Henderson-Hasselbalch Calculator
Find buffer pH from pKa and the base/acid ratio.
Results update as you type.
How it works
The Henderson-Hasselbalch equation is the essential tool for working with buffer solutions, and this calculator applies it. You enter the pKa of the weak acid and the concentrations of the conjugate base and the weak acid, and it returns the buffer's pH and the base-to-acid ratio. The equation states that pH equals the pKa plus the logarithm of the ratio of conjugate base to weak acid. This reveals the elegant behavior of buffers: when the concentrations of the acid and its conjugate base are equal, the ratio is one, its logarithm is zero, and the pH equals the pKa exactly. This is the point of maximum buffering capacity, where the solution best resists pH changes from added acid or base. Adjusting the ratio shifts the pH predictably, allowing chemists and biologists to prepare a buffer at a desired pH by choosing an acid with a suitable pKa and mixing the right proportions. Buffers are vital in biochemistry, where they keep the pH of blood and cellular environments stable, and in laboratory and industrial processes. The equation assumes the buffer is not too dilute and the ratio is within a reasonable range, typically within a factor of ten either way, where it is most accurate. Both concentrations must be positive. Enter the pKa and the two concentrations to compute the buffer pH.
pH = pKa + log₁₀([A⁻] / [HA])
The Henderson-Hasselbalch equation gives the pH of a buffer as the acid's pKa plus the base-10 logarithm of the ratio of conjugate base to weak acid concentration. When the two concentrations are equal, the log term is zero and the pH equals the pKa, the point of maximum buffering capacity.
Source:Acid-base chemistry
We review all our tools from time to time to ensure accuracy.
Worked example
How to use this calculator
- 1
Enter your values
Enter pka of the weak acid, conjugate base [a⁻] and weak acid [ha].
- 2
Adjust the options
Fine-tune any options to match your situation.
- 3
Read your result
Buffer pH updates instantly as you type — no button to press.
Frequently asked questions
What is the Henderson-Hasselbalch equation?
It is an equation that gives the pH of a buffer solution as the pKa of the weak acid plus the base-ten logarithm of the ratio of conjugate base concentration to weak acid concentration. It is fundamental for preparing and understanding buffers, which resist pH change, and is widely used in chemistry, biochemistry, and pharmacology.
Why does pH equal pKa when concentrations are equal?
When the conjugate base and weak acid concentrations are equal, their ratio is one, and the logarithm of one is zero. That makes the log term vanish, so the pH equals the pKa. This is also the point of maximum buffering capacity, where the buffer most effectively neutralizes added acid or base.
How do I make a buffer at a specific pH?
Choose a weak acid whose pKa is close to your target pH, ideally within one unit, then adjust the ratio of conjugate base to acid using the equation. If you need a pH above the pKa, use more conjugate base; below the pKa, use more acid. The calculator lets you test ratios to hit your desired pH.
What is buffering capacity?
Buffering capacity is a buffer's ability to resist changes in pH when acid or base is added. It is greatest when the conjugate base and weak acid concentrations are equal, at pH equal to pKa, and when the total buffer concentration is high. Outside a ratio range of about ten to one either way, the buffer becomes much less effective.
What are the limits of this equation?
The Henderson-Hasselbalch equation assumes the buffer components are present in reasonable concentrations and that the base-to-acid ratio is not extreme, typically between about 0.1 and 10. It also assumes ideal behavior and ignores activity effects. For very dilute solutions, very strong acids or bases, or extreme ratios, more rigorous calculations are needed for accuracy.