Buffer Calculator
Calculate buffer pH from pKa and the amounts of weak acid and conjugate base.
Buffer pH
5.24
Base-to-acid ratio: 3.00 : 1
pH is 0.48 above the pKa
- pKa
- 4.76
- Target pH
Within the usual buffering range
Try an example:
How it was calculated
Find the base-to-acid ratio
[A⁻] / [HA] = 0.150 / 0.050 = 3.00
Use the Henderson–Hasselbalch equation
pH = pKa + log₁₀([A⁻]/[HA])
Substitute the values
pH = 4.76 + log₁₀(3.00)
pH = 5.24
What is a buffer?
A buffer is a solution that resists large changes in pH when a small amount of acid or base is added.
A typical acid buffer contains a weak acid and its conjugate base.
The formula
pH = pKa + log₁₀([A⁻]/[HA])
- pH —
- buffer pH
- pKa —
- A measure of the acid's strength
- [A⁻] —
- conjugate-base concentration
- [HA] —
- weak-acid concentration
Why does the ratio matter?
The pH depends on the balance between the weak acid and its conjugate base. When the two are present in equal amounts, the pH equals the pKa.
[A⁻] = [HA], therefore pH = pKa
What is the useful buffer range?
A buffer generally works best when its pH is within about 1 pH unit of its pKa.
pKa − 1 to pKa + 1
Far outside this range, one component becomes much more abundant than the other, so the buffer becomes less effective.
Important note
The Henderson–Hasselbalch equation gives an estimate and assumes the usual buffer conditions. Actual pH can differ because of temperature, concentration, ionic strength, and chemical activity.
Watch out
Common mistakes
Using the wrong ratio
Use conjugate base ÷ weak acid, not the other way around.
Mixing units
If using concentrations, use the same concentration unit for both components.
Using pH instead of pKa
The equation needs the pKa of the weak acid, not the pH of the solution.
Using the wrong acid/base pair
The weak acid and conjugate base must belong to the same buffer system.
Keep going
Related tools
Working with acids and bases?
Need molar amounts?
Need molar mass?
Need to calculate a solution concentration?
Need to dilute a solution?
FAQ
Frequently asked questions
What is a buffer?
A buffer is a solution that resists large changes in pH when small amounts of acid or base are added.
What equation does this calculator use?
It uses the Henderson–Hasselbalch equation: pH = pKa + log₁₀([A⁻]/[HA]).
What happens when the acid and conjugate base are equal?
When [A⁻] = [HA], the ratio is 1, so the pH equals the pKa.
Can I enter moles instead of concentration?
Yes. If both components are in the same solution, their mole ratio can be used directly because the common volume cancels.
What is the best pH for a buffer?
A buffer generally works best when the desired pH is close to its pKa, usually within about 1 pH unit.
Can this calculator prepare a buffer recipe?
No. This calculator gives the pH or component ratio. Preparing a specific laboratory buffer may require additional information such as final volume, stock concentrations, reagent form, purity, and temperature.