Buffer Solution Calculator
A buffer resists changes in pH, and its pH is set by the acid's pKa and the balance between the weak acid and its conjugate base. The Henderson-Hasselbalch equation captures this exactly for ideal weak-acid buffers. This calculator applies it: enter the pKa of your acid from a reference table, then the concentrations of the conjugate base and the weak acid in the same units. It returns the buffer pH, the base-to-acid ratio, and the pOH. Buffers are most effective within about one pH unit of the pKa, where the ratio stays moderate.
Henderson-Hasselbalch formula
pH = pKa + log10([conjugate base] / [weak acid])
Base / acid ratio = [conjugate base] / [weak acid]
pOH = 14 - pH (at 25 degrees Celsius)
Only the ratio of base to acid matters, so any consistent concentration units work. When the concentrations are equal, the logarithm is zero and the pH equals the pKa, the centre of the buffer's range.
Buffer chemistry context
- The equation assumes a weak acid, ideal (dilute) behaviour, and activity coefficients near one.
- Buffers work best within about one pH unit of the pKa, where the base-to-acid ratio is between roughly 0.1 and 10.
- Acetic acid has a pKa near 4.76; look up the pKa for your specific acid in a reference table.
- The pOH relation pH plus pOH equals 14 holds at 25 degrees Celsius, where the water ion product is 1.0e-14.
- This is an estimate; precise work measures pH directly with a calibrated meter.
Buffer solution: frequently asked questions
What is the Henderson-Hasselbalch equation?
It relates the pH of a buffer to the acid's pKa and the ratio of conjugate base to weak acid: pH equals pKa plus the base-ten logarithm of the conjugate base concentration divided by the weak acid concentration. It is the standard equation for estimating buffer pH in chemistry and biochemistry.
Why must I enter the pKa?
The pKa is specific to the acid in your buffer, for example acetic acid or phosphate, and there are many possible buffers. Rather than assume one, the calculator asks you to enter the pKa from a reference table for your acid so the result matches the system you are preparing.
What is the ratio of base to acid for a buffer?
When the conjugate base and weak acid concentrations are equal, their ratio is 1, the logarithm is zero, and the pH equals the pKa. This is the centre of the buffer's effective range. Buffers work best within about one pH unit of the pKa, where the ratio stays between roughly 0.1 and 10.
Does the Henderson-Hasselbalch equation have limits?
Yes. It assumes the acid is weak, the buffer is reasonably concentrated, and activity coefficients are close to one (ideal behaviour). It becomes less accurate in very dilute or very concentrated solutions, at extreme pH, or where ionic strength effects matter. For most laboratory buffers it is a good estimate.
Can I use molar concentrations or moles?
You can use either, because only the ratio of conjugate base to weak acid appears in the equation. As long as both are expressed in the same units, whether moles per litre or moles, the ratio and therefore the pH are unchanged. Enter both in consistent units.
Official sources
- NIST Chemistry WebBook: Thermochemical and acid dissociation data.
- U.S. National Library of Medicine, PubChem: Compound pKa data.
Reviewed by the CalculatorHub team, edited by James Graham, 16 June 2026. See our methodology.