EXPERIMENT 6 Properties of Buffers INTRODUCTION A chemical buffer is any substance in a solution that tends to stabilize the hydronium ion concentration by neutralizing any added acid or base. Buffers are used in many ways: to keep the ph level in living organisms constant when drastic changes would imperil life; to control the ph of biological growth media; to maintain ph control in manufacturing processes; or to study the effects of drugs. In this lab you will (1) measure the ph of solutions containing acetic acid, sodium acetate and a combination of acetic acid and sodium acetate; (2) measure the ph of these solutions again after adding a small amount of strong acid or base, and (3) decide whether or not the solutions tested are buffers. To more thoroughly explore how buffers work, we must explore the common ion effect. The equilibrium reaction, for any buffer, may be written as the acid ionization (K a ) equilibrium: HA (aq) + H 2 O (l) H 3 O + (aq) + A (aq) The presence of the conjugate base shifts the equilibrium concentrations to the left, decreasing the hydronium ion concentration compared to a pure weak acid. The ph of the buffer solution can be calculated with the K a of the weak acid, and the concentrations of weak acid and conjugate base in the solution. When adding base, OH, to this mixture, the weak acid neutralizes the strong base and the conjugate base concentration increases while the weak acid concentration decreases: HA (aq) + OH (aq) H 2 O (l) + A (aq) This leaves the hydronium ion concentration in the buffer solution virtually unchanged. The ph of the resulting solution can be calculated with the K a of the weak acid, and new concentrations of weak acid and conjugate base. Similarly when strong acid is added to the buffer, the base neutralizes the hydronium ions producing more weak acid while decreasing the conjugate base concentration: A (aq) + H 3 O + (aq) HA (aq) + H 2 O (l) Here again the hydronium ion concentration in the buffer solution virtually unchanged. The ph of the resulting solution can be calculated with the K a of the weak acid, and new concentrations of weak acid and conjugate base. In this experiment you will (1) investigate the components of a solution that are necessary to produce a buffer, and (2) test the effect of adding HCl and NaOH to buffer and non-buffer solutions. 51
PROCEDURE 1. Students will work individually for this experiment. Except for the laboratory handout, remove all books, purses, and such items from the laboratory bench top, and placed them in the storage area by the front door. For laboratory experiments you should be wearing closed-toe shoes. Tie back long hair, and do not wear long, dangling jewelry or clothes with loose and baggy sleeves. Open you lab locker. Put on your safety goggles, your lab coat, and gloves. 2. If the laptop computer is turned on, proceed to step 3. If the computer is off, turn it on, and when the Log On dialogue box appears, for User Name: type in your Saddleback (or IVC) email address, and for Password: type in your Saddleback (or IVC) password. If any other dialogue boxes appears, select No or Cancel or Close. Double click on Logger Pro. If a Setup Interface dialogue boxes appears, under Port: select LabPro-USB, then select OK. 3. Open Logger Pro if it is not already open. Go to the Experiment menu and under Calibrate choose the channel with the ph probe (ex: CH1:pH ). In the window that appears make sure the Calibration tab is chosen. Click on Calibrate Now. Rinse the ph meter with copious amounts of deionized water. Carefully blot dry. CAUTION: Do not let the ph meter hit against anything. The thin glass bulb inside of the plastic protective shell is very fragile. Place the ph meter in the ph 4 standard solution. Observe the voltage reading, found in the middle of the new window that opened up, and wait for the voltage reading to stabilize. In the field beneath Enter Value enter the ph value of the solution (4.00, not the voltage reading!) and click. Rinse the ph meter with copious amounts of deionized water. Carefully blot dry. Place the ph meter in the ph 10 standard solution and wait for the voltage reading to stabilize. In the field beneath Enter Value enter the ph value of the solution (10.00, not the voltage reading!) and click. When finished with this step click Done to close the window. 4. When not in use, place the ph sensor back into its aqueous potassium chloride storage solution. PART A ACETIC ACID SOLUTION 5. Prepare a 0.1 M acetic acid solution by measuring out 5.0 ml of the 1.0 M acetic acid solution in a 100 ml graduated cylinder, diluting the 5.0 ml to a final volume of 50.0 ml with deionized water, and then stirring. Pour this solution into a clean, dry 100 ml breaker. NOTE: Do not under any circumstance put equipment into reagent bottles or pour any unused reagents back into bottles. Use a small beaker to obtain the quantity of solution needed, and refill the beaker as required. Do not waste the chemicals. As you perform the lab, collect all waste solutions and pour them into a large beaker. All waste from this experiment can be disposed of down the sink at the end of the laboratory period. By knowing that the K a for acetic acid is 1.8 x 10-5, calculate the theoretical ph of the solution. Record this value in your Data Table. 6. Rinse the ph electrode thoroughly with deionized water and gently pat it dry before using it to measure the ph of a solution. Use only deionized water for rinsing the ph electrodes. Do not use the ph probe to stir solutions. Collect all rinsing s in your waste beaker, which can be disposed of down the sink at the end of the laboratory period. 7. Place the ph probe into the solution. Allow 15 to 30 seconds for the reading to stabilize, and then record your measurement. Read and record the ph values from the computer to two decimal places. 52
8. Measure out 1.00 ml of 1.0 M hydrochloric acid and add it to the acetic acid solution. Stir the mixture. Calculate the molarities of the acetic acid and the hydrochloric acid in the 51.0 ml solution, and after their reaction, calculate the theoretical ph of the solution. Record this value in your Data Table. 9. Measure and record the ph of the solution, following the directions in steps 6 and 7. 10. Prepare another 50.0 ml sample of the solution 0.10 M acetic acid, following the directions in step 5. 11. Measure out 1.00 ml of 1.0 M sodium hydroxide and add it to the acetic acid solution. Stir the mixture. Calculate the molarities of the acetic acid and the sodium hydroxide in the 51.0 ml solution, and after their reaction, calculate the theoretical ph of the solution. Record this value in your Data Table. 12. Measure and record the ph of the solution, following the directions in steps 6 and 7. PART B SODIUM ACETATE SOLUTION 13. Prepare a 0.1 M sodium acetate solution by measuring out 5.0 ml of the 1.0 M sodium acetate solution in a 100 ml graduated cylinder, diluting the 5.0 ml to a final volume of 50.0 ml with deionized water, and then stirring. Pour this solution into a clean, dry 100 ml breaker. By determining the K b for the acetate ion, calculate the theoretical ph of the solution. Record this value in your Data Table. 14. Measure and record the ph of the solution, following the directions in steps 6 and 7. 15. Measure out 1.00 ml of 1.0 M hydrochloric acid and add it to the sodium acetate solution. Stir the mixture. Calculate the molarities of the sodium acetate and the hydrochloric acid in the 51.0 ml solution, and after their reaction, calculate the theoretical ph of the solution. Record this value in your Data Table. 16. Rinse the ph electrode thoroughly with deionized water and gently pat it dry before using it to measure the ph of a solution. 17. Place the ph probe into the solution. Allow 15 to 30 seconds for the reading to stabilize, and then record your measurement. Read and record the ph values from the computer to two decimal places. 18. Measure and record the ph of the solution, following the directions in steps 16 and 17. 19. Prepare another 50.0 ml sample of the 0.10 M sodium acetate solution, following the directions in step 12. 20. Measure out 1.00 ml of 1.0 M sodium hydroxide and add it to the sodium acetate solution. Stir the mixture. Calculate the molarities of the sodium acetate and the sodium hydroxide in the 51.0 ml solution, and calculate the theoretical ph of the solution. 21. Measure and record the ph of the solution, following the directions in steps 16 and 17. 53
PART C ACETIC ACID AND SODIUM ACETATE SOLUTION 22. Prepare a solution that is 0.1 M acetic acid and 0.1 M sodium acetate by measuring out 5.0 ml of the 1.0 M acetic acid solution and 5.0 ml of the 1.0 M sodium acetate solution in a 100 ml graduated cylinder, diluting the 10.0 ml to a final volume of 50.0 ml with deionized water, and then stirring. Pour this solution into a clean, dry 100 ml breaker. By knowing that the K a for acetic acid is 1.8 x 10-5, calculate the theoretical ph of the solution. Record this value in your Data Table. 23. Rinse the ph electrode thoroughly with deionized water and gently pat it dry before using it to measure the ph of a solution. 24. Place the ph probe into the solution. Allow 15 to 30 seconds for the reading to stabilize, and then record your measurement. Read and record the ph values from the computer to two decimal places. 25. Measure out 1.00 ml of 1.0 M hydrochloric acid and add it to the acetic acid/sodium acetate solution. Stir the mixture. Calculate the molarities of the acetic acid, sodium acetate and the hydrochloric acid in the 51.0 ml solution, and after their reaction, calculate the theoretical ph of the solution. Record this value in your Data Table. 26. Measure and record the ph of the solution, following the directions in steps 23 and 24. 27. Prepare another 50.0 ml sample of the solution 0.10 M in acetic acid and 0.10 M in sodium acetate, following the directions in step 22. 28. Measure out 1.00 ml of 1.0 M sodium hydroxide and add it to the acetic acid/sodium acetate solution. Stir the mixture. Calculate the molarities of the acetic acid, sodium acetate and the sodium hydroxide in the 51.0 ml solution, and after their reaction, calculate the theoretical ph of the solution. Record this value in your Data Table. 29. Measure and record the ph of the solution, following the directions in steps 23 and 24. 30. All solutions can be washed down the sink with water. 31. Clean and wipe dry your laboratory work area and all apparatus. When you have completed your lab report have the instructor inspect your working area. Once your working area has been checked your lab report can then be turned in to the instructor. 54
EXPERIMENT 6 LAB REPORT Name: Student Lab Score: Date/Lab Start Time: Lab Station Number: DATA TABLE PART A 1 Theoretical ph of 0.10 M Acetic Acid. Experimental ph of 0.10 M Acetic Acid. 2 Theoretical ph with the Addition of HCl. Experimental ph with the Addition of HCl. 3 Theoretical ph with the Addition of NaOH. Experimental ph with the Addition of NaOH. PART B 4 Theoretical ph of 0.10 M Sodium Acetate. Experimental ph of 0.10 M Sodium Acetate. 5 Theoretical ph with the Addition of HCl. Experimental ph with the Addition of HCl. 6 Theoretical ph with the Addition of NaOH. Experimental ph with the Addition of NaOH. PART C 7 Theoretical ph of 0.10 M Acetic Acid/0.10 M Sodium Acetate. Experimental ph of 0.10 M Acetic Acid/0.10 M Sodium Acetate. 8 Theoretical ph with the Addition of HCl. Experimental ph with the Addition of HCl. 9 Theoretical ph with the Addition of NaOH. Experimental ph with the Addition of NaOH. 55
CALCULATIONS 1. 2. 56
3. 4. 57
5. 6. 58
7. 8. 59
9. 60
QUESTIONS 1. A buffer is a solution that is resistant to a change in the hydronium ion concentration, and thus resistant to a change in the ph. Which one of the three solutions tested in Parts A, B, and C acted as a buffer? 2. What two components must be generally present to enable a solution to act as a buffer? (1) (2) 3. Consider a solution containing equal concentrations of HCl and NaCl. Could this solution be used as a buffer? Explain. 61
3. A solution of nitrous acid and potassium nitrite acts as a buffer due to reactions that occur within the solution when a strong acid or a strong base is added. (a) Write the net ionic equation for the reaction that occurs in this buffer to react away any added HCl (aq). (b) Write the net ionic equation for the reaction that occurs in this buffer to react away any added NaOH (aq). 62