Buffer solutions

Study Guide on Buffer Solutions for AP Chemistry

Definition

  • Buffer Solution: A buffer is a solution that resists changes in pH when small amounts of an acid or a base are added.

Components of a Buffer

  • Buffers typically consist of:

    • A weak acid and its conjugate base (e.g., acetic acid and sodium acetate)

    • A weak base and its conjugate acid (e.g., ammonia and ammonium chloride)

How Buffers Work

  • Buffers work through the equilibrium established between the weak acid/base and its conjugate.

  • When an acid (H⁺) is added:

    • The weak base in the buffer reacts with the H⁺, minimizing the change in pH.

  • When a base (OH⁻) is added:

    • The weak acid in the buffer donates a proton (H⁺) to counteract the pH increase.

Henderson-Hasselbalch Equation

  • Used to calculate the pH of a buffer solution:

    • pH = pKa + log([A⁻]/[HA])

    • Where [A⁻] is the concentration of the conjugate base and [HA] is the concentration of the weak acid.

Preparing a Buffer

  • Choose a weak acid and its salt or a weak base and its salt.

  • Adjust the concentrations of the weak acid and conjugate base to the desired ratio using the Henderson-Hasselbalch equation.

Applications of Buffers

  • Biological systems: Maintain pH in blood and cells.

  • Industrial processes: Control pH for reaction efficiency.

  • Laboratory settings: Stabilize pH during experiments.

Key Concepts

  • Buffers are effective within a certain pH range, typically within one pH unit of their pKa.

  • The capacity of a buffer depends on the concentrations of the acid/base components; more concentrated buffers resist pH changes better than dilute buffers.


The pH scale is a logarithmic scale that measures the acidity or basicity of a solution. It typically ranges from 0 to 14, with the following values indicating:

  • pH < 7: Acidic solution

  • pH = 7: Neutral solution

  • pH > 7: Basic (alkaline) solution

In the context of buffer solutions:

  • Buffers are most effective at maintaining pH within 1 unit of their pKa, which is why understanding the pH scale is essential in buffer preparation and application.