.76 = pKa 4.76 + log₁₀(1)
  • Ka = 10^−pKa = 1.738e-5 · pKa = −log₁₀(Ka)
  • Ratio 0.1 ÷ 0.1 = 1
  • At pH = pKa the buffer is half acid, half base and its resistance to added acid or base is greatest.

The Henderson-Hasselbalch equation is an approximation. It assumes the acid is weak, that the equilibrium concentrations equal the amounts you weighed out, and that activity coefficients are 1 — reasonable in dilute solution near the pKa, but less so in concentrated or high-salt buffers. pKa values in the list are for 25 °C; Tris in particular shifts by about −0.03 pH units per °C, so a Tris buffer set at room temperature reads noticeably higher in the cold room.

What is the Henderson-Hasselbalch Calculator?

The Henderson-Hasselbalch equation is pH = pKa + log10([A-]/[HA]). It gives the pH of a buffer from the ratio of conjugate base to weak acid, so a buffer with equal amounts of both sits exactly at its pKa.

  • Solves for pH, pKa or the ratio needed to hit a target pH
  • Built-in pKa table covering acetate, phosphate, carbonate, Tris, HEPES and more
  • Accepts molarities or moles in a shared volume
  • Converts between Ka and pKa alongside the answer
  • Warns outside the pKa plus or minus 1 window where buffer capacity collapses
  • Runs locally in your browser with nothing uploaded

How to use the Henderson-Hasselbalch Calculator

  1. 1

    Choose whether you want the pH, the pKa, or the base-to-acid ratio for a target pH.

  2. 2

    Pick a buffer system from the list, or select Custom pKa and type your own value.

  3. 3

    Enter the conjugate base and weak acid amounts as concentrations or as moles in a shared volume.

  4. 4

    Read the answer, the [A-]/[HA] ratio and the percentage of buffer sitting in the base form.

  5. 5

    Watch for the range warning, which appears when the buffer is more than one pH unit from its pKa.

About the Henderson-Hasselbalch Calculator

The ByteTools Henderson-Hasselbalch Calculator solves pH = pKa + log10([A-]/[HA]) for whichever term you are missing. Give it the concentrations of a weak acid and its conjugate base and it returns the pH; give it a measured pH instead and it returns the pKa; or ask it for the base-to-acid ratio that will hold a buffer at a pH you choose.

A pKa list covers the buffers people actually mix — acetate at 4.76, the second phosphate dissociation at 7.21, HEPES at 7.55, Tris at 8.06, ammonium at 9.25 — and any value can be overridden. Amounts can be entered as molarities or as moles sharing one volume, because the volume cancels out of the ratio either way.

Everything runs client-side in your browser, so no lab data leaves your machine. The page also tells you when your target pH sits more than one unit from the pKa, where a buffer has very little capacity left and a different system would work better.

Frequently asked questions

What is the Henderson-Hasselbalch equation used for?

It predicts the pH of a buffer solution from the ratio of conjugate base to weak acid, and works in reverse to tell you what ratio to mix for a pH you want. It is the everyday tool for preparing biological buffers and for exam questions on buffer chemistry.

Why is a buffer strongest when the pH equals the pKa?

At that point the solution holds equal amounts of acid and conjugate base, so it has the most of both forms available to soak up whatever you add. Move away from the pKa and one form runs low, which is why buffers are only useful within about one pH unit of it.

Does the volume of the solution matter?

Not for the pH. The equation depends only on the ratio of base to acid, and dividing both by the same volume leaves that ratio unchanged. Volume does affect buffer capacity, though — a bigger volume of the same buffer resists more added acid or base.

How accurate is the Henderson-Hasselbalch equation?

It is an approximation. It assumes the acid is weak, that equilibrium concentrations equal what you weighed out, and that activity coefficients are 1. That holds well for dilute buffers near the pKa but drifts in concentrated or high-salt solutions.

Why does my Tris buffer read a different pH in the cold room?

Tris has an unusually strong temperature dependence, shifting roughly 0.03 pH units for every degree Celsius. A Tris buffer adjusted to pH 8.0 on the bench will read noticeably higher at 4 degrees, so set the pH at the temperature you intend to use it.

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