LAB 11 Determination of Buffer Capacity. Purpose Students will determine the buffer capacity of...
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Transcript of LAB 11 Determination of Buffer Capacity. Purpose Students will determine the buffer capacity of...
LAB 11
Determination of Buffer Capacity
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 Solution
CH3COOH
CH3COOH
CH3COOHCH3COO-
CH3COO-
CH3COO-
Your buffer solution will have an equimolar
concentration of acetic acid and the acetate ion.
Na+
Na+
Na+
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.
Equilibrium Expression and theHenderson-Hasselbalch Equation
CH3COOH CH3COO- + H+
Ka =
pH = pKa + log
3
3
H CH COO
CH COOH
3
3
CH COO
CH COOH
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 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.
Cb =
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.
NaOH NaOH
buffer,L
M VpH V
Safety ConcernsReagents:
• 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
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.
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!