Determination of Buffer Capacity Lab 5

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

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

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.

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

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.

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 is called buffer capacity. 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.

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.

Solutions and Waste  Your solutions are located in the back of the lab.  Please conserve distilled water during cleanup.  Dispose of waste in the appropriate waste receptacles. Acidic and basic solutions / waste need to be disposed in the acid/base waste container in the fume hood. Solutions with a pH between 6 and 8 can be disposed down the drain.

Lab 6 Reminder  Lab 6 next week.