How a buffer resists pH change
A buffer contains a weak acid, HA, and its conjugate base, A−, in similar amounts. Acetic acid with sodium acetate is a common example.
Added strong acid reacts with the conjugate base: H+ + A− → HA. The added H+ ions are removed, so the pH falls only slightly.
Added strong base reacts with the weak acid: OH− + HA → A− + H2O. The added OH− ions are removed, so the pH rises only slightly.
The pH depends on the log of the base-to-acid ratio: pH = pKa + log([A−]/[HA]). A small change in the ratio gives a small change in pH.
Buffer vs water: pH change after adding acid or base
Each solution has a volume of 1.0 L. The buffer contains 0.10 mol acetic acid and 0.10 mol sodium acetate, with pKa 4.76.
| Solution | Starting pH | Final pH | pH change |
|---|---|---|---|
| Water + 0.010 mol HCl | 7.00 | 2.00 | −5.00 |
| Buffer + 0.010 mol HCl | 4.76 | 4.67 | −0.09 |
| Water + 0.010 mol NaOH | 7.00 | 12.00 | +5.00 |
| Buffer + 0.010 mol NaOH | 4.76 | 4.85 | +0.09 |
Buffer capacity and buffer range
- A buffer works only while both HA and A− remain. Buffer capacity is the amount of acid or base it can neutralize.
- Higher concentrations of HA and A− give a higher buffer capacity.
- A buffer works best within 1 pH unit of its pKa, where [A−]/[HA] is between 0.1 and 10.
- Blood uses the carbonic acid and bicarbonate buffer to stay near pH 7.4.
- The phosphate buffer, H2PO4− and HPO4 2−, works near pH 7.2.
How to use this calculator
- Enter pKa and the initial weak-acid and conjugate-base amounts.
- Select the added strong acid or strong base.
- Enter the added amount and calculate.
- Compare the new pH with the pKa to see how little the pH changed.
Formula and variables
pH = pKa + log10(nbase/ nacid) after stoichiometry
| Symbol | Meaning | Unit |
|---|---|---|
| nbase | post-reaction base amount | mol |
| nacid | post-reaction acid amount | mol |
Worked example
Adding 0.10 mol strong acid to 1.0 mol each of acid and base changes them to 1.10 and 0.90 mol before pH is calculated.
Interpretation
The result shows the post-addition composition; exhausted buffer components are rejected rather than sent into a logarithm.
Common mistakes
- Apply strong-addition stoichiometry before equilibrium.
- Do not continue the buffer equation after a component is exhausted.
Frequently asked questions
How does a buffer solution resist change in pH?
The conjugate base neutralizes added acid. The weak acid neutralizes added base. Only the base-to-acid ratio changes, so pH changes little.
Why does a buffer need a weak acid instead of a strong acid?
A strong acid ionizes completely. Its conjugate base is too weak to react with added acid, so it cannot buffer.
Why does the calculator apply stoichiometry first?
A strong acid or base reacts with a buffer component before the remaining pair reaches equilibrium.
What happens when one buffer component is exhausted?
The buffer equation no longer applies. The calculator rejects that result.
Can I enter concentrations instead of moles?
Convert each component and the added reagent to moles before you use this calculator.
Can I use scientific notation?
Yes. Enter values such as 6.022e23 or 1.0e-6 in a numeric field.
How should I round the result?
Keep extra digits during the calculation. Round the final result to the precision required by your data.