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Determination of Buffer Capacity

Determination of Buffer Capacity. Lab 11. Outline. Purpose Buffers Buffer Example Equilibrium Expression and the Henderson- Hasselbalch Equation Things to Remember Buffer Capacity Solutions and Waste Skill Evaluations Reminder. Purpose.

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Determination of Buffer Capacity

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  1. Determination of Buffer Capacity Lab 11

  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 Your buffer solution will have an equimolar concentration of acetic acid and the acetate ion! CH3COOH Na+ CH3COO- CH3COO- Na+ CH3COOH CH3COO- Na+ CH3COOH

  6. Buffer Example • Acetic Acid / Acetate Ion: CH3COOH CH3COO-+ H+ • Weak acid neutralizes OH-: OH- + CH3COOH CH3COO-+ H2O • Weak base neutralizes H+: H+ +CH3COO- CH3COOH Thus, a change in pH is resisted.

  7. Equilibrium Expression and theHenderson-Hasselbalch Equation CH3COOH CH3COO- + 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. Cb = • 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. Thanksgiving Week and Skill Evaluations • Next week – no labs. • Your instructor will let you know when to submit your Lab 11 Report for preliminary grading. • Skill Evaluations in 2 weeks. • Study for the quiz. • You may be evaluated on any of the posted skills. • Read through pages 285 – 304 in your lab manual. • Remember your goggles and lab manual!

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