Transcript Determination of Buffer Capacity Lab 5 Purpose Students will determine the buffer capacity of several acetic acid / acetate buffer solutions using a pH probe.

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:
CH3COOH
CH3COO- + H+
Weak acid neutralizes OH-:
OH- + CH3COOH
CH3COO- + H2O
Weak base neutralizes H+:
H+ + CH3COOCH3COOH
Thus, a change in pH is resisted.
Equilibrium Expression and the
Henderson-Hasselbalch Equation
CH3COOH
CH3COO- + H+
H  CH3COO 
Ka =
CH3COOH
 CH3COO  

pH = pKa + log  
 CH3COOH 


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.
Cb =
MNaOH  VNaOH 
 pH  Vbuffer,L 
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.