Henderson–Hasselbalch Calculator

Find the pH of a buffer from its acid and base forms, or how much of each you need for a target pH. Phosphate, Tris, HEPES and other common buffers are built in.

Calculator

Make or check a buffer

Find

Make a bufferFind the pH

Buffer

pKa

At 25 °C.

Target pH

Buffer concentrationOptional

Both forms together.

Final volumeOptional

Stock concentrationOptional

If you mix two stocks of the same strength.

Enter the target pH to see how much of each form you need.

Worked example

100 mL of 0.1 M phosphate buffer at pH 7.4

01[A⁻] ÷ [HA] = 10^(7.4 − 7.21) = 1.549
02base form = 1.549 ÷ 2.549 = 60.8% of the buffer
03from 0.2 M stocks: 30.38 mL Na₂HPO₄, 19.62 mL NaH₂PO₄, 50 mL water

Mix 30.38 mL of 0.2 M Na₂HPO₄ with 19.62 mL of 0.2 M NaH₂PO₄, add 50 mL of water, then check the pH.

How it works

The reasoning behind it

A buffer is a weak acid (HA) together with its conjugate base (A⁻). Added acid is taken up by A⁻ and added base by HA, so the pH changes little.

The Henderson–Hasselbalch equation links the pH to the ratio of the two forms. When they are equal the log term is zero and the pH equals the pKa. Each tenfold change in the ratio moves the pH by one unit.

To make a buffer, the calculator turns your target pH into that ratio, splits the total concentration between the two forms, and gives the amount of each, or the volumes of two equal-strength stocks to mix.

Questions

Frequently asked

What is the Henderson-Hasselbalch equation?

It gives the pH of a buffer from its pKa and the ratio of its base form to its acid form: pH = pKa + log10([A⁻] ÷ [HA]). It is the equilibrium of a weak acid, rearranged and written in logarithms.

Why doesn't my buffer read the pH I calculated?

The equation leaves out ionic strength and temperature, and published pKa values differ a little. Use the calculated amounts as a starting point and adjust with acid or base while reading a pH meter.

Which buffer should I use?

One with a pKa within about one unit of your target pH that doesn't interfere with your experiment. Phosphate and HEPES suit pH 7 to 7.5; Tris suits about pH 7.5 to 9, but its pH changes with temperature.

How is the equation used for blood pH?

Blood is buffered mainly by bicarbonate. With bicarbonate in mmol/L and carbon dioxide as its partial pressure in mmHg, pH = 6.1 + log10([HCO₃⁻] ÷ (0.03 × pCO₂)). The normal values, 24 mmol/L and 40 mmHg, give pH 7.40.

Lab Tools