Capacity of Buffers

Learning Objective 8.10.A Explain the relationship between the buffer capacity of a solution and the relative concentrations of the conjugate acid and conjugate base components of the solution.

Quick Notes

Buffer capacity refers to how much acid or base a buffer can neutralize before a significant pH change occurs.
Buffer capacity increases with higher concentrations of the acid/base pair.
A balanced buffer (equal [HA] and [A⁻]) has equal capacity to neutralize added acid or base.
Unbalanced buffers favor neutralization of either added acid or added base, depending on which component is in excess.

Full Notes

What Is Buffer Capacity?

Buffer capacity is the amount of strong acid or base that a buffer can absorb without a large change in pH. It is a measure of the effectiveness of the buffer.

Effect of Concentration

The absolute concentrations of the conjugate acid ([HA]) and conjugate base ([A⁻]) determine how much strong acid or base the buffer can neutralize.

If you increase both [HA] and [A⁻] proportionally, the pH stays the same (since the ratio is constant), but the capacity increases.

Example: A 1.0 M acetic acid/sodium acetate buffer can absorb more added HCl or NaOH than a 0.10 M buffer of the same ratio.

Effect of Ratio

If [HA] > [A⁻], the buffer can better neutralize added base (because more HA is available to react with OH⁻).

If [A⁻] > [HA], the buffer can better neutralize added acid (because more A⁻ is available to react with H₃O⁺).

A 1:1 ratio gives balanced buffering.

Buffer Capacity Summary

Ratio [A⁻]/[HA]	Buffer Capacity Is Best For...
Greater than 1	Neutralizing added acid
Less than 1	Neutralizing added base
Equal (1:1)	Neutralizing both equally

Worked Example

Q: You are preparing a buffer solution with acetic acid (pK_a = 4.76). Which buffer will have a higher capacity to neutralize added base?
Option A: 0.50 M CH₃COOH and 0.10 M CH₃COO⁻
Option B: 0.10 M CH₃COOH and 0.50 M CH₃COO⁻

A: Option A has more acid (HA), so it can better neutralize added base. Option B has more base (A⁻), so it would be better for added acid. Correct answer: Option A.

Summary

Buffer capacity depends on both concentration and ratio of the buffer components.
A higher concentration of buffer components = greater capacity.
The dominant species (acid or base) determines whether the buffer resists pH changes more effectively against added acid or base.
For general use, a 1:1 ratio provides optimal balance.