E3.8 Describe the chemical characteristics of buffer solutions

Ontario Grade 12 Chemistry

Chemical Systems and Equilibrium

How certain mixtures limit changes in pH

A small addition of acid can cause a large pH change in plain water. In a buffer solution, the same kind of addition usually causes a smaller change, as long as the amount added is not too large. A buffer does not keep pH perfectly fixed. It contains particles that react with some of the added acid or base. This lesson focuses on those chemical characteristics.

What you will learn

Describe what a buffer solution does when a small amount of acid or base is added.
Identify the weak acid and its conjugate base in an acidic buffer.
Use particle-level explanations and balanced equations to describe how a buffer responds.
Explain why a buffer has a limited capacity to resist pH change.

1. Prerequisite bridge: acids, bases, and conjugate pairs

An acid is a substance that can donate a hydrogen ion, written as

\mathrm{H^+}

. A base can accept a hydrogen ion. In water, a strong acid such as hydrochloric acid forms hydrogen ions and chloride ions. A weak acid ionizes only partly, so a mixture contains both un-ionized acid particles and ions.

When a weak acid donates a hydrogen ion, it forms its conjugate base. A conjugate base is the particle left after the acid loses a hydrogen ion. The acid and its conjugate base differ by one hydrogen ion. For example, ethanoic acid and the ethanoate ion form a conjugate pair.

A reversible reaction can proceed in both directions. At equilibrium, the forward and reverse changes continue, but the amounts of the substances stay steady overall. Equilibrium does not mean that the amounts of all substances are equal.

\mathrm{HA(aq) + H_2O(l) \rightleftharpoons H_3O^+(aq) + A^-(aq)}

A conjugate acid–base pair differs by one hydrogen ion.
A weak acid is only partly ionized in water.
Equilibrium means the amounts remain steady overall, not that they are equal.

2. What makes a solution a buffer?

A buffer solution resists a large change in pH when a small amount of acid or base is added. The word “resists” matters: the pH can change, but the change is usually limited compared with the change in an unbuffered solution receiving the same small addition.

A common acidic buffer contains a weak acid and its conjugate base in useful amounts. The weak acid can react with some added base. The conjugate base can react with some added acid. The two components therefore help the solution respond to additions in either direction.

In a mixture of ethanoic acid and ethanoate ions, the acid is present as molecules and the conjugate base is present as ions. These particles are available to react. An acidic buffer is not simply any solution containing an acid: the paired weak acid and conjugate base are important to its response.

A buffer can also be made from a weak base and its conjugate acid. In that kind of pair, the weak base responds to added acid, and the conjugate acid responds to added base. The same general idea applies: the mixture contains related particles that can react with small additions.

\mathrm{CH_3COOH(aq) / CH_3COO^-(aq)}

A buffer limits, but does not eliminate, pH change.
A common acidic buffer contains a weak acid and its conjugate base.
A weak base and its conjugate acid can also form a buffer.

3. How an acidic buffer responds

Consider an acidic buffer containing ethanoic acid and ethanoate ions. If a small amount of acid is added, it brings extra hydrogen ions into the solution. Ethanoate ions can accept those hydrogen ions and form ethanoic acid. This removes some of the added hydrogen ions from the solution.

If a small amount of base is added, hydroxide ions can react with the weak acid. The acid donates hydrogen ions to the hydroxide ions, forming water. The weak acid is changed into its conjugate base. This uses up some of the added hydroxide ions.

These reactions explain the buffer’s chemical action at the particle level. One buffer component reacts with added acid, and the other reacts with added base. Since the additions are reduced by reaction, the pH changes less than it would in a solution without the buffer. The buffer does not remove every added particle, so its pH is not guaranteed to stay exactly constant.

The buffer has a limited capacity. Buffer capacity is the amount of added acid or base a buffer can handle while still limiting the pH change. If too much acid or base is added, one of the buffer components can be used up. Once there is too little of that component to react effectively, the solution can no longer limit the change as well.

\mathrm{CH_3COO^-(aq) + H_3O^+(aq) \rightarrow CH_3COOH(aq) + H_2O(l)}

Added acid is partly taken up by the conjugate base.
Added base is partly taken up by the weak acid.
Buffer capacity is limited; a buffer can be overwhelmed.

4. Reading the equations and describing the response

The equations show which particles react; they do not mean that every added particle disappears. In the acid-response equation, ethanoate ions accept hydrogen ions from hydronium ions. Hydronium, written as

\mathrm{H_3O^+}

, is a form in which hydrogen ions are present in water. The reaction forms ethanoic acid and water.

For added base, the weak acid reacts with hydroxide ions to make water and ethanoate ions. Both equations are balanced: the atoms and total charge are the same on each side. The state symbols indicate that the ions and ethanoic acid are in aqueous solution and that water is liquid.

When describing a buffer, name the buffer components and state what happens after an addition. For example: “The ethanoate ions react with some of the added hydronium ions, so the amount of added acid remaining in solution is reduced. The pH changes less than it would without the buffer.” This connects the particle change to the observed property without claiming that pH stays unchanged.

Do not call a buffer a solution that prevents all pH change. Its effect depends on having enough of the reacting buffer component for the amount added. A large addition can exceed the buffer’s capacity.

\mathrm{CH_3COOH(aq) + OH^-(aq) \rightarrow CH_3COO^-(aq) + H_2O(l)}

Use particles and reactions to explain why the pH change is limited.
A buffer reduces the effect of a small addition; it does not make pH unchangeable.
A large addition can exceed the amount the buffer can handle.

Worked example

Explaining an ethanoic acid buffer

A solution contains ethanoic acid and ethanoate ions. Describe what happens when a small amount of hydrochloric acid is added. Then describe what happens when a small amount of sodium hydroxide is added. Explain why the solution resists a large pH change.

Identify the buffer pair
Ethanoic acid is the weak acid, and ethanoate is its conjugate base. The solution has particles that can react with either added acid or added base.
$\mathrm{CH_3COOH(aq) / CH_3COO^-(aq)}$
Consider added acid
Hydrochloric acid supplies hydrogen ions in water. Ethanoate ions accept some of these ions, forming ethanoic acid and water. This uses up some of the added acidic particles.
$\mathrm{CH_3COO^-(aq) + H_3O^+(aq) \rightarrow CH_3COOH(aq) + H_2O(l)}$
Consider added base
Sodium hydroxide supplies hydroxide ions in water. Ethanoic acid reacts with some of the hydroxide ions to form water and ethanoate ions.
$\mathrm{CH_3COOH(aq) + OH^-(aq) \rightarrow CH_3COO^-(aq) + H_2O(l)}$
State the buffer’s limit
In both cases, a buffer component reacts with some of the added particles. The pH change is therefore limited compared with an unbuffered solution. The effect is not unlimited: a sufficiently large addition can use up much of the component needed to respond.

Answer: Ethanoate ions react with some added acid, while ethanoic acid reacts with some added base. These reactions reduce the effect of small additions on pH. The buffer can be overwhelmed by a large addition.

Check: The equations conserve atoms and net charge. They show the reacting particles and the products formed.

Common mistakes and how to avoid them

A buffer keeps its pH perfectly constant.

Correction: A buffer limits pH change when a small amount of acid or base is added. The pH may still change.

Any weak acid by itself is a buffer.

Correction: A common acidic buffer contains a weak acid and its conjugate base in useful amounts, so both types of addition can be addressed.

A buffer can handle any amount of added acid or base.

Correction: Buffer capacity is limited. A large addition can use up a component needed to react.

The acid and its conjugate base are the same substance.

Correction: They are related particles that differ by one hydrogen ion. They have different formulas and play different roles in the buffer.

Lesson summary

A buffer solution resists a large pH change when a small amount of acid or base is added.
A common acidic buffer contains a weak acid and its conjugate base.
The conjugate base reacts with some added acid; the weak acid reacts with some added base.
Buffer capacity is limited, so a large addition can overwhelm the buffer.

Check your understanding

Question 1

In an ethanoic acid–ethanoate buffer, which component reacts with some added hydroxide ions?

Ethanoic acid
Ethanoate ions
Water only
Neither component

Show answer and explanation

Ethanoic acid

Ethanoic acid donates a hydrogen ion to hydroxide ions. The products are water and ethanoate ions.

Question 2

Which statement best describes buffer capacity?

The amount of acid or base a buffer can handle while limiting pH change
The claim that a buffer’s pH never changes
The amount of water in a buffer
The claim that a buffer contains equal amounts of all particles

Show answer and explanation

The amount of acid or base a buffer can handle while limiting pH change

Buffer capacity describes the limited amount of added acid or base the solution can handle while still limiting the pH change.

Key terms

Acid: A substance that can donate a hydrogen ion.
Base: A substance that can accept a hydrogen ion.
Conjugate base: The particle formed when an acid loses a hydrogen ion.
Buffer solution: A solution that limits a large pH change when a small amount of acid or base is added.
Buffer capacity: The amount of added acid or base a buffer can handle while still limiting pH change.
Equilibrium: A condition in which forward and reverse changes continue while the amounts of substances remain steady overall.

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Published by DoAssignment. This reviewed lesson follows Ontario Grade 12 Chemistry (SCH4U), expectation E3.8. It is a study resource, not an official curriculum publication.

Before publication, content is checked for structure, mathematical or chemical notation, calculations, course boundaries, and readability. Errors can still occur, so corrections are welcomed.

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E3.8 · Describe the chemical characteristics of buffer solutions