Lab 6: Buffer Solutions
Objective : Learn to make buffer solutions and study buffer characteristics.
Introduction:
The pH scale is used to indicate the acidity of a solution, where values near 1 are very acidic, values around 7 are neutral, and values near 14 are basic. A normal range for the pH of blood is 7.35 to 7.45. Values outside of this range can negatively impact critical functions of the body. For example, if a patient’s blood is to acidic, with a pH value below 7.35, they will experience acidosis and proteins can denature. If blood pH values are too far outside the normal range, such as a pH of 6.8, life cannot be sustained.
Blood needs to maintain a narrow pH range, however many of the waste products of metabolism are acidic. For example, blood transports uric acid, a by-product of protein metabolism, to the kidneys. To maintain the ideal pH a buffer system of carbonic acid (H2CO3) and bicarbonate (HCO3-) is present in blood plasma.
H2CO3 + H2O H3O+ + HCO3-
A buffer is composed of a weak acid and it’s conjugate base and is capable of reacting with either strong acids or strong bases that need to be transported by the blood to prevent changes in pH. As acidic waste products react with the conjugate base (bicarbonate) of the buffer system more of the weak acid (carbonic acid) of the buffer system forms. As carbonic acid forms it will decompose into carbon dioxide (CO2) and water (H2O), the CO2 is then removed from the blood through respiration.
2 H2O + CO2 H2CO3
This connection to the respiratory system allows the blood’s buffer system to avoid hitting its buffer capacity and remain in a normal pH range.
Pre Lab Assignment: (must be completed before starting lab)
Ø Name, date, title of lab
Ø Summary of lab procedure (brief paragraph in your own words)
Ø Answers to pre-lab Questions:
1. (In your own words) What is a buffer and how does it work?
2. Refer to the buffer system in your blood that is described above. What is the chemical formula of the weak acid in the buffer system? What is the chemical formula of the weak base in the buffer system?
3. Write a chemical equation for the Acetic Acid buffer system you will be working with in lab. Label the weak acid and the conjugate base.
Procedure :
Part 1: Buffer capacity when adding a strong acid
1. Make acetic acid / acetate buffer solution in a small 50 mL beaker #1 by mixing 5 mL of 0.100 M acetic acid with 5 mL of 0.100M sodium acetate.
2. Make carbonic acid / bicarbonate buffer solution in a 100 mL beaker #2 by mixing 5 mL of 0.100 M carbonic acid with 5 mL 0.100 M sodium bicarbonate.
3. Obtain 10 ml of distilled water in beaker #3.
4. Starting with beaker #1
5. Measure and record pH
6. Add 2 drops of 0.500M HCl, measure and record pH
7. Add additional 2 drops of 0.500M HCl, measure and record pH. Repeat until total of 12 drops are added.
8. Repeat steps 5-8 for beaker #2, and then for beaker #3. Make sure to rinse the probe between beakers.
Part 2: Buffer capacity when adding a strong base
1. Make acetic acid / acetate buffer solution in a small 50 mL beaker #1 by mixing 5mL of 0.100M acetic acid with 5mL of 0.100M sodium acetate.
2. Make carbonic acid / bicarbonate buffer solution in a 100 mL beaker #2 by mixing 5 mL of 0.100 M carbonic acid with 5 mL 0.100 M sodium bicarbonate.
3. Obtain 10 ml of distilled water in beaker #3.
4. Starting with beaker #1
5. Measure and record pH
6. Add 2 drops of 0.500 M NaOH, measure and record pH
7. Add additional 2 drops of 0.500M NaOH, measure and record pH. Repeat until total of 12 drops are added.
8. Repeat 6-8 for beaker 2, and then for beaker 3. Make sure to rinse the probe between beakers.
Lab 6: Data & Observations Worksheet
Part 1: Data Table & Graph
(record data using correct significant figures)
Drops of HCl added / pH Beaker 1Acetic Acid / pH Beaker 2
Carbonic Acid / pH Beaker 3
Water
0 drops
Plot drops of acid vs pH for all three buffers in the graph below:
Part 2: Data Table & Graph
(record data using correct significant figures)
Drops of NaOH added / pH Beaker 1Acetic Acid / pH Beaker 2
Carbonic Acid / pH Beaker 3
Water
0 drops
Plot drops of base vs pH for all three buffers in the graph below:
Lab 6: Post Lab Report
Results: Complete Data & Observations Worksheet, including graphs of each buffer solution’s pH as acid or base is added.
Error Analysis Discussion: Compare the 3 buffer solutions and their effectiveness as acid or base is added. Discuss any sources of error for your results, please be specific.
Post lab Questions: State the answer to the question followed by a brief description of the evidence supporting that answer, unless the question is asking for you to write a chemical reaction or give a numerical answer.
1. According to your graphed data is one buffer (beaker 1-3) better or worse than the other(s)? Explain.
2. How can you tell if the buffer is working?
3. What happens when you go beyond the buffer capacity? How is buffer capacity indicated on your graph(s)?
4. Write out the chemical equation for the reaction that occurs in the Carbonic Acid / Bicarbonate buffer (blood buffer) when
a. NaOH is added
b. HCl is added