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Henderson-Hasselbalch Calculator

Calculate the pH of a buffer solution from the pKa and the ratio of conjugate base to weak acid concentrations.

Reviewed by Christopher FloiedPublished Updated

This free online henderson-hasselbalch calculator provides instant results with no signup required. All calculations run directly in your browser — your data is never sent to a server. Enter your values below and see results update in real time as you type. Perfect for everyday calculations, homework, or professional use.

Negative log of the acid dissociation constant.

Results

pH

4.76

How to Use This Calculator

1

Enter your input values

Fill in all required input fields for the Henderson-Hasselbalch Calculator. Most fields include unit selectors so you can work in your preferred unit system — metric or imperial, whichever matches your problem.

2

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Double-check that all values are correct and that you have selected the right units for each field. Incorrect units are the most common source of calculation errors and can produce results that are off by factors of 2, 10, or more.

3

Read the results

The Henderson-Hasselbalch Calculator instantly computes the output and displays results with units clearly labeled. All calculations happen in your browser — no loading time and no data sent to a server.

4

Explore parameter sensitivity

Try adjusting individual input values to see how the output changes. This is a quick and effective way to develop intuition about how different parameters influence the result and to identify which inputs have the largest effect.

When to Use This Calculator

  • •Use the Henderson-Hasselbalch Calculator when you need accurate results quickly without the risk of manual computation errors or unit conversion mistakes.
  • •Use it to verify calculations made by hand or in spreadsheets — an independent check can catch errors before they lead to costly decisions.
  • •Use it to explore how changing input parameters affects the output — a quick way to develop intuition and identify the most influential variables.
  • •Use it when collaborating with others to ensure everyone is working from the same numbers and applying the same assumptions.

About Henderson-Hasselbalch Calculator

The Henderson-Hasselbalch calculator determines the pH of a buffer solution from the acid's pKa and the molar ratio of conjugate base to weak acid. Buffer solutions resist pH changes upon addition of small amounts of acid or base, and they are essential in virtually every area of chemistry and biology — from maintaining physiological pH in blood (pH 7.4) to controlling reaction conditions in synthesis. The Henderson-Hasselbalch equation is the most practical tool for designing buffers and predicting their pH. When the concentrations of acid and base are equal, pH = pKa, which is the point of maximum buffering capacity.

The Math Behind It

The Henderson-Hasselbalch equation is derived from the Ka expression for a weak acid HA: Ka = [H⁺][A⁻]/[HA]. Taking the negative log of both sides: pKa = pH − log([A⁻]/[HA]), which rearranges to pH = pKa + log([A⁻]/[HA]). This equation has two key implications: (1) When [A⁻] = [HA], pH = pKa — this is the midpoint of a titration curve and the pH of maximum buffer capacity. (2) The buffer resists pH changes most effectively within ±1 pH unit of the pKa; beyond this range, one component is depleted and buffering capacity drops sharply. To design a buffer, choose an acid with a pKa close to the desired pH, then adjust the ratio of conjugate base to acid. Common biological buffers include acetate (pKa 4.76), phosphate (pKa₂ = 7.20), and Tris (pKa 8.06). The equation assumes that the equilibrium concentrations of HA and A⁻ are approximately equal to the initial (analytical) concentrations, which is valid when Ka is much smaller than the analytical concentrations.

Formula Reference

Henderson-Hasselbalch Equation

pH = pKa + log₁₀([A⁻]/[HA])

Variables: pH = solution pH; pKa = acid dissociation constant; [A⁻] = conjugate base concentration; [HA] = weak acid concentration

Worked Examples

Example 1: Acetate buffer

Prepare a pH 5.0 acetate buffer. pKa = 4.76, using 0.1 M acetic acid and sodium acetate.

Step 1:5.0 = 4.76 + log([A⁻]/[HA]).
Step 2:log([A⁻]/[HA]) = 0.24.
Step 3:[A⁻]/[HA] = 10^0.24 = 1.74.
Step 4:For 0.1 M total: [A⁻] ≈ 0.064 M, [HA] ≈ 0.036 M (adjustable).

A ratio of about 1.74:1 (base:acid) gives pH 5.0.

Common Mistakes & Tips

  • !Choosing a buffer whose pKa is far from the target pH — effectiveness drops rapidly beyond ±1 unit.
  • !Confusing Ka with pKa — the equation uses pKa (the negative log).
  • !Assuming buffer pH never changes — buffers resist change but do not prevent it; large additions of acid/base will overwhelm the buffer.

Related Concepts

pH Calculator

For solutions without buffering, use the direct pH calculation.

Titration

The Henderson-Hasselbalch equation describes the buffer region of a weak acid titration curve.

Used in These Calculators

Calculators that build on or apply the concepts from this page:

pH Calculator

Titration Calculator

Frequently Asked Questions

What is buffer capacity?

Buffer capacity is the amount of acid or base that can be added before the pH changes significantly (usually defined as ±1 pH unit). It increases with higher buffer concentration and is maximized when pH = pKa.

Can I use this for polyprotic acids?

Yes, but apply it one dissociation at a time. For phosphate buffers, use pKa₂ = 7.20 for the H₂PO₄⁻/HPO₄²⁻ pair in the physiological pH range.