1. LAB 11 Determination of Buffer Capacity

2. Outline Purpose Buffers Buffer Example Equilibrium Expression and the Henderson-Hasselbalch Equation Things to Remember Buffer Capacity Solutions and Waste Skill Evaluations Reminder

3. Purpose Students will determine the buffer capacity of several acetic acid / acetate buffer solutions using a pH probe.

4. Buffers A buffered solution is a solution that resists a change in pH. In order to have a buffer, two components are essential: a weak acid a weak base These two must have a common ion: called a conjugate acid-base pair

5. Buffer Solution CH 3 COOH CH 3 COO - Your buffer solution will have an equimolar concentration of acetic acid and the acetate ion! Na +

6. Buffer Example Acetic Acid / Acetate Ion: CH 3 COOH CH 3 COO - + H + Weak acid neutralizes OH - : OH - + CH 3 COOH CH 3 COO - + H 2 O Weak base neutralizes H + : H + + CH 3 COO - CH 3 COOH Thus, a change in pH is resisted.

7. Equilibrium Expression and the Henderson-Hasselbalch Equation CH 3 COOH CH 3 COO - + H + Ka = pH = pKa + log

8. Things to remember: Buffers are most effective when the pH of the buffered system is equal to the pKa of the conjugate acid. Buffers are only effective when the pH is within 1 unit from the pKa. pH = pKa ± 1 When this pH is exceeded, we have exceeded the buffer capacity.

9. Buffer Capacity The amount of acid or base that can be absorbed by a buffered system without changing the pH by more than one unit. C b = The concentrations of all your buffers for today were set at a 1:1 ratio. We will titrate to a change in pH of exactly 1 unit.

10. Safety Concerns Reagents: Acetic Acid (1 N) Acetate Buffers Sodium Hydroxide (0.1 N) / Potassium Hydroxide (0.1 N) Eye Contact: Irritation, tearing, redness, pain, impaired vision, severe burns and irreversible eye injury. Skin Contact: Severe skin irritation, soreness, redness, destruction of skin (penetrating ulcers). May cause sensitization and / or allergic reaction. Inhalation: May cause coughing, serious burns, pneumonitis, pulmonary edema, and coma. Ingestion: Toxic. Corrosive to mucous membranes. May cause perforation of the esophagus and stomach, abdominal pain, nausea, vomiting, diarrhea, general gastro-intestinal upset.

11. Solutions and Waste Please conserve distilled water during cleanup. Only solutions with a pH between 6 and 8 can go down the drain. All other solutions need to go in the acid/base waste container in the fume hood.

12. Next Week – Skill Evaluations Study for the quiz. You may be evaluated on any of the posted skills. Read through pages 283 – 302 in your lab manual. Remember your goggles, lab manual, and Lab 11 Report!