Chapter 18: Aqueous Ionic Equilibrium

Introduction to Buffers

Buffers are essential solutions in chemistry that help maintain a relatively constant pH when small amounts of acid or base are added. Understanding their composition, function, and calculation methods is crucial for mastering acid-base equilibria.

• Buffer Definition: A buffer is a solution containing significant amounts of both a weak acid and its conjugate base (or a weak base and its conjugate acid). It resists changes in pH by neutralizing added acids or bases.

• Example: A solution of 0.100 M CH3COOH (acetic acid) and 0.100 M CH3COONa (sodium acetate) forms a buffer.

Buffer Action and pH Calculation

Buffers work by shifting equilibrium to neutralize added H+ or OH–. The pH of a buffer can be calculated using equilibrium expressions or the Henderson–Hasselbalch equation.

• Acid Dissociation Reaction:

  • for acetic acid

• ICE Table Setup: Used to determine equilibrium concentrations.

• Equilibrium Expression:

• Example Calculation: For 0.100 M acetic acid and 0.100 M acetate:

  • (since x is small)

• Comparison: Without the buffer (just 0.100 M acetic acid), pH = 2.87.

Henderson–Hasselbalch Equation

The Henderson–Hasselbalch equation simplifies buffer pH calculations and is derived from the acid dissociation equilibrium.

• General Form:

• Application: Use when both weak acid and conjugate base concentrations are known.

Preparing a Buffer

To prepare a buffer with a specific pH, follow these steps:

1. Select the desired pH.

2. Choose a weak acid with a pKa near the desired pH (or a weak base whose conjugate acid has a pKa near the desired pH).

3. Calculate the required ratio of base to acid using the Henderson–Hasselbalch equation.

4. Use the dilution equation () to determine the volumes and concentrations needed.

Buffer Response to Addition of Strong Acid or Base

Buffers neutralize added strong acids or bases, minimizing pH changes. The new pH can be calculated by adjusting the concentrations and applying the Henderson–Hasselbalch equation.

• Example: Adding 0.100 moles of HCl to 1.00 L of buffer ([HA] = 0.50 M, [A–] = 0.50 M, pKa = 3.00):

  • New [HA] = 0.60 M, new [A–] = 0.40 M

  • pH decreases only slightly from 3.00 to 2.82.

Choosing the Best Acid for a Buffer

The most effective buffer occurs when pH ≈ pKa, meaning the ratio of [Base] to [Acid] is close to 1. Buffers with pKa far from the desired pH are less effective.

• Example: Preparing a buffer at pH = 4.00 with acids of pKa = 4.30 and 5.00:

  • For pKa = 4.30: [A–] = 0.13 M, [HA] = 0.27 M

  • For pKa = 5.00: [A–] = 0.04 M, [HA] = 0.36 M

  • After adding 0.010 moles NaOH, the buffer with pKa = 4.30 shows a smaller pH change (0.75%) compared to the one with pKa = 5.00 (3.75%).

Buffer Range

The buffer range is the pH interval over which the buffer effectively resists changes in pH. It is determined by the ratio of [Base] to [Acid].

• Effective Range:

• Corresponding pH Range:

  • Lowest pH:

  • Highest pH:

  • Effective buffer range:

Buffer Capacity

Buffer capacity is the amount of strong acid or base a buffer can neutralize before its pH changes significantly (i.e., outside the buffer range).

• Factors Affecting Buffer Capacity:

  • Higher concentrations of buffer components increase capacity.

  • Capacity is greatest when [Base] ≈ [Acid].

• Example: For a buffer with [A–] = 0.18 M, [HA] = 0.10 M, pKa = 3.7447:

  • Buffer range: 2.7447 to 4.7447

  • Maximum acid that can be added: 0.15 moles (before [Base]/[Acid] < 0.1)

  • Maximum base that can be added: 0.075 moles (before [Base]/[Acid] > 10)

Summary Table: Buffer Preparation and Properties

Key Equations

• Acid Dissociation Constant:

• pH Calculation:

• Henderson–Hasselbalch Equation:

• Buffer Range:

• Dilution Equation:

Practice and Application

• Practice calculating buffer pH before and after addition of strong acid/base.

• Evaluate buffer capacity by determining the maximum amount of acid or base that can be added before the buffer is exhausted.

Additional info: For more examples and acid/base tables, refer to your textbook's appendices (e.g., Tables C1 and C3).