Moderate dilution of an acidic buffer can leave its pH approximately unchanged because the ratio of conjugate base to weak acid stays the same. A fixed volume of the diluted buffer contains less of both components, so the same addition of strong acid can cause a larger change in pH.
The 2027 H2 Chemistry 9476 syllabus covers buffer action and pH calculations. This example compares equal-volume portions before and after a tenfold dilution, using the usual concentration-ratio approximation.
- Check the acid-to-base ratio before adding acid.
- Compare equal volumes of the two buffers.
- Update the component amounts after neutralisation.
- Calculate the new pH from the new ratio.
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Begin with two samples at the same approximate pH
Take 100 cm³ of a buffer containing 0.100 mol dm⁻³ weak acid HA and 0.100 mol dm⁻³ conjugate base A⁻. Take a separate 100 cm³ portion of the same buffer after tenfold dilution, so each component is 0.0100 mol dm⁻³. The weak acid has pKₐ = 4.76. Each portion starts with a base-to-acid ratio of one, giving pH ≈ 4.76.
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Worked example | Add the same amount of strong acid
Add 0.500 mmol of H⁺ to each portion. It reacts with A⁻ to form HA. The concentrated portion starts with 10.0 mmol of each component and ends with 9.50 mmol A⁻ and 10.50 mmol HA. Its pH is approximately 4.76 + log₁₀(9.50/10.50) = 4.72.
The dilute portion starts with 1.00 mmol of each and ends with 0.500 mmol A⁻ and 1.50 mmol HA. Its approximate pH is 4.76 + log₁₀(0.500/1.50) = 4.28. The same acid addition consumes a greater fraction of the available base in the dilute portion.
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CHAPTER 3 OF 4 · Back to contents
Explain why equal volume matters
Adding water to an entire sample leaves its original amounts of HA and A⁻ in the larger vessel. The comparison here takes the same volume from each concentration. That is why the dilute portion has fewer moles available to react.
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CHAPTER 4 OF 4 · Back to contents
Frequently asked questions
Why can I use the amounts directly in the ratio?
Both components share the same final volume within a sample. The volume cancels when one concentration is divided by the other.
What if all the conjugate base is consumed?
The buffer approximation no longer applies. Determine the species present after the reaction before calculating pH.