Buffers. The ph range in which a buffer solution is effective is generally considered to be ±1 of the pk a

Computer Buffers 19 A buffer is a mixture of a weak acid and its conjugate base, or a weak base and its conjugate acid. A buffer’s function is to a...
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Buffers

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A buffer is a mixture of a weak acid and its conjugate base, or a weak base and its conjugate acid. A buffer’s function is to absorb acids (H+ or H3O+ ions) or bases (OH– ions) so that the pH of the system changes very, very little. In many systems, buffers are critical. Blood plasma, a natural example in humans, is a bicarbonate buffer that keeps the pH of blood between 7.2 and 7.6. By design, a buffer is an equilibrium system. For example, a buffer can be prepared with nitrous acid, HNO2. The weak acid establishes an aqueous equilibrium as shown below. HNO2 (aq) ↔ H+ (aq) + NO2– (aq) The equilibrium constant expression is shown below. Ka =

[H + ][ NO -2 ] [HNO 2 ]

To prepare a buffer system with nitrous acid, a conjugate base is added, such as sodium nitrite (NaNO2). The resulting system is a mixture of HNO2 and NO2– ions. The nitrous acid molecule will neutralize hydroxide ions and the nitrite ion from the conjugate will neutralize hydrogen ions. A variation of the equilibrium expression above, called the Henderson-Hasselbalch equation, is the best reference in preparing a buffer solution. For our nitrous acid/sodium nitrate buffer example, the Henderson-Hasselbalch equation is shown below. pH = pK a + log

[ NO −2 ] [ HNO 2 ]

The pH range in which a buffer solution is effective is generally considered to be ±1 of the pKa. In this experiment, you will use the Henderson-Hasselbalch equation to determine the amount of acetic acid and sodium acetate needed to prepare two acidic buffer solutions. You will then prepare the buffers and test their buffer capacities by adding solutions of NaOH and HCl.

OBJECTIVES In this experiment, you will • •

Prepare and test two acid buffer solutions. Determine the buffer capacity of the prepared buffers.

Advanced Chemistry with Vernier

© 2006 Vernier Software & Technology

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Computer 19

MATERIALS Vernier computer interface computer Vernier pH Sensor magnetic stirrer and stirring bar three 250 mL beakers 100 mL graduated cylinder 25 mL graduated cylinder two 50 mL burets and two buret clamps balance

0.5 M sodium hydroxide, NaOH, solution 0.5 M hydrochloric acid, HCl, solution 0.1 M acetic acid, HC2H3O2, solution 1.0 M acetic acid, HC2H3O2, solution solid sodium acetate, NaC2H3O2 ring stand utility clamp distilled water

PRE-LAB EXERCISE Use the Henderson-Hasselbalch equation to perform the following calculations. The Ka of acetic acid is 1.8 × 10–5. Review your calculations with your instructor before preparing the buffer solutions. •

Buffer A: Calculate the mass of solid sodium acetate required to mix with 100.0 mL of 0.1 M acetic acid to prepare a pH 4 buffer. Record the mass in your data table. • Buffer B: Calculate the mass of solid sodium acetate required to mix with 100.0 mL of 1.0 M acetic acid to prepare a pH 4 buffer. Record the mass in your data table.

PROCEDURE Part I Prepare and Test Buffer Solution A

1. Obtain and wear goggles. 2. Use your calculations from the Pre-Lab Exercise to prepare 100 mL of Buffer A. Weigh out the precise mass of sodium acetate and dissolve it in 100.0 mL of 0.1 M acetic acid solution. 3. Set up two burets, buret clamps, and ring stand (see Figure 1). Rinse and fill one buret with 0.5 M NaOH solution. Rinse and fill the second buret with 0.5 M HCl solution. CAUTION: Sodium hydroxide solution is caustic. Avoid spilling it on your skin or clothing. Handle the hydrochloric acid with care. It can cause painful burns if it comes in contact with the skin. 4. Use a graduated cylinder to measure out 10.0 mL of the Buffer A solution into a 250 mL beaker and add 15 mL of distilled water. Place the beaker on a magnetic stirrer, beneath the buret of NaOH, and add a stirring bar. If no magnetic stirrer is available, you will stir with a stirring rod during the testing. 5. Connect a pH Sensor to Channel 1 of the Vernier computer interface. Connect the interface to the computer using the proper interface cable. Suspend the pH Sensor in the pH 4 buffer solution, as shown in Figure 1. Make sure that the sensor is not struck by the stirring bar. 6. Start the Logger Pro program on your computer. Open the file “19 Buffers” from the Advanced Chemistry with Vernier folder.

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Advanced Chemistry with Vernier

Buffers

Figure 1 7. You are now ready to test Buffer A. You will slowly and carefully add 0.5 M NaOH solution to the buffer solution. a. Take an initial pH reading of the buffer solution. Click and monitor pH for 5-10 seconds. Once the displayed pH reading has stabilized, click . In the edit box, type “0” (for 0 mL added). Press the ENTER key to store the first data pair. Record the initial pH value in your data table. b. Add a small amount of the NaOH solution, up to 0.50 mL. When the pH stabilizes click . Enter the current buret reading and press ENTER to store the second data pair. c. Continue adding the NaOH solution in small increments that raise the pH consistently and enter the buret reading after each increment. Your goal is to raise the pH of the buffer by precisely 2 pH units. d. When the pH of the buffer solution is precisely 2 units greater than the initial reading, continue to add the NaOH solution in small increments until you have reached, and passed, the equivalence point of the titration. e. Click . Print a copy of the first trial. 8. Dispose of the reaction mixture as directed. Rinse the pH sensor with distilled water in preparation for the second titration. 9. Repeat Steps 7 and 8, using a fresh 10.0 mL sample of the Buffer A solution. For this second trial, titrate the buffer with 0.5 M HCl solution. Carefully add HCl in small increments until the pH of the solution has been lowered by precisely 2 units. Record, in your data table, the volume of HCl that was used. There is no need to print a copy of the graph. Part II Prepare and Test Buffer Solution B

10. Use your calculations from the Pre-Lab Exercise to prepare 100 mL of Buffer B. Weigh out the precise mass of sodium acetate and dissolve it in 100.0 mL of 1.0 M acetic acid solution. If necessary, refill the burets of NaOH and HCl solution. 11. Use a graduated cylinder to measure out 10.0 mL of the Buffer B solution and add 15 mL of distilled water. Repeat the necessary steps to test Buffer B in a manner similar to the Part I trials. Print a copy of your graph of the titration using the NaOH solution. Record the volume of HCl that was used to lower the pH of Buffer B by 2 units; no printed graph is needed.

Advanced Chemistry with Vernier

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Computer 19

DATA TABLE Buffer A

Buffer B

100.0

100.0

0.1

1.0

Mass of NaC2H3O2 used to prepare buffer (g) Volume of buffer prepared (mL) Molar concentration of HC2H3O2 in buffer (M) Initial pH of buffer Volume of 0.5 M NaOH to raise pH by 2 units (mL) Volume of 0.5 M HCl to lower pH by 2 units (mL) Volume of 0.5 M NaOH at equivalence point (mL)

DATA ANALYSIS 1. Write reaction equations to explain how your acetic acid-acetate buffer reacts with an acid and reacts with a base.

2. Buffer capacity has a rather loose definition, yet it is an important property of buffers. A commonly seen definition of buffer capacity is: “The amount of H+ or OH– that can be neutralized before the pH changes to a significant degree.” Use your data to determine the buffer capacity of Buffer A and Buffer B.

3. Say, for example, that you had prepared a Buffer C, in which you mixed 8.203 g of sodium acetate, NaC2H3O2, with 100.0 mL of 1.0 M acetic acid. a. What would be the initial pH of Buffer C? b. If you add 5.0 mL of 0.5 M NaOH solution to 20.0 mL each of Buffer B and Buffer C, which buffer’s pH would change less? Explain.

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Advanced Chemistry with Vernier

Experiment

19

TEACHER INFORMATION

Buffers 1. This experiment conforms to the guidelines for the 19th laboratory experiment listed in the College Board AP Chemistry guide (the Acorn book). 2. Prepare the 0.50 M sodium hydroxide solution by dissolving 20.0 g of solid NaOH in about 800 mL of distilled water, and then dilute the solution to 1.0 L. 3. Prepare the 0.50 M hydrochloric acid solution by diluting 41 mL of concentrated HCl in sufficient distilled water to make 1.0 L of solution. 4. Prepare the 0.10 M acetic acid solution by adding 5.8 mL of glacial acetic acid to sufficient water to make 1.0 L of solution. 5. Prepare the 1.0 M acetic acid solution by adding 58 mL of glacial acetic acid to sufficient water to make 1.0 L of solution. 6. The sodium acetate may be either the hydrate or anhydrous form. Inform your students of the type they are using so that they will successfully complete the pre-lab calculations. The molar mass of anhydrous NaCH3COO is 82.03 g/mol. The formula mass of NaCH3COO•3H2O is 136.08 g/mol. 7. The stored calibration for the pH Sensor in the data collection programs (Logger Pro, DataMate, or Data Pro) will provide satisfactory results. 8. In this experiment, your students will not be conducting a full titration. They will add NaOH or HCl solution to a buffer solution until the pH changes by 2 units from the initial pH reading. Tabular data is important. You may require your students to complete a full titration, especially if they have conducted previous acid-base titrations. The results of a buffer titration provide interesting information when compared to a standard acid-base titration. 9. You may include a basic buffer in this experiment. A good choice is an ammonia/ammonium chloride solution.

HAZARD ALERTS Sodium hydroxide: Corrosive solid; skin burns are possible; much heat evolves when added to water; very dangerous to eyes; wear face and eye protection when using this substance. Wear gloves. Hazard code: B—Hazardous. Acetic acid: Corrosive to skin and tissue; moderate fire risk (flash point: 39°C); moderately toxic by ingestion and inhalation. Hazard Code: A—Extremely hazardous. Sodium acetate: Skin, eye, and respiratory irritant. Hazard Code: D—Relatively non-hazardous. The hazard information reference is: Flinn Scientific, Inc., Chemical and Biological Catalog Reference Manual P.O. Box 219, Batavia, IL 60510, (800) 452-1261, www.flinnsci.com.

Advanced Chemistry with Vernier

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Experiment 19

ANSWERS TO THE PRE-LAB EXERECISE Buffer A: The mass of anhydrous sodium acetate needed to prepare 100 mL of pH 4 buffer solution is 0.15 g. By solving the Henderson-Hasselbalch equation, the concentration of sodium acetate was calculated as 0.018 M. X represents the molarity of sodium acetate. 4.0 = 4.74 + log

XM 0.10 M

Using the quotient property of logarithms, this expression can be written as shown below. –0.74 = log X – log 0.10 X = 0.018 M Mass of Na C2H3O2 = (82.03 g/mol) × (0.018 mol/L) × (0.100 L) = 0.15 g Buffer B: The mass of anhydrous sodium acetate needed to prepare Buffer B is 1.5 g. The calculation is preformed as shown above, using 1.0 M as the acetic acid molarity.

ANSWERS TO THE DATA ANALYSIS QUESTIONS

Answers have been removed from the online versions of Vernier curriculum material in order to prevent inappropriate student use. Graphs and data tables have also been obscured. Full answers and sample data are available in the print versions of these labs.

SAMPLE DATA

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Buffer A

Buffer B

Mass of NaC2H3O2 used to prepare buffer (g)

xxxx

xxxx

Volume of buffer prepared (mL)

xxxx

xxxx

Molar concentration of HC2H3O2 in buffer (M)

xxxx

xxxx

Initial pH of buffer

xxxx

xxxx

Volume of 0.5 M NaOH to raise pH by 2 units (mL)

xxxx

xxxx

Volume of 0.5 M HCl to lower pH by 2 units (mL)

xxxx

xxxx

Volume of 0.5 M NaOH at equivalence point (mL)

xxxx

xxxx

Advanced Chemistry with Vernier

Buffers

SAMPLE GRAPH

Advanced Chemistry with Vernier

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