Exercise 4-4 Titration of a Buffer Solution EXERCISE OBJECTIVE Titrate a buffer solution, plot a graph using the titration data, and analyze the titration curve. DISCUSSION OUTLINE The Discussion of this exercise covers the following points: Buffer solutions Sodium bicarbonate DISCUSSION Buffer solutions A buffer solution consists in an aqueous solution containing a high concentration of a weak acid,, and its conjugate base,. The dissociation equation of the weak acid can be written as: (4-45) The equations giving the dissociation constant of the weak acid can be rewritten as follows: (4-46) Similarly, the equilibrium equation describing the reaction of the conjugate base with water can be written as: (4-47) The dissociation constant of the conjugate base reaction can be rewritten as: (4-48) In both cases, the concentration of hydronium,, and hydroxyl,, 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 remains approximately the same, even if a small quantity of acid or a base is added to the buffer solution. The hydronium, produced by a small quantity of acid added to the buffer solution, reacts with the conjugate base to form weak acid. Consequently, the ratio Festo Didactic 86013-00 101
Ex. 4-4 Titration of a Buffer Solution Discussion remains approximately constant. Likewise, if a small quantity of base is added to the buffer solution, the hydroxyl reacts with the weak acid to form the conjugate base and water. Again, the ratio remains 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: (4-49) where is the variation of the concentration of strong base is the variation of the concentration of strong acid is the variation of the ph The variation of the concentration and produce a variation 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. ), an approximate solution can be used. This approximate solution is: (4-50) is the natural logarithm. That is, the logarithm having base e=2.718281... is approximately equal to 2.3. Remember that the ph of a buffer solution is given by the equation: (4-51) Take, for example, a buffer solution made of ammonia,, and ammonium,. The dissociation constant of ammonium is. To have a ph 10 buffer solution, the ratio of ammonia to ammonium to use must be calculated as follows: The ratio to use to create such a buffer solution is: A buffer solution always has a ph value of 10 if this proportion is respected. However, it does not have the same buffer capacity. A solution with a 102 Festo Didactic 86013-00
Ex. 4-4 Titration of a Buffer Solution Discussion concentration of ammonia equal to 5.6 mol/l and a concentration of ammonium of 1 mol/l has a buffer capacity of: Which means that it requires 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 is Sodium bicarbonate A solution of sodium bicarbonate can be used as a buffer solution. As mentioned earlier, a buffer solution usually contains 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 too: Figure 4-47. Sodium bicarbonate (2D). When in solution, carbon dioxide reacts with water to form carbonic acid, : Figure 4-48. Sodium bicarbonate (3D). 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,, and bicarbonate,, providing the conjugate base (A - ) required to create a buffer solution: Thus, the dissolved carbon dioxide reacting with water provides the weak acid ( ) and the dissolved sodium bicarbonate provides the conjugate base ( ). Hence the pair HA/A - required to create a buffer is present in the solution. The equilibrium equations describing the / buffer solution are: Festo Didactic 86013-00 103
Ex. 4-4 Titration of a Buffer Solution Procedure Outline PROCEDURE OUTLINE The Procedure is divided into the following sections: Set up and connections Titration of a buffer solution Cleaning the process water Titration curve analysis PROCEDURE Set up and connections a After this exercise, the system must either be rinsed thoroughly with fresh water or the water filtered using the setup and procedure of Appendix F to get rid of the effect of sodium bicarbonate in the process water. Before using ANY of the chemicals provided with the ph Process Control Training System, read the chemical MSDS and wear the appropriate personal protective equipment. 1. Connect the equipment as the piping and instrumentation diagram (P&ID) in Figure 4-49 shows. Use Figure 4-50 to position the equipment correctly on the frame of the training system from the 3532 series or use Figure 4-51 to position the equipment on the frame from the 3531 series. Use the basic setup presented in the Familiarization with the Training System manual. Table 4-15 lists the equipment you must add to the basic setup in order to set up your system for this exercise. This setup is exactly the same as for Ex. 4-2 only the chemicals used are different. Table 4-15. Equipment required for this exercise. Name Part number Identification Volumetric flask 38406 Scopulla 38442 Phenol red 38446 Graduated cylinder 38447 Precision scale 38485 Latex gloves 40331-54 Pipette 40331-65 Safety glasses 40348 ph transmitter 46945 AIT Metering pumps 46956 Paperless recorder 46972 UR Water analyzer 46989 Chemical tanks 46994 Acetic acid 5% (v/v)(vinegar) 76764 Sodium bicarbonate (backing soda) 76765 104 Festo Didactic 86013-00
Ex. 4-4 Titration of a Buffer Solution Procedure Figure 4-49. P&ID. Festo Didactic 86013-00 105
Ex. 4-4 Titration of a Buffer Solution Procedure Figure 4-50. Setup (series 3532). 106 Festo Didactic 86013-00
Ex. 4-4 Titration of a Buffer Solution Procedure Figure 4-51. Setup (series 3531). 2. Wire the emergency push-button so that you can cut power in case of an emergency. The Familiarization with the Training System manual covers the security issues related to the use of electricity with the system, as well as the wiring of the emergency push-button. 3. Wire the paperless recorder to record the output of the ph transmitter. Festo Didactic 86013-00 107
Ex. 4-4 Titration of a Buffer Solution Procedure 4. Do not power up the instrumentation workstation yet. Do not turn the electrical panel on before your instructor has validated your setup that is not before step 9. 5. To titrate an alkaline buffer solution with a weak acid, one of the chemical tanks must be filled with a solution of 0.08 mol/l of sodium bicarbonate and the other chemical tank with a solution of 0.08 mol/l of acetic acid. If you have only two chemical tanks available, empty the chemical tank containing sodium hydroxide, rinse it, and fill it with the sodium bicarbonate solution (see caution below). Be sure to drain the metering pump tubing as well. Refer to the Familiarization with the Training System manual for instructions to drain the metering pump tubing. Dispose of all solutions according to local environmental regulation. 6. Make sure there is enough acetic acid solution left in the other tank and follow the procedure of Ex. 3-1 to prepare the sodium bicarbonate solution. 7. Before proceeding further, complete the following checklist to make sure you have set up the system properly. The points on this checklist are crucial elements for the proper completion of this exercise. This checklist is not exhaustive. Be sure to follow the instructions in the Familiarization with the Training System manual as well. f All unused male adapters on the column are capped and the flange is properly tightened. The hand valves are in the positions shown in the P&ID. The chemical tanks are filled with the appropriate solutions and are carefully labeled. You are wearing the appropriate PPE. The vent tube is properly installed. The paperless recorder is set up and configured to record the output of the ph transmitter. 8. Ask your instructor to check and approve your setup. Titration of a buffer solution 9. Power up the electrical unit. This starts all electrical devices. 10. Make sure the ph probe is properly inserted into the connection port on the process workstation. 108 Festo Didactic 86013-00
Ex. 4-4 Titration of a Buffer Solution Procedure 11. Test your system for leaks. Use the drive to make the pump run at low speed in order to produce a small flow rate. Gradually increase the flow rate up to 50% of the maximum flow rate the pumping unit can deliver (i.e., set the drive speed to 30 Hz). Repair all leaks. 12. Start the pump and set the drive speed to 30 Hz. This setup uses a telescopic pipe, which can extend if the pump runs at an excessive speed. Be sure to set the drive speed to a maximum of 30 Hz and secure the tubing with as much attach brackets, Model 85444, as possible. 13. Fill the column up to 25 cm of water. Then, close HV1 and open HV6 to put the process workstation into recirculation mode. 14. Remove one of the caps from the top of the column and, using a funnel, add about 20 ml of phenol red to the process water. 15. Using the metering pump connected to the chemical tank containing the solution of 0.08 mol/l of acetic acid, start injecting acid solution into the process water. The purpose of this operation is to drop the ph of the process water down to a value around 4.0. Set the metering pump to 50% of its maximum delivery rate. 16. Make sure the paperless recorder is recording the output of the ph transmitter. 17. Closely monitor the ph of the process water on the transmitter. Once the ph of the process water has dropped to a value of 4.0, stop the metering pump injecting the acid solution and wait two minutes to make sure the process water is perfectly mixed. 18. Once the ph value is stable, inject the buffer solution into the process water using the second metering pump. Set the metering pump delivery rate at around 50%. This stretches the titration process and provides more data for analysis. 19. Closely monitor the ph of the process water on the transmitter. Once the ph of the process water reaches a value of 6.5, stop the metering pump injecting the buffer solution and wait two minutes to make sure the process water is perfectly mixed. 20. Again, inject the acid solution into the process water and let the ph drop to down to a value around 4.5. Festo Didactic 86013-00 109
Ex. 4-4 Titration of a Buffer Solution Conclusion 21. Watch the titration process by observing both the tint of the process water given by the phenol red and the ph value measured by the paperless recorder. 22. Wait until the ph of the process water has decreased to a value of 4.5 and stop the metering pump injecting the acid solution. The titration of a buffer solution (made with sodium bicarbonate) with a weak acid (acetic acid) is completed. 23. Follow the procedure in the Familiarization with the Training System manual to transfer the data of the titration of a buffer solution from the paperless recorder to a computer. Cleaning the process water 24. Adding sodium bicarbonate to the process water creates a buffer effect in the process water. Trying to neutralize the process water using an acid does not remove this effect. Therefore, it is strongly recommended that you either drain the process water from the system and rinse it thoroughly with fresh water, or that you use the setup and procedure of Appendix F to filter the process water. This last setup requires the optional conductivity process add-on. Titration curve analysis 25. Plot the data using spreadsheet software. 26. Compare the portion of curve where the ph drops to a value of 4.0 before the addition of sodium bicarbonate with the portion of the curve where the ph drops to a value of 4.5 after the addition of sodium bicarbonate. CONCLUSION In this exercise, you learned how to titrate an alkaline buffer solution with a weak acid solution. You analyzed the titration curve obtained and observed the buffer region of the alkaline solution. REVIEW QUESTIONS 1. What is a buffer solution? 110 Festo Didactic 86013-00
Ex. 4-4 Titration of a Buffer Solution Review Questions 2. What is the buffer capacity of a solution? 3. How can the buffer capacity of a buffer solution be increased? 4. How can you decrease the ph of a buffer solution made of ammonia,? 5. Is it possible to make an alkaline buffer solution with a weak acid and its salt? Festo Didactic 86013-00 111