Henderson-Hasselbalch Calculator
Toggle between calculating buffer pH from concentrations or determining the base amount needed to reach a specific pH.
Buffer pH
7.2
[base]/[acid]
1
How to Use This Calculator
Choose calculation mode
Compute pH from known concentrations or determine the base amount required for a target pH.
Enter pKa
Use the dissociation constant of the buffering acid at your working temperature.
Provide concentrations
Supply current acid/base concentrations or the acid concentration plus target pH as required.
Apply the result
Use the resulting pH or required base-to-acid ratio to formulate or adjust your buffer.
Formula
pH = pKa + log₁₀([base]/[acid])
[base]/[acid] = 10^(pH - pKa)
The classic Henderson-Hasselbalch form relates buffer pH to the ratio of conjugate base to acid. Rearranging yields the ratio needed for a target pH.
Example
For a phosphate buffer (pKa = 7.2) targeting pH 7.4, ratio = 10^(7.4 − 7.2) ≈ 1.58, meaning base concentration should be 1.58 times the acid concentration.
Full Description
The Henderson-Hasselbalch equation provides a fast way to design and analyze buffer solutions. It connects pH, pKa, and the ratio of conjugate base to acid in a simple logarithmic relationship.
This calculator supports two common tasks: predicting pH from buffer composition and determining how much base relative to acid is required to reach a desired pH. Both are staples of buffer preparation in labs.
Remember that the approximation assumes the acid and base concentrations greatly exceed any added strong acid or base and that the buffer operates within about one pH unit of the pKa.
Frequently Asked Questions
Does ionic strength matter?
High ionic strength can shift pKa slightly. Adjust pKa if literature values indicate significant ionic strength effects.
Can I mix units for concentration?
The ratio is unitless, so any consistent units cancel. Keep acid and base in the same units.
What happens if acid concentration is zero?
The buffer collapses to pure base and the equation no longer applies. Include both species to form a buffer.
Does temperature change pKa?
Yes. Consult temperature-dependent pKa tables to refine calculations when working away from standard conditions.
Is this valid for polyprotic systems?
Use the pKa corresponding to the buffering pair. Treat each buffering range separately for polyprotic acids.