BUFFERS LAB –
Preparation of a Buffer and Buffer Capacity
INTRODUCTION
Many important biochemical reactions occur only over a small range in pH. Living organisms dependent upon these reactions rely on chemical systems called buffers to maintain a relatively constant pH when acids or bases are added to their environment. The
company contracting the students’ chemistry class aims to produce a variety of bacteria designed to destroy harmful living agents. The bacteria must be grown in a medium with a pH similar to that of the environment they will be functioning in. This medium must be able to maintain a pH within plus or minus one unit of the target pH for the bacteria it will support when strong acid or base is added. The students’ job will be to produce a buffer for such a medium. As buffers must neutralize both acids and bases, they must contain both a base and an acid. The question is, how do we prevent the acid and base in the buffer from simply neutralizing one another, thus rendering the buffer useless? Students must also consider how much acid and base their buffer should be able to neutralize. Buffers are most effective when they have been produced so they may neutralize a reasonable amount of either acid or base. The quantity of acid or base that may be added to a buffer while maintaining a relatively constant pH is a function of the buffer’s capacity.
PRE-LAB QUESTIONS
1. How does a buffer solution resist a change in pH?
2. Why would HCl and NaOH be a poor choice for an acid-base pair to make a buffer? 3. Consider the three acid and salt combinations listed in the table below:
a. Which of the following combinations would produce a buffer with an acidic pH? b. Which of the following combinations would produce a buffer with a basic pH? c. Which of the following combinations would be an ineffective buffer? Why?
4. What change would result from making the acid and salt combination each 10 times more concentrated than they were originally?
BACKGROUND
A buffer is a solution designed to resist the changes in pH that occur when small amounts of acid or base are added. Many important chemical reactions occur only over a small pH range. If it were not for the presence of buffer systems in the blood of human beings, the intake of a small glass of orange juice could lead to a dangerous condition called acidosis that would lead to seizures, loss of consciousness, coma, and eventually death. Clearly buffers are very important.
A conjugate acid-base pair is a set of two species that differ from one another by one easily removable hydrogen ion (a proton). Should the members of a conjugate pair neutralize one another, they will simply reform the same species, hence there is no neutralization. It is critical that both members of the conjugate pair are weak as the conjugate of a strong species will be so weak it will be essentially neutral and could not effectively neutralize any added acid or base. A good example of a conjugate pair for a buffer would be hydrofluoric acid (HF) and fluoride ion (F-),
as HF can neutralize added base:
HF(aq) + OH-(aq) → F-(aq) + H
2O (l) and F– can neutralize added acid:
F- (aq) + H+ (aq) → HF(aq)
It is also important to remember that we cannot simply obtain an independent anion (or cation) from our stockroom shelf. Rather we require a soluble salt as the source of the required ion. In the case of our sample HF/F- buffer, we might combine hydrofluoric acid with some sodium fluoride salt. In this experiment, you will combine different pairs of chemicals in an effort to create a buffer of a particular pH.
Since buffers contain weak acids, it is possible to calculate the pH as you would for any weak acid, using the Ka expression, with one significant difference: There is some common ion (actually the conjugate base of the weak acid) in the solution. Consequently, use the general weak acid equation:
HA(aq) + H2O(l) H3O+(aq) + A-(aq) produces the expression
Ka = [H3 O ] [A+ - ]
[HA]
As there is already conjugate base, A-, present in the solution, when equilibrium concentrations are inserted into the expression, the [H3O+] will not be equal to the [A-].
Buffers are most effective when they are equally prepared to neutralize acids or bases. For this to be true, the [HA] should approximately equal the [A–]. Examining the expression for the general buffer equation above, we see that such a
circumstance would result in the Ka of the conjugate acid in the buffer being equal to the [H3O+]. The negative
logarithmic form of this equation indicates that the pKa is equal to the pH of the buffer. In summary, when [HA] = [A–]:
Ka = [H3O+] and pKa = pH
MATERIALS
0.10 M acetic acid, HC2H3O2 (Ka = 1.8 x 10-5) Sodium acetate, NaC2H3O2
0.10 M ammonia, NH3 (Kb = 1.8 x 10-5) Ammonium chloride
0.10 M sodium dihydrogen phosphate, NaH2PO4 (Ka = 6.2 x10-8) Sodium hydrogen phosphate, Na2HPO4
0.10 M citric acid, H3C6H5O7 (Ka = 8.4 x 10-4) Sodium dihydrogen citrate, NaH2C6H5O7
0.10 M sodium monohydrogen citrate, Na2HC6H5O7 (Ka = 4.0 x 10-6) Sodium citrate, Na3C6H5O7
pH 4 buffer pH 7 buffer
0.20 M hydrochloric acid 0.20 M sodium hydroxide
pH probes computer with Logger Pro
PROCEDURE
Preparing the Equipment:
1. Rinse a burette with 10 mL or so of 0.20 M HCl and rinse three times. Then fill the burette and clamp it to the ring stand. Repeat this process with a second burette and 0.20 M NaOH.
2. A pH probe will be calibrated with two standard buffer solutions of pH 4.00 and 7.00 and available at the front of the room for you to test the pH of your prepared buffer solution. You should know how to calibrate this from before!
Preparing the Buffer:
Using the card at your lab station, prepare a 100 mL sample of your buffer solution and test it in hopes that it falls within ± 0.5 of the target pH stated and that a 50 mL sample can maintain a relatively constant pH (within one unit of the initial value) with the addition of at least 20 mL of 0.20 M HCl or 0.20 M NaOH.
Capacity Testing:
1. Each lab group must follow the same procedure for capacity testing. Split your buffer sample into two different 50.0 mL samples. One will be used to test the capacity as a strong base is added quantitatively, and the other will be used to test the capacity as a strong acid is added.
2. You’ll also need to use an indicator for capacity testing to visually determine when the buffering capacity has been reached. For most groups, this will be thymol blue. Thymol blue is yellow at a pH between 3-8, but red at a pH below 2 and blue at a pH above 9. A significant and permanent color change will indicate that one part of the buffer has been stoichiometrically removed. [If you have the buffer in a basic range, pH = 9.00, you will use alizarin, which is red from pH 7-12, but purple above 12 and yellow below 5).
3. The purpose of this step is to generate data to produce a significant change in pH, adding smaller amounts of acid when the pH changes noticeably in terms of color. Begin your capacity testing by using the buret and the drop counter to measure the volume of acid that it takes to reach the buffering capacity with its ability to neutralize acid. Record this volume, and explain whether your buffer meets the standard for acid neutralization.
ANALYSIS
Students should answer the following questions after completing their procedure.
1.
Why are the pH changes so noticeable with the last additions of strong acid and strong base?
2.
Suppose, during preparation, an additional 10 mL of distilled water was added to your buffer by mistake.
a.
What effect would this have on your buffer’s pH? Explain.
b.
Would this affect your buffer’s capacity? Explain.
3.
Given a solution of hydrocyanic acid (HCN), what additional reagent or reagents is/are needed to prepare a
buffer from the hydrocyanic acid solution?
a.
Explain how this buffer solution resists a change in pH when moderate amounts of strong acid (less
than the equivalence point) are added. Use a chemical equation in your explanation.
b.
Explain how this buffer solution resists a change in pH when moderate amounts of strong base (less
than the equivalence point) are added. Again, provide an equation with your explanation.
4.
A buffer solution contains 0.20 moles of methanoic acid, HCOOH, and 0.30 moles of sodium methanoate,
NaCOOH, in 1.00 L of the buffer. The acid ionization constant, K
a,of methanoic acid is 1.8 x 10
-4.
a.
Calculate the pH of this solution.
b.
Compare the capacity of this buffer to neutralize added acid to its capacity to neutralize added base.
Which one would it have a higher capacity to neutralize? Explain your answer completely.
c.
If 0.10 moles of HCl gas solution were bubbled through a liter of the buffer:
i. What would happen to the pH? Show the stoichiometric reaction and explain its effect.
ii. How would this addition affect the buffer’s capacity to neutralize added acid and base in the
TO BE INCLUDED IN WRITEUP:
1) Pre-lab questions and analysis
2) All calculations and explanation of calculations involved in the preparation of your
buffer
3) Graphs and data (volumes) associated with the capacity testing of your buffer.
Explain the graphs, and use the data to explain whether your buffer met the
standard set for its capacity to neutralize both acid and base.
YOUR BUFFER
Prepare a buffer for an antibiological
agent. Your agent is designed for use in
the human body. Its buffer should have
a pH of 7.00 ± 0.5 with the ability to
stay within one pH unit of this target
when 20.0 mL of strong acid or base is
added.
YOUR BUFFER
Prepare a buffer for an antifungal agent.
Your agent is designed for use against a
fungus that attacks food sources that
grow in acidic soil. Its buffer should
have a pH of 4.75 ± 0.5 with the ability
to stay within one pH unit of this target
when 20.0 mL of strong acid or base is
added.
YOUR BUFFER
Prepare a buffer for an antiviral agent.
Your agent is designed for use against a
strain of virus that attacks
drug-producing bacteria that grow in pH just
below neutral environments. Its buffer
should have a pH of 6.25 ± 0.5 with the
ability to stay within one pH unit of this
target when 20.0 mL of strong acid or
base is added.
YOUR BUFFER
Prepare a buffer for an antifungal agent.
Your agent is designed for use against a
fungus that attacks food sources that
grow in basic soil. Its buffer should have
a pH of 9.00 ± 0.5 with the ability to
stay within one pH unit of this target
when 20.0 mL of strong acid or base is
added.
YOUR BUFFER
Prepare a buffer for an antiviral agent.
Your agent is designed for use against a
strain of virus that attacks