Calculate buffer capacity in mol/L per pH unit from total buffer concentration, pKa and pH.
Buffer capacity (mol/L per pH)
How to use this tool
Enter the total buffer concentration (acid + conjugate base) in mol/L.
Enter the acid pKa and the solution pH.
Read buffer capacity and the acid/base split.
Try pH values near and far from pKa to see capacity peak and fall.
Formula & worked example
A 0.1 mol/L buffer with pKa 4.76 at pH 4.76 has a capacity of about 0.0576 mol/L per pH unit, its maximum for that concentration.
Limitations — read this before you rely on it
Ideal-dilute-solution formula using concentrations, not activities; real buffers deviate at high ionic strength, and polyprotic systems need a sum over all dissociation steps.
Frequently asked questions
How much strong acid or base (moles per litre) the solution can absorb per unit change in pH. Higher capacity means pH changes less when acid or base is added.
When pH equals pKa, so acid and conjugate base are present in equal amounts. Capacity is already much lower one pH unit away from pKa.
Near pKa, yes — capacity is proportional to total buffer concentration. Diluting a buffer weakens it in proportion.
No. It follows a bell-shaped curve centred on pKa, which is why buffers are chosen with a pKa close to the target pH.
At pH extremes the water terms ([H+] and Kw/[H+]) dominate, so even pure water resists pH change there; this tool includes those terms.