Henderson-Hasselbalch Calculator

The Henderson-Hasselbalch Calculator works out the pH of a buffer solution from the pKa of the weak acid and the ratio of conjugate base [A⁻] to weak acid [HA]. Unlike a one-way formula tool, this solver runs in three directions: find the buffer pH, find the exact base-to-acid ratio (and a mixing recipe) needed to hit a target pH, or back-calculate the pKa from a measured pH. It also shows your buffer on an interactive 0–14 pH scale with its effective buffering window, so you can see at a glance whether the buffer actually works at your pH.

The Henderson-Hasselbalch Equation

The equation links a buffer's pH to the pKa of its weak acid and the ratio of the two buffer species. It is derived from the acid-dissociation equilibrium of a weak acid HA ⇌ H⁺ + A⁻.

Where [A⁻] is the concentration of the conjugate base, [HA] is the concentration of the weak acid, and pKa = −log₁₀(Ka). Because the equation uses a ratio, you can enter molarity, moles, or millimoles for the two species — the units cancel.

Solving in Three Directions

The Effective Buffering Window (pKa ± 1)

A buffer resists pH change best when there is plenty of both the acid and its conjugate base. That happens within about one pH unit of the pKa. Inside this window the base-to-acid ratio stays between 1:10 and 10:1, and the buffer can soak up added acid or base before the pH drifts. Right at pH = pKa the ratio is 1:1 and buffer capacity is at its absolute maximum. Outside pKa ± 2 one species nearly vanishes and the buffer is effectively spent — that is exactly what the highlighted band on the pH scale shows you.

Worked Example

Suppose you mix an acetate buffer with pKa 4.76 using 0.10 M sodium acetate (the base, A⁻) and 0.10 M acetic acid (HA):

1. Ratio = [A⁻]/[HA] = 0.10 / 0.10 = 1.0
2. log₁₀(1.0) = 0
3. pH = 4.76 + 0 = 4.76 — the pH equals the pKa, the strongest possible buffer.

To instead reach pH 5.0 with the same buffer, ratio = 10^(5.0 − 4.76) = 10^0.24 ≈ 1.74, so you would use about 1.74 parts acetate to 1 part acetic acid.

Common Buffer Systems and Their pKa

What Affects Buffer pH?

How to Use This Calculator

1. Choose what to solve for: Find pH, Find ratio for a target pH, or Find pKa.
2. Pick a buffer or enter the pKa: Choose a preset such as acetate or phosphate to auto-fill the pKa, or type your own.
3. Enter your concentrations or target: Provide [A⁻] and [HA], or a target pH, depending on the mode. In ratio mode you can add a total concentration to get a mixing recipe.
4. Click Calculate: Review the result, the buffer's position on the pH scale, the base-vs-acid balance, the buffer-capacity gauge, and the full step-by-step breakdown.

Frequently Asked Questions

What is the Henderson-Hasselbalch equation?

The Henderson-Hasselbalch equation relates the pH of a buffer to the pKa of the weak acid and the ratio of conjugate base to weak acid: pH = pKa + log₁₀([A⁻]/[HA]). It lets you predict a buffer's pH, or work backwards to find the ratio or pKa.

How do I calculate buffer pH from pKa and the acid-base ratio?

Take the base-10 logarithm of the conjugate base concentration divided by the weak acid concentration, then add it to the pKa. For example, with pKa 4.76 and a 1:1 ratio, log₁₀(1) = 0, so the pH equals the pKa of 4.76.

What ratio of base to acid do I need for a target pH?

Rearrange the equation to ratio = 10^(pH − pKa). The base-to-acid ratio equals 10 raised to the difference between your target pH and the pKa. If the target pH is above the pKa you need more conjugate base; if below, more weak acid.

What is the effective buffering range?

A buffer works best within about one pH unit of its pKa (pKa ± 1), where the base-to-acid ratio stays between 1:10 and 10:1. Buffer capacity is greatest exactly at pH = pKa, where the ratio is 1:1.

When does the Henderson-Hasselbalch equation break down?

It assumes the buffer concentrations are much larger than the hydrogen ion concentration and uses concentrations instead of activities. It becomes inaccurate for very dilute buffers, very strong acids or bases, or pH values far from the pKa where one species nearly disappears.

Can I use moles or grams instead of molarity?

Yes. Because the equation uses the ratio of base to acid, the units cancel as long as both are expressed the same way. You can use molarity, moles, or millimoles for [A⁻] and [HA] and get the same pH, since both share the same volume.

Additional Resources

• Henderson-Hasselbalch equation - Wikipedia
• Buffer solution - Wikipedia