Buffer pH Calculator
Estimate the pH of an acid or base buffer using the concentrations of a conjugate pair.
01 Set up your calculation
- Select acid or base buffer.
- Enter pKa for the weak acid or pKb for the weak base, at the relevant temperature.
- Enter both concentrations after mixing; base mode also needs pKw.
02 The relationship
For the assumptions behind these results, read the explanation below. You can also explore Henderson–Hasselbalch and pKa.
Follow the numbers
For an acid buffer with pKa 4.76 and equal 0.1 mol/L acid and conjugate-base concentrations, the ratio is one and the estimated pH is 4.76.
03 A conjugate pair controls the estimate
An acid buffer contains a weak acid and its conjugate base; a base buffer contains a weak base and its conjugate acid. The ratio of the two forms shifts the pH relative to the relevant dissociation constant. Equal concentrations place an acid buffer near pKa.
Use concentrations after any neutralization and dilution. If strong acid or base has been added, first account for its stoichiometric reaction with the buffer components. Entering the original stock concentrations would ignore that change.
04 Useful range and model limits
The concentration form of the Henderson–Hasselbalch equation approximates activities with concentrations. It is most useful when appreciable amounts of both partners remain and their ratio is reasonably balanced. The tool flags ratios outside 0.1–10, but does not certify buffer capacity.
High ionic strength, very dilute solutions, multiple overlapping acid-base equilibria, and substantial ionization can require a fuller equilibrium model. Temperature also affects pKa, pKb, and pKw. The familiar pKw value 14 is appropriate near 25 °C, not every temperature.
Questions from the notebook
Is pKa the same as Ka?
No. pKa is −log₁₀(Ka). Enter the logarithmic value in this calculator.
Can I use moles instead of concentrations?
The ratio is equivalent if both amounts occupy the same final volume and no unaccounted reaction changes them.
Does this calculate buffer capacity?
No. pH depends primarily on the ratio, while resistance to added acid or base also depends on total concentration.
References & reading
- OpenStax, Rice University — Buffers ↗
Explains conjugate pairs, buffer behavior, and the Henderson–Hasselbalch relationship.
- OpenStax, Rice University — pH and pOH ↗
Defines the concentration approximation to pH, pOH, and their relationship to water ionization.
- NIST — Guide to SI conversion factors ↗
Supports physical unit conversions. Input values are converted to the formula’s base units before calculation.
Our explanations are written for ChemistryCalc. Read how we build and check our tools. These calculations are educational estimates where the underlying model requires assumptions.