---
title: "Buffer action and acidic buffer mechanism"
subject: "Engineering Chemistry"
chapter: 1
key: "semester_2/engineering_chemistry:buffer-action"
source: https://www.bctengineer.com/q/semester_2/engineering_chemistry/buffer-action
site: BCT Engineering
---

# Buffer action and acidic buffer mechanism

Engineering Chemistry · Chapter 1: Electro-chemistry and Buffer

- Asked in: 2075 Baisakh, 3 marks; 2068 Bhadra, 1+4 marks (asked 2 times)
- Video: [Acidic and Basic Buffers](https://www.youtube.com/watch?v=4wSZkM50CBw) — Allery Chemistry
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_2/engineering_chemistry/buffer-action
- Whole chapter: https://www.bctengineer.com/md/semester_2/engineering_chemistry/chapter-1

## Question

What is buffer action? Describe the mechanism of acidic buffer solution with a suitable example.

## Short answer

Buffer action is the resistance of a solution to pH change upon adding small amounts of strong acid or base. In an acidic buffer like CH₃COOH/CH₃COONa, added H⁺ reacts with CH₃COO⁻ to form CH₃COOH, while added OH⁻ is neutralized by CH₃COOH, keeping pH nearly constant.

## Answer

#### Buffer action
The ability of a buffer solution to resist a change in its pH on adding a small amount of a strong acid or a strong base (or on dilution) is called **buffer action**. A buffer is made of a weak acid and its salt with a strong base (acidic buffer, e.g. CH₃COOH + CH₃COONa), or a weak base and its salt with a strong acid (basic buffer, e.g. NH₄OH + NH₄Cl).

#### Mechanism of an acidic buffer (CH₃COOH / CH₃COONa)
In the solution:

```text
CH₃COOH  ⇌  CH₃COO⁻ + H⁺      (weak acid, slightly ionised)
CH₃COONa →  CH₃COO⁻ + Na⁺     (salt, fully ionised)
```
The salt gives a large reserve of acetate ions (CH₃COO⁻), which suppresses the ionisation of acetic acid (common ion effect). So the solution contains a large amount of undissociated CH₃COOH (acid reserve) and CH₃COO⁻ (base reserve).

- **On adding a small amount of a strong acid (H⁺):** the extra H⁺ combines with the acetate ions to form the weak acid.
  CH₃COO⁻ + H⁺ → CH₃COOH
  Free H⁺ is removed, so pH is almost unchanged.
- **On adding a small amount of a strong base (OH⁻):** OH⁻ is neutralised by the acetic acid (acid reserve), and more acetic acid ionises to replace the H⁺ used up.
  CH₃COOH + OH⁻ → CH₃COO⁻ + H₂O
  Free OH⁻ is removed, so pH is almost unchanged.

The pH of the buffer is given by Henderson's equation: pH = pKa + log([salt]/[acid]). Since the ratio [salt]/[acid] changes only slightly, the pH stays nearly constant.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. What is meant by buffer action?
   A. Ability to keep the pH exactly equal to 7
   B. Ability to neutralise any amount of acid or base completely
   C. Ability to increase the ionisation of a weak acid
   D. Ability to resist a change in pH on adding small amounts of strong acid or base
   Answer: D. Buffer action is the resistance of a solution to pH change when a small amount of strong acid or base is added (or on dilution). (Key point: Definition of buffer action)

2. In CH3COOH/CH3COONa, why does the solution contain mostly undissociated CH3COOH?
   A. Sodium ions react with acetic acid
   B. Acetic acid is a strong acid
   C. The salt is only slightly ionised
   D. The acetate ions from the salt suppress ionisation of the acid (common ion effect)
   Answer: D. The fully ionised salt supplies a large reserve of CH3COO-, which pushes the acid equilibrium back and suppresses its ionisation. (Key point: Role of the common ion)

3. When a small amount of strong acid is added to the acetate buffer, what removes the extra H+?
   A. CH3COOH ionises further
   B. Na+ ions combine with H+
   C. Water combines with H+ to form H2
   D. CH3COO- combines with H+ to form CH3COOH
   Answer: D. Acetate ions (the base reserve) react with H+ to give weak CH3COOH, so free H+ is removed and pH barely changes. (Key point: Response to added H+)

4. According to Henderson's equation, why does the pH of the buffer stay nearly constant?
   A. pKa changes to compensate
   B. The acid concentration becomes zero
   C. The salt concentration becomes equal to the Ka
   D. The ratio [salt]/[acid] changes only slightly
   Answer: D. pH = pKa + log([salt]/[acid]); a small addition changes the ratio only slightly, so pH changes very little. (Key point: Why pH stays nearly constant)

## Related in Engineering Chemistry

- [Buffer action; pH after adding KOH to acetic acid buffer](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Aacidic-buffer-koh-ph) — Engineering Chemistry, chapter 1
- [Buffer action; pH change on adding KOH](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Abuffer-koh-change-ph) — Engineering Chemistry, chapter 1
- [Buffer solution; pH of NH₄OH-NH₄Cl mixture](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Abuffer-ph-nh4oh-nh4cl) — Engineering Chemistry, chapter 1
- [Buffer solution; sodium formate for pH 3.80](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Abuffer-sodium-formate) — Engineering Chemistry, chapter 1
- [Buffer criteria; sodium benzoate calculation](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Abuffer-sod-benzoate) — Engineering Chemistry, chapter 1
- [Buffer criteria; NH₄Cl weight calculation](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Abuffer-nh4cl) — Engineering Chemistry, chapter 1