Henderson-Hasselbalch Equation Calculator
Use the Henderson-Hasselbalch equation to solve for buffer pH, pKa, or one missing acid-base concentration. This is the broader equation page in the buffer cluster, while the direct buffer-pH page stays focused on the most common pH-only workflow.
Edited by Gail Joyce
This calculator page is maintained by the Chemistry Calculators editorial team. The Henderson-Hasselbalch relationship, concentration-ratio handling, and worked examples on this page are reviewed against standard acid-base and buffer reference material before major updates.
Henderson-Hasselbalch Equation Calculator
Enter the values you know and solve one missing Henderson-Hasselbalch quantity. This page is broader than the direct buffer-pH page because it can work backward to pKa or to one missing concentration.
Scope: this page uses the Henderson-Hasselbalch approximation for weak-acid buffer systems. It is not a full speciation solver and it works best when the buffer pair is present in a reasonable concentration range.
Optional helpers
If you know the ratio directly, you do not have to enter both concentrations. If you also know total buffer concentration and volume, the calculator can show a simple recipe split for [A⁻] and [HA].
How to Use the Henderson-Hasselbalch Calculator
Use this page when you need one missing Henderson-Hasselbalch variable rather than just the direct pH of a known buffer.
Enter the values you already know
This page works best when pH, pKa/Ka, and the acid/base concentrations describe the same weak-acid buffer system.
Use ratio input when that is what you know first
If you know the base-to-acid ratio directly, enter that ratio instead of forcing the calculator to infer it from two separate concentrations.
Add total concentration and volume for recipe output
If you know the overall buffer concentration and how much solution you want to prepare, the calculator can split that total into acid and base amounts.
Review the ratio logic
The result should make chemical sense: more conjugate base pushes pH upward relative to pKa, while more weak acid pushes it downward.
Table of Contents
Understanding the Henderson-Hasselbalch Equation
The Henderson-Hasselbalch equation connects pH, pKa, and the ratio of conjugate base to weak acid in a buffer. It is especially useful because it lets you move between direct buffer-pH calculation and the reverse planning steps used in buffer preparation.
What this page is best for
Use it when you need to solve one missing Henderson-Hasselbalch variable, not just the pH of an already-defined buffer.
Important limit
This is still an approximation. It is not a full equilibrium solver and can mislead if the system is too dilute or too far from the buffer range.
Formula and Equation
pH = pKa + log([A⁻]/[HA])
Rearrangements let you solve for pKa, [A⁻], or [HA] when the other values are known.
Worked Examples
Example 1: Solve pH
If pKa = 4.76, [A⁻] = 0.10 M, and [HA] = 0.10 M, then the ratio is 1.
Answer: pH = 4.76 + log(1) = 4.76.
Example 2: Solve pKa
If pH = 5.20, [A⁻] = 0.20 M, and [HA] = 0.10 M, the ratio is 2.
Answer: pKa = 5.20 - log(2) ≈ 4.90.
Common Mistakes
Most Henderson-Hasselbalch mistakes come from invalid ratio inputs or using the approximation outside a true buffer situation.
Mixing incompatible concentration units
The acid and base concentrations must describe the same chemical basis before their ratio is taken.
Using it for strong-acid or strong-base systems
The Henderson-Hasselbalch equation is a weak-acid buffer approximation, not a general strong-acid pH formula.
Ignoring the valid buffer range
The approximation is strongest in real buffer conditions where both species are meaningfully present and pH is near pKa.
Using the direct pH page for reverse solving
If you need pKa or a missing concentration, this broader page is the right one; the direct buffer-pH page is intentionally narrower.
Frequently Asked Questions (FAQs)
What does the equation tell me chemically?
It shows how far pH sits above or below pKa based on the base-to-acid ratio in the buffer pair.
When is pH equal to pKa?
When the conjugate base and weak acid concentrations are equal, the ratio is 1 and pH ≈ pKa.
Can this page solve for pKa?
Yes. That is one of the main reasons to use this page instead of the more direct buffer-pH page.
What if I only need buffer pH from known concentrations?
Use the pH of a Buffer Calculator for the simpler direct workflow.
References
| Resource | Description |
|---|---|
| Chemistry LibreTexts: Henderson-Hasselbalch Approximation | Strong reference for where the equation comes from and when the approximation is appropriate. |
| OpenStax Chemistry 2e | Textbook support for weak acids, conjugate bases, and buffer-equation reasoning. |