What is the Henderson-Hasselbalch equation?

It relates the pH of a buffer to the pKa of the weak acid and the ratio of conjugate base to acid: pH = pKa + log([A-]/[HA]). It lets you predict the pH of any buffer or work out the ratio needed to hit a target pH.

What happens when acid and base are equal?

When [A-] equals [HA] the ratio is 1, its logarithm is 0, and the pH equals the pKa exactly. This is the point of maximum buffering capacity, which is why buffers are chosen so their pKa is close to the desired working pH.

What is the useful buffering range?

A buffer works well within about plus or minus one pH unit of its pKa, where the base-to-acid ratio runs from roughly 1:10 to 10:1. Outside that window one species is nearly exhausted and the buffer can no longer resist pH change.

Does this account for ionic strength or temperature?

No. The equation uses concentrations rather than activities and assumes a fixed pKa, so it ignores ionic-strength and temperature effects. For precise work calibrate the final buffer with a pH meter and adjust at the working temperature.

Can I solve for the ratio I need?

Yes, indirectly. Rearranged, the base-to-acid ratio equals 10 raised to the power (pH minus pKa). Adjust the two concentrations until the displayed pH matches your target, and the shown ratio is what you must mix.

What is the Henderson-Hasselbalch Buffer pH Calculator?

Compute the pH of a buffer using the Henderson-Hasselbalch equation, pH = pKa + log([A-]/[HA]), from an entered pKa, weak acid concentration, and conjugate base concentration. For biochemists, pharmacists, and analytical chemists. It runs free in your browser on Gera Tools, with nothing uploaded.

Henderson-Hasselbalch Buffer pH Calculator

Name: Henderson-Hasselbalch Buffer pH Calculator
Creator: Gera Tools
License: https://creativecommons.org/licenses/by/4.0/

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The Henderson-Hasselbalch equation is the workhorse for buffer preparation. Given a weak acid’s pKa and the concentrations of its acid and conjugate-base forms, this calculator returns the buffer pH and tells you whether you are inside the effective buffering window.

How it works

Buffer pH depends on the pKa and the logarithm of the base-to-acid ratio:

pH = pKa + log10( [A-] / [HA] )

where [A-] is the conjugate base concentration and [HA] is the weak acid concentration. When the two are equal the log term is zero and pH equals pKa. Rearranged, the ratio you must mix is [A-]/[HA] = 10^(pH − pKa).

Worked examples with common buffer systems

Acetate buffer at pH 4.76

Acetic acid / sodium acetate has a pKa of 4.76. Mixing 0.1 M acetic acid and 0.1 M sodium acetate gives:

pH = 4.76 + log10(0.1 / 0.1) = 4.76 + 0 = 4.76

Equal concentrations produce a pH exactly equal to the pKa. To shift the buffer to pH 5.06 (half a unit higher), you need a ratio of 10^(5.06 − 4.76) = 10^0.30 ≈ 2, so twice as much acetate as acetic acid.

Phosphate buffer at pH 7.20

Monobasic/dibasic phosphate is the standard physiological buffer. The relevant pKa for the transition between H2PO4⁻ and HPO4²⁻ is 7.20. At equal concentrations of both species, the buffer sits exactly at pH 7.20 — which is why phosphate-buffered saline (PBS) is prepared by combining specified amounts of NaH2PO4 and Na2HPO4.

To target pH 7.4 (blood pH):

[A-]/[HA] = 10^(7.4 − 7.20) = 10^0.20 ≈ 1.58

Use about 1.58 parts dibasic to 1 part monobasic.

Choosing the right buffer system

A buffer resists pH change most effectively within ±1 pH unit of its pKa. Use this as your primary criterion for selection:

Target pH range	Common buffer system	pKa
3.6 – 5.6	Acetic acid / acetate	4.76
5.8 – 7.8	Phosphate (H2PO4⁻ / HPO4²⁻)	7.20
7.2 – 9.2	Tris (tris-HCl / Tris base)	~8.06
6.0 – 8.0	HEPES (biological)	7.48

The pKa values above are for 25 °C at low ionic strength. Temperature and salt concentration shift the effective pKa, sometimes by several tenths of a unit for systems like Tris (roughly −0.03 per °C).

The equation assumes ideal dilute solutions. Real buffers deviate because:

Ionic strength — increases in ionic strength lower the effective pKa for many weak acids, shifting the true pH from the calculated value. The Davies or extended Debye-Hückel correction accounts for this.
Temperature — especially important for Tris-HCl, which shifts about 0.3 pH units when warmed from 4 °C to 37 °C.
Concentration effects — high buffer concentrations amplify these deviations.

Always verify the final pH with a calibrated pH meter, equilibrated at your working temperature, rather than relying solely on the calculated result. This tool gives the ideal-solution prediction, which is the correct starting point for recipe design before experimental adjustment.