1. LAB 11 Determination of Buffer Capacity

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

3. 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

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

5. 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.

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

7. 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.

8. Buffer Capacity  The moles of acid or base that can be absorbed by a buffered system without changing the pH by more than one unit is called buffer capacity. C b =  The concentrations of all your buffers for this lab have a 1:1 ratio.  We will titrate to a change in pH of exactly 1 unit.

9. 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.

10. Solutions and Waste 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.

11. Next Week: Skill Evaluations Study for the quiz. You may be evaluated on any of the posted skills. Read through pages 281 – 300 in your lab manual. Remember your goggles and lab manual!