Buffer pH Calculator
Find buffer pH with the Henderson-Hasselbalch equation.
Results update as you type.
How it works
A buffer resists changes in pH when small amounts of acid or base are added, and this calculator finds a buffer's pH using the Henderson-Hasselbalch equation. You enter the pKa of the weak acid and the concentrations of its conjugate base and the acid itself, and it returns the pH. The equation shows that a buffer's pH is centred on the acid's pKa and shifts up or down according to the ratio of conjugate base to acid: when the two are present in equal amounts, the pH equals the pKa exactly, which is the point of maximum buffering capacity. Adding more of the conjugate base raises the pH, while adding more acid lowers it, but only modestly, because the logarithm compresses large ratio changes into small pH changes. This is precisely what makes buffers effective. Buffers are essential in biology, where blood is buffered near pH 7.4, and throughout chemistry, biochemistry, pharmacy, and industry, where reactions and formulations must hold a steady pH. To design a buffer for a target pH, choose a weak acid whose pKa is close to that pH, then adjust the base-to-acid ratio. Enter the pKa and the two concentrations to compute the buffer pH.
pH = pKa + log₁₀([A⁻] / [HA])
The Henderson-Hasselbalch equation gives a buffer's pH as the acid's pKa plus the base-ten logarithm of the ratio of conjugate base to weak acid concentrations. When the two concentrations are equal, the log term is zero and the pH equals the pKa. More conjugate base raises the pH above the pKa, while more acid lowers it, so the ratio fine-tunes the buffer around the pKa value.
Source:Chemistry (Henderson-Hasselbalch equation)
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Worked example
How to use this calculator
- 1
Enter your values
Enter pka of the 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?
The Henderson-Hasselbalch equation calculates the pH of a buffer as the pKa of the weak acid plus the base-ten logarithm of the ratio of conjugate base concentration to weak acid concentration. It provides a quick, reliable way to find or design buffer pH without solving full equilibrium expressions, and it is one of the most used equations in biochemistry and analytical chemistry.
How does a buffer resist pH change?
A buffer contains both a weak acid and its conjugate base. When acid is added, the conjugate base neutralises it; when base is added, the weak acid neutralises it. This dual capacity soaks up added hydrogen or hydroxide ions, so the pH barely moves. The Henderson-Hasselbalch equation shows why: large changes in the base-to-acid ratio produce only small pH shifts.
When does buffer pH equal the pKa?
Buffer pH equals the pKa exactly when the concentrations of the conjugate base and the weak acid are equal, because the logarithm of one is zero. This is also the point of maximum buffering capacity, where the buffer resists pH change most effectively in both directions. Choosing an acid with a pKa near your target pH gives the strongest buffer.
How do I design a buffer for a specific pH?
First select a weak acid whose pKa is within about one unit of your desired pH, since buffers work best near their pKa. Then adjust the ratio of conjugate base to acid using the Henderson-Hasselbalch equation to hit the exact pH. This calculator lets you test ratios quickly: change the concentrations until the reported pH matches your target.
Why is blood pH buffered?
Human blood is buffered near pH 7.4 mainly by the carbonic acid and bicarbonate system, which keeps the pH within a narrow, safe range despite the acids and bases produced by metabolism. Even small deviations can be dangerous, so this buffering is vital. The Henderson-Hasselbalch equation is used clinically to relate blood pH to bicarbonate and carbon dioxide levels.