Exercise 2-4. Titration of a Buffer Solution EXERCISE OBJECTIVES
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1 Exercise 2-4 Titration of a Buffer Solution EXERCISE OBJECTIVES To define the terms buffer solution and buffer capacity; To titrate a buffer solution with a weak acid solution; To plot a graph using the titration data; To analyze the curve of a buffer solution titrated with a weak acid solution. DISCUSSION Buffer solutions A buffer solution consists in an aqueous solution containing a high concentration of a weak acid, HA, and its conjugate base, A -. The dissociation equation of the weak acid can be written as: The equations giving the dissociation constant of the weak acid can be rewritten as follows: Similarly, the equilibrium equation describing the reaction of the conjugate base with water can be written as: And the dissociation constant of the conjugate base reaction can be rewritten as: In both cases, the concentration of hydronium, [H 3 O + ], and hydroxyl, [OH - ], depends on the ratio of the concentration of weak acid to the concentration of conjugate base (and vice versa). Consequently the ph of the solution depends on the same ratio. With high concentrations of the weak acid and conjugate base, the ratio will remain approximately the same, even if a small quantity of acid or a base is added to the buffer solution. The hydronium, H 3 O +, produced by a small quantity of acid added to the buffer solution, will react with the conjugate base, A -, to form weak acid, HA. Consequently, the ratio will remain approximately constant. 2-63
2 Likewise, if a small quantity of base is added to the buffer solution, the hydroxyl, OH -, will react with the weak acid, HA, to form the conjugate base, A -, and water. Again, the ratio will remain approximately constant. The quantity of acid or base added to a buffer solution that can be absorbed without significantly affecting the ph is limited. The buffer capacity,, of the solution quantifies this absorption limit. It gives the ratio of strong acid (or strong base) that can be added to a buffer solution before a significant change in ph is observed. The buffer capacity is measured in mol/l. This represents the number of mole(s) of acid or base necessary to change the ph of the buffer solution by one unit. The formal definition of the buffer capacity is: Where dc acid is the variation of the concentration of strong acid; dc base is the variation of the concentration of strong base; dph is the variation of the ph. The variation of the concentration dc acid and dc base produce a variation dph of the ph of the buffer solution. The exact solution to this equation can be complex to calculate and use. However, when working with a buffer solution around the half-equivalence point (i.e. ph = pk a) ) an approximate solution can be used. This approximate solution is: Note: ln 10 is the natural logarithm of 10 and is approximately equal to 2.3. Remember that the ph of a buffer solution is given by the equation: Take for example a buffer solution made of ammonia, NH 3, and ammonium,. The dissociation constant of ammonium is pk a = To have a ph 10 buffer solution, the ratio of ammonia to ammonium must be calculated: The ratio to use to create such a buffer solution is: 2-64
3 A buffer solution will always have a ph value of 10 if this proportion is respected. However, it will not have the same buffer capacity. A solution with a concentration of ammonia equal to 5.6 mol/l and a concentration of ammonium of 1 mol/l will have a buffer capacity of: Which means that it will take 1.95 mole of strong acid to change the ph value of one liter of buffer solution from 10 to 9. If the concentration of ammonia and the concentration of ammonium are ten times smaller, the buffer capacity of the solution will be = Sodium bicarbonate In this exercise, a solution of sodium bicarbonate will be used as a buffer solution. As it has been mentioned in Unit 1, buffer solution usually contain a weak acid and its conjugate base or a weak base and its conjugate acid. To understand how a sodium bicarbonate solution (a salt) can act as a buffer, one must remember that there is carbon dioxide in the air and that carbon dioxide is soluble in water. Since water is almost always in contact with air, carbon dioxide is present in aqueous solutions: When in solution, carbon dioxide reacts with water to form carbonic acid, H 2 CO 3 : Hence, the carbon dioxide dissolved into water provides the weak acid (HA) required to create a buffer. When sodium bicarbonate is added to an aqueous solution, it dissolves into a sodium ion, Na +, and bicarbonate, to create a buffer:, providing the conjugate base (A - ) required Thus, dissolved carbon dioxide reacting with water provide the weak acid (H 2 CO 3 ) and dissolved sodium bicarbonate provide the conjugate base (HCO 3 - ). Hence the pair HA/A - required to create a buffer is present in the solution. The equilibrium equations describing the buffer solution are: 2-65
4 Procedure summary In this exercise, you will use the Process Control Training System to titrate water containing an alkaline buffer solution with a weak acid solution. You will use the acquired data to plot a titration curve. EQUIPMENT REQUIRED Refer to the Equipment Utilization Chart in Appendix A of the manual to obtain the list of equipment required to perform this exercise. PROCEDURE Preliminary setup G 1. Get the Expanding Work Surface from your storage location and mount it vertically to the Main Work Surface (at an angle of 90 ), if this has not already been done. G 2. Use the same setup as for Exercise 2-2 (refer to Figure 2-15 and Figure 2-16). Note: Refer to Figure B-2 of Appendix B for details on how to connect the Lab-Volt Process Control and Simulation Software (LVPROSIM), Model 3674, to the ph Transmitter, Model 6544, the Set Point Device, Model 6561, and the Metering Pump Drive, Model CAUTION! Mount the Chemical Tanks and the Column as shown in Figure Place electrical components as far as possible from them. Failure to do so may result in water entering the modules upon disconnection of the hoses, which in turn might cause damage to electrical components. CAUTION! Mount the 24-V DC Power Supply and the ph Transmitter in such a manner that water cannot enter their components and electrical terminals upon disconnection of the hoses. 2-66
5 G 3. Make the following settings: On the Metering Pump Drive: S1 switch SC 1 manual control knob turned fully counterclockwise S2 switch pulsed mode SC 1 pulse width adjustment knobs % S3 switch SC 2 manual control knob turned fully counterclockwise S4 switch pulsed mode SC 2 pulse width adjustment knobs % On the ph Transmitter: SELECTOR switch ph PROBE CALIBRATION SELECTOR switch FIXED Preparation of the CH 3 COOH and NaHCO 3 solutions G 4. If there is any liquid left in one of the Chemical Tanks, dispose of it safely and wash the tank carefully. Note: Make sure to expel any solution remaining in the tubing of the tanks and Metering Pumps using water. G 5. Calculate the volume of Acetic Acid 5% (Vinegar) required to make 2000 ml of a 0.08 mol/l solution of CH 3 COOH. Required volume of Acetic Acid 5% (Vinegar): ml Note: Confirm this value with your instructor before proceeding further. G 6. Prepare 2000 ml of a 0.08 mol/l solution of CH 3 COOH. G 7. Fill the first Chemical Tank with the CH 3 COOH solution and carefully identify the contents of the Chemical Tank with a HMIG paper label. G 8. Carefully wash the glassware. G 9. Calculate the quantity of sodium bicarbonate required to make 2000 ml of a 0.08 mol/l solution of NaHCO 3. Required quantity of Sodium bicarbonate: g Note: Confirm this value with your instructor before proceeding further. 2-67
6 G 10. Weigh the proper amount of sodium bicarbonate and transfer it into the volumetric flask. Add a small amount of water and swirl it in the flask until the sodium bicarbonate is dissolved. Add water into the volumetric flask until it almost reaches the etched mark on the neck. Use a pipette to add water until the bottom of the meniscus reaches the mark. The flask should now be filled with 2000 ml of a 0.08 mol/l solution of sodium bicarbonate. G 11. Fill the second Chemical Tank with the NaHCO 3 solution and carefully identify the contents of the Chemical Tank with a HMIG paper label. Filling the Column with water CAUTION! To avoid water and chemical spills all over the Process Control Training System, make sure the ph probe is properly inserted into the port at the top of the Flow Chamber before starting the Pumping Unit. G 12. Make sure the reservoir of the Pumping Unit is filled with about 12 liters (3.2 gallons US) of water. Make sure the baffle plate is properly installed at the bottom of the reservoir. G 13. Use Figure 2-15 and 2-16 and connection diagram B-2 of Appendix B to make the appropriate setup. G 14. Turn on the Pumping Unit by setting its POWER switch at I. G 15. On the Pumping Unit, adjust valves HV1 to HV3 as follows: close HV1 completely (turn handle fully clockwise); close HV2 completely (turn handle fully clockwise); set HV3 for directing the full reservoir flow to the pump inlet (turn handle fully clockwise). G 16. Adjust the pump speed to 60-70% of its maximum by setting the Set Point Device output between 3.00 V and 3.50 V. G 17. Allow the level of water to rise in the Column until it reaches 38 cm (15 in). 2-68
7 Placing the system in recirculating mode G 18. Once the proper water level is reached, rapidly adjust HV3 to stop water flow from the reservoir and direct the full return flow to the pump inlet (turn the handle fully counterclockwise). G 19. The Column is now in recirculating mode. Water is pumped to the Pumping Unit outlet, passes through the Flow Chamber, goes into the Column, and flows out of the Column through one of the bottom outlets to be directed to the pump inlet again. G 20. Make sure the two Chemical Tanks are filled with the proper chemicals. G 21. On the Pumping Unit, open HV2 and let the water level in the Column decrease to 15 cm (6 in). As soon as the water reaches the proper level, close HV2. Operation of the ph Transmitter in the fixed calibration mode G 22. Power up the ph Transmitter using the DC Power Supply. G 23. Turn on the Metering Pump Drive. G 24. Make sure the water is properly circulating through the system and that the Metering Pumps are not running (the SC1 and SC2 manual control knobs are turned fully counterclockwise). G 25. Use a funnel to add about 5 ml of Phenol Red Aqueous solution 0.05% in the Column. G 26. Have the signal at the 0-5 V OUTPUT of the ph Transmitter and the ANALOG OUTPUT 1 of the I/O Interface plotted on the trend recorder. Note: Refer to Figure B-2 of Appendix B for details on how to connect the LVPROSIM computer to the ph Transmitter. On the I/O Interface, make sure the RANGE switch of ANALOG INPUT 1 is set at 5 V. In LVPROSIM, select Analog Input 1 from the Trend Recorder selection list to have the ph Transmitter signal plotted on the trend recorder. Set the LVPROSIM sampling interval at 500 ms. Access the Configure Analog Inputs window and set the minimum and maximum range values of Analog Input 1 at a ph value of 0 and 12 respectively, which corresponds to the measurement range of the ph Transmitter. Set the filter time constant of this input at 0.5 second. Make sure the square root extracting function is unselected. Accept setup and return to main screen. 2-69
8 G 27. On the trend recorder, observe the ph Transmitter output signal. Since neither acid nor base has been added to the water in the Column, theoretically, the water ph value should be 7.0. Record below the initial ph value as detected by the ph probe. Initial ph: G 28. On the Metering Pump Drive, make sure the S1 switch is set to 1. G 29. On the controller, initiate data saving in order to start saving the data used to plot the controller output and ph Transmitter output signals. Note: If the controller you are using is the Lab-Volt Process Control and Simulation Software (LVPROSIM), Model 3674, start data saving by clicking the box next to the disk icon in the upper left-hand corner of the trend recorder display area. The data will be saved to disk as a.txt format file and later be imported into a spreadsheet software. Refer to Appendix H of the manual for details on how to use the LVPROSIM data saving function. G 30. Set the Controller Output to 40%. Small volumes of CH 3 COOH solution should be added to the water already present in the Column. On the trend recorder, observe what happens to the ph value of the water. The ph of the water in should decrease. Is this your observation? G Yes G No G 31. Let the ph of the water in the Column decrease down to 4.5. This should take about 1 minute. G 32. Set the Controller Output to 0%. This should stop the flow of the CH 3 COOH solution. G 33. The Phenol Red should give a light tint to the solution. Record the color of the solution below. Color of the solution in the Column: G 34. On the Pumping Unit, open HV2 and let the water level in the Column decrease to 15 cm (6 in). As soon as the water reaches the proper level, close HV
9 G 35. Make sure SC 1 manual control knob of the Metering Pump Drive is turned fully counterclockwise and set the S1 switch to 2. G 36. On the Metering Pump Drive, set the S3 switch to 1. G 37. Set the Controller Output to 40%. Small volumes of NaHCO 3 solution should be added into the Column. On the trend recorder, observe what happens to the ph value of the solution. G 38. Let the ph of the water in the Column increase up to 6.5. This should take about 2 minutes. G 39. Set the Controller Output to 0%. This should stop the flow of the NaHCO 3 solution. G 40. On the Pumping Unit, open HV2 and let the water level in the Column decrease to 15 cm (6 in). As soon as the water reaches the proper level, close HV2. G 41. On the Metering Pump Drive, set the S3 switch to 2 and the S1 switch to 1. G 42. Set the Controller Output to 40%. Small volumes of CH 3 COOH solution should be added to the Column. G 43. Let the ph of the water in the Column decrease to 4.5. G 44. Is the titration faster or slower than the first time CH 3 COOH has been added to the Column. G 45. Once the proper ph is reached, stop data saving on the controller. Note: If the controller you are using is LVPROSIM, stop data saving by deselecting the box next to the disk icon. G 46. Set the Controller Output to 0%. This should stop the flow of the CH 3 COOH solution. G 47. Stop the variable-speed drive of the Pumping Unit by setting the Set Point Device output to 0.00 V. 2-71
10 G 48. Import the saved data into a spreadsheet program to plot the titration curve. Note: If the controller you are using as ATC1 is LVPROSIM, the saved data has been stored in a file named Trendrec.txt located in the LVPROSIM application folder. G 49. Open valve HV1 of the Pumping Unit completely and let the water in the Column drain back to the reservoir. G 50. Turn off the Pumping Unit and the 24-V DC Power Supply by setting their POWER switch at O. G 51. Disconnect the hoses of the Pumping Unit from the system and safely dispose of the solution in the reservoir. CAUTION! Before disposing of the reservoir contents, always neutralize the solution to avoid acid or alkaline products from being released into the environment. After neutralization, only water and salts should remain in the reservoir. Refer to the neutralization procedure in Appendix I for details. G 52. Disconnect the system. Return all leads, hoses, and components to their storage location. CAUTION! Water may remain in the hoses and components. Be careful not to allow water to enter the electrical components and their terminals upon disconnection of the hoses. G 53. Thoroughly wash the glassware. G 54. Store the ph probe in the flow chamber filled with storage solution. Refer to Appendix K for details. G 55. Wipe up any water from the floor and the Process Control Training System. G 56. Remove and dispose of your protection gloves before leaving the classroom. Carefully wash your hands. 2-72
11 CONCLUSION In this exercise, you learned how to titrate an alkaline buffer solution with a weak acid solution using the Process Control Training System. You analyzed the titration curve obtained and observed the buffer region of the alkaline solution. REVIEW QUESTIONS 1. What is a buffer solution? 2. What is the buffer capacity of a solution? 3. How can the buffer capacity of a buffer solution can be increased? 4. How can you decrease the ph of a buffer solution made of ammonia, NH 3? 5. Is it possible to make an alkaline buffer solution with a weak acid and its salt? 2-73
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