Buffer Solution Preparation and Testing

Overview

This lecture demonstrates how to prepare a buffer solution using a weak acid (benzoic acid) and its conjugate base (sodium benzoate), including steps for measuring, dissolving, mixing, and pH testing.

Equipment & Materials

• Use a 100 mL volumetric flask for solution preparation.
• Weigh benzoic acid using a scale, weighing boat, and spatula.
• Sodium benzoate serves as the conjugate base.
• Deionized water is used to dissolve reagents and rinse equipment.
• Required tools: beaker, pipette with bulb, stirring rod, pH probe, iPad (for reading), Kimwipes, and squirt bottle.

Preparation of Weak Acid Solution

• Weigh 0.31 g benzoic acid and transfer to the volumetric flask.
• Add small increments of deionized water and swirl to dissolve the acid.
• Use a hot plate if the acid does not fully dissolve.
• Bring the flask volume up to 100 mL using a pipette for accuracy.
• Calculate the molarity based on the mass and volume.

Measuring Initial pH

• Transfer the solution to a 150 mL beaker (allows pH probe use).
• Optional: add universal indicator for a color estimate of pH.
• Rinse and insert pH probe to measure and record initial pH (found to be 2.89).

Addition of Conjugate Base (Sodium Benzoate)

• Add 0.011 g sodium benzoate to the solution, stir, and dissolve completely.
• Record pH after each increment: first addition raises pH to 3.76.
• Add 0.13 g sodium benzoate, dissolve, and pH increases to 4.31.
• Further addition (0.12 g) dissolves and pH rises to 4.62, with change slowing.

Buffer Testing & Practical Notes

• Buffer is a mixture of weak acid and its conjugate base resisting pH changes.
• To test buffer capacity, split solution and add acid to one part, base to the other.
• Observe the buffer’s ability to resist pH changes after additions.

Key Terms & Definitions

• Buffer — a solution that resists changes in pH when small amounts of acid or base are added.
• Weak Acid — an acid that partially dissociates in water (e.g., benzoic acid).
• Conjugate Base — the species formed when a weak acid loses a proton (e.g., sodium benzoate from benzoic acid).
• Molarity — concentration of a solute in a solution, in moles per liter.

Action Items / Next Steps

• Test your prepared buffer by adding small amounts of acid and base to separate portions and record pH changes.
• Calculate the initial acid concentration if using a stock solution rather than a solid acid.