Exercise 2-2. Titration of a Strong Acid EXERCISE OBJECTIVES

Transcription

1 Exercise 2-2 Titration of a Strong Acid EXERCISE OBJECTIVES To describe the effect of a ph variation on a chemical indicator; To titrate water containing a strong base solution with a strong acid solution; To plot a graph using the titration data; To analyze a titration curve; To observe the effect of bad mixing inside a reactor; To calculate the ph of a strong acid solution; To calculate the ph of a strong base solution. DISCUSSION Strong acid When in aqueous solution, a strong acid completely dissociates into ions. After the dissociation there is no undissociated acid molecule in the solution, all that is left are hydronium ions and a conjugate base. Since there is complete dissociation, the dissociation constant is almost infinite. It is sometimes said that the reaction goes completely to the right side of the equation: Figure 2-14 shows the proportion of the different chemical species in solution before and after dissociation (at equilibrium). 2-33

2 Figure Proportion of the chemical species before and after dissociation. Table 2-1 lists common strong acids. If an acid cannot be found in this table, it is probably a weak acid (or it is not a single compound acid). Note that sulfuric acid has two hydrogens, thus each sulfuric acid molecule can be ionized twice. For the first ionization, the sulfuric acid acts as a strong acid, which means that it completely dissociates to form H 3 O + and HSO 4 -. Once ionized a first time the HSO 4 - ions can be ionized too, but this time there is no complete dissociation, thus HSO 4 - ions act like a weak acid. Acids able to donate more than one proton per molecule are named polyprotic acids; sulfuric acid is a diprotic acid since it can donate two protons per acid molecule. Because it can be ionized twice, sulfuric acid has two dissociation constants. The first acid-ionization constant, K a1, is almost infinite and the second acid-ionization constant, K a2, is equal to 1.2x10-2. Calculation of the ph of a strong acid solution A strong acid completely dissociates in an aqueous solution. Each mole of acid dissolved in water will result in a mole of hydronium, H 3 O +. For example, a solution of 0.1 mol/l of hydrochloric acid, HCl, will produce 0.1 mol/l of H 3 O +. The equation of this dissociation is: As mentioned in Unit 1, ph is given by the equation: Therefore, the ph of a 0.1 mol/l solution of hydrochloric acid is: Note: Even if it is very unusual, theoretically ph can be negative. 2-34

3 Strong base As for strong acids, when in aqueous solution a strong base completely dissociates into ions. After the dissociation, the aqueous solution is solely constituted of hydroxyl ions and of a conjugate acid. There is complete dissociation of the base, thus the dissociation constant is almost infinite and the equation of dissociation can be written as: Table 2-3 lists the most common strong bases. As for acids, some bases have more than one hydroxyl group and can be ionized more than one time. They are identified as polybasic. Such polybasic molecules will have as many dissociation constants as they have hydroxyl groups. Oxides of metals from the group I are monobasic while oxides of metals from group II are all dibasic. Calculation of the ph of a strong base solution Each mole of strong base dissolved into water will give a mole of hydroxyl ions. For example, a solution of 0.1 mol/l of sodium hydroxide, NaOH, will produce 0.1 mol/l of OH -. The dissociation equation is: First, the poh must be calculated: The ph of the solution is then given by: Procedure summary In the first part of the exercise, you will use the Process Control Training System to titrate water containing a strong base solution with a strong acid solution. You will use the acquired data to plot a titration curve. In the second part of the exercise, you will redo the previous titration with more water and insufficient mixing. This will allow you to observe the effect of mixing on a chemical process by comparing the titration curve obtained in this part of the exercise with the titration curve obtained in the first part of the exercise. 2-35

4 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. Make the setup according to Figure 2-15 and Figure 2-16, take the same precautions as in Exercise 2-1. 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

5 Figure Measuring ph value with a ph Transmitter. 2-37

6 Figure Suggested setup for the diagram of Figure 2-15 (see Table below Figure 2-13 for the detail of the components). 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. 2-38

7 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. 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 HCl and NaOH 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: In step 5 to 18 you will make a 0.08 mol/l solution of HCl and a 0.08 mol/l solution of NaOH. G 5. Calculate the volume of Hydrochloric Acid Solution 1.0 N required to make 2000 ml of a 0.08 mol/l solution of HCl. Required volume of Hydrochloric Acid Solution 1.0 N: ml Note: Confirm this value with your instructor before proceeding further. G 6. Measure the required volume of Hydrochloric Acid Solution 1.0 N using a graduated cylinder. G 7. Half fill the volumetric flask with water (about 1000 ml). G 8. Use a funnel to pour the HCl solution in the 2000-ml volumetric flask. 2-39

8 G 9. 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. G 10. The flask should now be filled with a 0.08 mol/l solution of HCl. Fill the first Chemical Tank with the contents of the volumetric flask. G 11. Using the HMIG (Hazardous Materials Identification Guide) paper labels, identify the Chemical Tank with the name of the chemical, the concentration, the date, your initials, and the possible hazard(s). G 12. Carefully wash the volumetric flask and half fill it with water (about 1000 ml). G 13. Calculate the volume of Sodium Hydroxide Standard Solution 1.0 N required to make 2000 ml of a 0.08 mol/l solution of NaOH. Required volume of Sodium Hydroxide Standard Solution 1.0 N: ml Note: Confirm this value with your instructor before proceeding further. G 14. Measure the required volume of Sodium Hydroxide Standard Solution 1.0 N using a graduated cylinder. G 15. Pour the NaOH solution in the flask and complete with water until the etched mark is reached. G 16. The flask should now be filled with a 0.08 mol/l solution of NaOH. Fill the second Chemical Tank with the contents of the volumetric flask 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 17. 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. 2-40

9 G 18. Use Figure 2-15 and 2-16 and connection diagram B-2 of Appendix B to make the appropriate setup. G 19. Turn on the Pumping Unit by setting its POWER switch at I. G 20. 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 21. 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 22. Allow the level of water to rise in the Column until it reaches 38 cm (15 in). Placing the system in recirculating mode G 23. 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 24. 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. With this setup, liquid in the Column is constantly stirred allowing chemicals to mix rapidly. G 25. Make sure the two Chemical Tanks are filled with the proper chemicals: First Chemical Tank: 0.08 mol/l solution of HCl. Second Chemical Tank: 0.08 mol/l solution of NaOH. G 26. 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

10 Operation of the ph Transmitter in the fixed calibration mode G 27. Power up the ph Transmitter using the DC Power Supply. G 28. Turn on the Metering Pump Drive. G 29. 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 30. Use a funnel to add about 5 ml of Phenol Red Aqueous solution 0.05% into the Column. Note: Phenol Red Aqueous solution 0.05% is a chemical indicator which changes color from yellow to red over the ph range 6.6 to 8.0. G 31. 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 the main screen. G 32. 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 33. On the Metering Pump Drive, make sure the S1 switch is set to 1. G 34. On the controller, initiate data saving in order to start saving the data used to plot the controller output and ph Transmitter output signals. 2-42

11 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 35. Set the Controller Output to 40%. Small volumes of HCl 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 36. Let the ph of the water in the Column decrease to a value of 3. This should take about 1 minute. Note: If the tubing between the Metering Pump and the Chemical Tank is filled with water, it may takes several seconds before the ph starts to change. G 37. Set the Controller Output to 0%. This should stop the flow of the HCl solution. G 38. 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 39. Make sure SC 1 manual control knob of the Metering Pump Drive is turned fully counterclockwise and set the S1 switch to 2. G 40. On the Metering Pump Drive, set the S3 switch to 1. G 41. Set the Controller Output to 40%. Small volumes of NaOH solution should be added into the Column. On the trend recorder, observe what happens to the ph value of the solution. G 42. Let the ph of the water in the Column increase to a value of This should take about 2 minutes. 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. 2-43

12 G 43. Set the Controller Output to 0%. This should stop the flow of the NaOH solution. G 44. Stop the variable-speed drive of the Pumping Unit by setting the Set Point Device output to 0.00 V. G 45. 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. The importance of proper mixing G 46. Disconnect the hose connected to the right connector at the top of the column and connect it to the left connector (recirculating water will pass through the transparent tube and will be injected at the bottom of the Column). Figure Install the hose on the left inlet. CAUTION! Be careful not to spill the water remaining in the hose. G 47. 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 48. On the Pumping Unit, set HV3 for directing the full reservoir flow to the pump inlet (turn handle fully clockwise). 2-44

13 G 49. Allow the level of water to rise in the Column until it reaches 38 cm (15 in). G 50. 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 51. On the Pumping Unit, open HV2 and let the water level in the Column decrease to 20 cm (8 in). As soon as the water reaches the proper level, close HV2. G 52. Repeat steps 29 to 45 with this setup and try to identify the stagnant region in the Column. Note: Make sure there is enough Phenol Red Aqueous solution 0.05% in the water to be able to observe the stagnant region. G 53. Compare the curve obtained with a water level of 15 cm (6 in) and the curve obtained with a water level of 20 cm (8 in). G 54. Stop the variable-speed drive of the Pumping Unit by setting the Set Point Device output to 0.00 V. G 55. Open valve HV1 of the Pumping Unit completely and let the water in the Column drain back to the reservoir. G 56. Turn off the Pumping Unit and the 24-V DC Power Supply by setting their POWER switch at O. G 57. 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. 2-45

14 G 58. 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 59. Thoroughly wash the glassware. G 60. Store the ph probe in the flow chamber filled with storage solution. Refer to Appendix K for details. G 61. Wipe up any water from the floor and the Process Control Training System. G 62. Remove and dispose of your protection gloves before leaving the classroom. Carefully wash your hands. CONCLUSION In this exercise, you learned how to titrate a strong base solution with a strong acid solution using the Process Control Training System. You analyzed the titration curve obtained and observed that the process takes more time to react when the tank content is improperly mixed. You also learned how to calculate the ph of a strong acid solution and the ph of a strong base solution. REVIEW QUESTIONS 1. In both sections calculate the ph of a strong acid solution and calculate the ph of a strong base solution an approximation has been made. What is this approximation? 2. What is the ph of a solution of 10 mol/l of hydrochloric acid, HCl. 2-46

15 3. What is a diprotic acid? 4. When titrating a strong base with a strong acid, which chemical species are present in solution at the equivalence point? 5. What is the ph of a solution of 0.2 mol/l of potassium hydroxide, KOH. 2-47