CHE 230
Lab #5
Buffers and Buffered Aspirin
Introduction:
Two species are required in a buffer solution. One is capable of reacting with OH- and the other will
react with H3O+. The two species must not react with each other. Many buffers are prepared by combining a
weak acid and its conjugate (acetic acid and sodium acetate) or a weak base and its conjugate (ammonia and
ammonium chloride). In general, the pH range in which a buffer solution is effective is +/- one pH unit on either
side of the pKa. The Henderson–Hasselbalch provides the information needed to prepare a buffer.
pH  pKa  log
conjugatebase
weakacid 
There is a limit to the amount of acid or base that can be added to a buffer solution before one of the
components is used up. This limit is called the buffer capacity and is defined as the moles of acid or base
necessary to change the pH of one liter of solution by one unit.
Buffer Capacity = (number of moles of OH- or H3O+ added)
(pH change)(volume of buffer in L)
Objectives:
 Prepare acetate buffers at various concentrations and at various pH levels using two different methods.
 Determine the relationship between concentration and buffer capacity as well as pH and buffer capacity.
 Compare the buffer capacity of regular and buffered aspirin.
Tools of the trade:
 Analytical balance
 Buret
 pH meter
Techniques:
 Titration
 Buffer preparation
Procedure – Buffer Preparation:


Pre-Experiment Calculations
Using the Henderson-Hasselbalch equation, determine the amount of acetic acid and sodium acetate required
to make:
o buffer #1
o buffer #2
Using the Henderson-Hasselbalch equation, determine the amount of acetic acid and NaOH required to make
buffer #3.
1. Buffer #1: Prepare 100 mL of an acetic acid – sodium acetate buffer at pH 4.7 using sodium acetate and 0.1
M acetic acid.
2. Buffer #2: Prepare 100 mL of an acetic acid – sodium acetate buffer at pH 4.7 using sodium acetate and 0.2
M acetic acid.
3. Buffer #3: Experimentally prepare 100 mL of an acetic acid – sodium acetate buffer at pH 4.0 using 0.1 M
NaOH and 0.1 M acetic acid.
Procedure – Buffer Capacity:
1. Transfer 10.00 mL of the solution to be tested to a 250 mL beaker and add 40 mL of distilled water.
2. Using a buret, add 0.1 M NaOH to the buffer solution in small increments. After each addition, record the
total volume of NaOH added and the pH of the solution.
3. Continue adding the NaOH solution until the pH has been raised at least one pH unit.
4. Test the buffer capacity of each buffer solutions: buffer #1, buffer #2, buffer #3
5. Calculate the buffer capacity for each sample.
Procedure – Buffering Ability of Aspirin and Buffered Aspirin:
1. Grind one aspirin pill using a mortar and pestle.
2. Dissolve crushed pill in 50 mL of distilled water. This may require gentle heating. (The pill will not dissolve
entirely – filter out any solid material.)
3. Test the solution’s buffer capacity using procedure from above.
4. Perform procedure on both an aspirin and buffered aspirin sample.
Discussion:
 Using LeChatlier’s Principle, describe how a buffer really “works.”
 Both methods of buffer preparation are acceptable methods. Which do you think is “better”? Why?
 Make a bar graph that compares the buffer capacity of each buffer.
 What is the trend observed with respect to buffer capacity and:
o Buffer concentration and
o Buffer pH?
Justify these trends.
 Make a bar graph that compares the buffer capacity of aspirin and buffered aspirin (of compiled class data).
Add error bars to the graph based on the 95% confidence interval.
 Are the buffer capacities of the two aspirin samples statistically different? Explain.
 How are aspirin samples “buffered”?
 Is buffered aspirin better for you? Why?
 Buffering is especially important for physiological processes. Cite some specific reasons for this. (This will
likely require some research.)