What are Buffers and Buffering Capacity?
A buffer is a solution that resists large pH changes when small amounts of acid or base are added. Buffers are made of a weak acid and its conjugate base (or weak base and its conjugate acid). Buffering capacity is the amount of acid or base a buffer can neutralize before pH shifts significantly.
Buffers work because the weak acid HA and its conjugate base A− exist in equilibrium. When H+ is added, A− neutralizes it. When OH− is added, HA neutralizes it. Buffering capacity depends on the concentrations of both components and is highest when [HA] = [A−] (pH = pKa).
- 1↓Initial equilibriumWeak acid HA and conjugate base A− in balance
- 2↓Acid added (H+)A− quickly neutralizes added H+: A− + H+ → HA
- 3↓HA increases slightlyequilibrium shifts, but pH change is small
- 4↓Base added (OH−)HA quickly neutralizes added OH−: HA + OH− → A− + H2O
- 5A− increases slightlyequilibrium shifts, but pH stays near pKa
Try it: interactive calculator
Step-by-step worked examples
A buffer contains 0.1 M acetic acid (CH3COOH, pKa = 4.75) and 0.1 M sodium acetate (NaCH3COO). Calculate the pH.
pH = pKa + log([A−]/[HA]) pH = 4.75 + log(0.1/0.1) pH = 4.75 + log(1) pH = 4.75 + 0 pH = 4.75 (≈ pKa when concentrations are equal)
In the same buffer, 0.05 M HCl is added. How does pH change? (Assume volume negligible)
Added H+ = 0.05 M H+ + A− → HA New [A−] ≈ 0.1 − 0.05 = 0.05 M New [HA] ≈ 0.1 + 0.05 = 0.15 M pH = 4.75 + log(0.05/0.15) = 4.75 + log(0.33) = 4.75 − 0.48 = 4.27 (Without buffer, 0.05 M HCl alone would give pH ≈ 1.3)
What is the buffering capacity range of an acetate buffer (pKa 4.75)?
Buffering capacity is best within ±1 pH unit of pKa Range: 4.75 − 1 = 3.75 to 4.75 + 1 = 5.75 Outside this range the buffer becomes ineffective
Flashcards
Quick quiz
Q1.What is the main component of a buffer?
Q2.pH = pKa when:
Q3.Buffer capacity is best within
Q4.Diluting a buffer solution:
The full card deck, worked steps and AI-tutor support for “What are Buffers and Buffering Capacity?” are in Notek — study by hand before your exam.
Common mistakes
A buffer prevents ANY pH change. — Correct: Buffers resist pH change within a limited range (typically ±1 unit of pKa).
A strong acid + strong base makes a good buffer. — Correct: Buffers require a weak acid/base and its conjugate form.
A buffer works equally well at any pH. — Correct: Buffer capacity is best near the pKa of the weak acid.
The amount of buffer doesn't matter, only the ratio [HA]:[A−]. — Correct: Both the concentrations AND the ratio matter; higher concentrations = higher buffering capacity.
FAQ
What is a buffer in chemistry?
A solution that resists large pH changes when acid or base is added. Typically made from a weak acid and its conjugate base.
How does Henderson-Hasselbalch equation help?
It shows the relationship between pH, pKa, and the ratio of conjugate base to weak acid, allowing you to calculate buffer pH.
What is buffering capacity?
The maximum amount of acid or base a buffer can neutralize before the pH changes by 1 unit.
Why is buffer capacity related to concentration?
More concentrated buffers have more HA and A− molecules, so they can neutralize more added acid or base.