Exercise Titration of a Strong Acid - Lab- · PDF file · 2016-04-18Titration of a...

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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 thedissociation there is no undissociated acid molecule in the solution, all that is left arehydronium ions and a conjugate base. Since there is complete dissociation, thedissociation constant is almost infinite. It is sometimes said that the reaction goescompletely to the right side of the equation:

Figure 2-14 shows the proportion of the different chemical species in solution beforeand after dissociation (at equilibrium).


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Figure 2-14. 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 isprobably a weak acid (or it is not a single compound acid). Note that sulfuric acid hastwo hydrogens, thus each sulfuric acid molecule can be ionized twice. For the firstionization, the sulfuric acid acts as a strong acid, which means that it completelydissociates to form H3O

+ and HSO4-. Once ionized a first time the HSO4

- ions can beionized too, but this time there is no complete dissociation, thus HSO4

- ions act likea weak acid. Acids able to donate more than one proton per molecule are namedpolyprotic acids; sulfuric acid is a diprotic acid since it can donate two protons peracid molecule. Because it can be ionized twice, sulfuric acid has two dissociationconstants. The first acid-ionization constant, Ka1, is almost infinite and the secondacid-ionization constant, Ka2, 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 aciddissolved in water will result in a mole of hydronium, H3O

+. For example, a solutionof 0.1 mol/l of hydrochloric acid, HCl, will produce 0.1 mol/l of H3O

+. The equation ofthis 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.


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Strong base

As for strong acids, when in aqueous solution a strong base completely dissociatesinto ions. After the dissociation, the aqueous solution is solely constituted of hydroxylions and of a conjugate acid. There is complete dissociation of the base, thus thedissociation constant is almost infinite and the equation of dissociation can be writtenas:

Table 2-3 lists the most common strong bases. As for acids, some bases have morethan one hydroxyl group and can be ionized more than one time. They are identifiedas polybasic. Such polybasic molecules will have as many dissociation constants asthey have hydroxyl groups. Oxides of metals from the group I are monobasic whileoxides 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. Forexample, a solution of 0.1 mol/l of sodium hydroxide, NaOH, will produce 0.1 mol/lof 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 totitrate water containing a strong base solution with a strong acid solution. You willuse the acquired data to plot a titration curve.

In the second part of the exercise, you will redo the previous titration with more waterand insufficient mixing. This will allow you to observe the effect of mixing on achemical process by comparing the titration curve obtained in this part of theexercise with the titration curve obtained in the first part of the exercise.


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EQUIPMENT REQUIRED

Refer to the Equipment Utilization Chart in Appendix A of the manual to obtain thelist of equipment required to perform this exercise.

PROCEDURE

Preliminary setup

G 1. Get the Expanding Work Surface from your storage location and mount itvertically to the Main Work Surface (at an angle of 90 ), if this has notalready been done.

G 2. Make the setup according to Figure 2-15 and Figure 2-16, take the sameprecautions as in Exercise 2-1.

Note: Refer to Figure B-2 of Appendix B for details on how toconnect the Lab-Volt Process Control and SimulationSoftware (LVPROSIM), Model 3674, to the pHTransmitter, Model 6544, the Set Point Device, Model 6561, andthe Metering Pump Drive, Model 6560.


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Figure 2-15. Measuring pH value with a pH Transmitter.


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Figure 2-16. 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 2-16. 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.


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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 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1SC 1 manual control knob . . . . . . . turned fully counterclockwiseS2 switch . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . pulsed modeSC 1 pulse width adjustment knobs . . . . . . . . . . . . . . . . . . . 50%S3 switch . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2SC 2 manual control knob . . . . . . . turned fully counterclockwiseS4 switch . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . pulsed modeSC 2 pulse width adjustment knobs . . . . . . . . . . . . . . . . . . . 50%

On the pH Transmitter:

SELECTOR switch . . . . . . . . . . . . . . . . . . . . . . . . . . pH PROBECALIBRATION 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 safelyand wash the tank carefully.

Note: In step 5 to 18 you will make a 0.08 mol/l solution of HCland a 0.08 mol/l solution of NaOH.

G 5. Calculate the volume of Hydrochloric Acid Solution 1.0 N required tomake 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 proceedingfurther.

G 6. Measure the required volume of Hydrochloric Acid Solution 1.0 N using agraduated 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.


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G 9. Add water into the volumetric flask until it almost reaches the etched markon the neck. Use a pipette to add water until the bottom of the meniscusreaches the mark.

G 10. The flask should now be filled with a 0.08 mol/l solution of HCl. Fill the firstChemical 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 requiredto 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 proceedingfurther.

G 14. Measure the required volume of Sodium Hydroxide Standard Solution 1.0 Nusing a graduated cylinder.

G 15. Pour the NaOH solution in the flask and complete with water until the etchedmark is reached.

G 16. The flask should now be filled with a 0.08 mol/l solution of NaOH. Fill thesecond Chemical Tank with the contents of the volumetric flask andcarefully identify the contents of the Chemical Tank with a HMIG paperlabel.

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 (3.2gallons US) of water. Make sure the baffle plate is properly installed at thebottom of the reservoir.


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G 18. Use Figure 2-15 and 2-16 and connection diagram B-2 of Appendix B tomake 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 PointDevice 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 waterflow from the reservoir and direct the full return flow to the pump inlet (turnthe handle fully counterclockwise)

G 24. The Column is now in recirculating mode. Water is pumped to the PumpingUnit outlet, passes through the Flow Chamber, goes into the Column, andflows out of the Column through one of the bottom outlets to be directed tothe pump inlet again.

With this setup, liquid in the Column is constantly stirred allowing chemicalsto 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 Columndecrease to 15 cm (6 in). As soon as the water reaches the proper level,close HV2.


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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 theMetering Pumps are not running (the SC1 and SC2 manual control knobsare turned fully counterclockwise).

G 30. Use a funnel to add about 5 ml of Phenol Red Aqueous solution 0.05% intothe Column.

Note: Phenol Red Aqueous solution 0.05% is a chemical indicatorwhich changes color from yellow to red over the pHrange 6.6 to 8.0.

G 31. Have the signal at the 0-5 V OUTPUT of the pH Transmitter and theANALOG 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 toconnect the LVPROSIM computer to the pH Transmitter. Onthe I/O Interface, make sure the RANGE switch of ANALOGINPUT 1 is set at 5 V.

In LVPROSIM, select Analog Input 1 from the Trend Recorderselection list to have the pH Transmitter signal plotted on thetrend recorder. Set the LVPROSIM sampling interval at 500 ms.Access the Configure Analog Inputs window and set the minimumand maximum range values of Analog Input 1 at a pH value of 0and 12 respectively, which corresponds to the measurementrange of the pH Transmitter. Set the filter time constant of thisinput at 0.5 second. Make sure the square root extracting functionis 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 usedto plot the controller output and pH Transmitter output signals.


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Note: If the controller you are using is the Lab-Volt ProcessControl and Simulation Software (LVPROSIM), Model 3674, startdata saving by clicking the box next to the disk icon in the upperleft-hand corner of the trend recorder display area. The data willbe saved to disk as a .TXT format file and later be imported intoa spreadsheet software. Refer to Appendix H of the manual fordetails on how to use the LVPROSIM data saving function.

G 35. Set the Controller Output to 40%. Small volumes of HCl solution should beadded 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 shouldtake about 1 minute.

Note: If the tubing between the Metering Pump and the ChemicalTank is filled with water, it may takes several seconds beforethe pH starts to change.

G 37. Set the Controller Output to 0%. This should stop the flow of the HClsolution.

G 38. The Phenol Red should give a light tint to the solution. Record the color ofthe solution below.

Color of the solution in the Column:

G 39. Make sure SC 1 manual control knob of the Metering Pump Drive is turnedfully 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 shouldbe added into the Column. On the trend recorder, observe what happens tothe pH value of the solution.

G 42. Let the pH of the water in the Column increase to a value of 10.0. Thisshould take about 2 minutes. Once the proper pH is reached, stop datasaving on the controller.

Note: If the controller you are using is LVPROSIM, stop datasaving by deselecting the box next to the disk icon.


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G 43. Set the Controller Output to 0%. This should stop the flow of the NaOHsolution.

G 44. Stop the variable-speed drive of the Pumping Unit by setting the Set PointDevice 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, thesaved data has been stored in a file named Trendrec.txt locatedin the LVPROSIM application folder.

The importance of proper mixing

G 46. Disconnect the hose connected to the right connector at the top of thecolumn and connect it to the left connector (recirculating water will passthrough the transparent tube and will be injected at the bottom of theColumn).

Figure 2-17. 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 PointDevice output between 3.00 V and 3.50 V.

G 48. On the Pumping Unit, set HV3 for directing the full reservoir flow to thepump inlet (turn handle fully clockwise).


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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 waterflow from the reservoir and direct the full return flow to the pump inlet (turnthe handle fully counterclockwise).

G 51. On the Pumping Unit, open HV2 and let the water level in the Columndecrease 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 regionin the Column.

Note: Make sure there is enough Phenol Red Aqueoussolution 0.05% in the water to be able to observe the stagnantregion.

G 53. Compare the curve obtained with a water level of 15 cm (6 in) and the curveobtained with a water level of 20 cm (8 in).

G 54. Stop the variable-speed drive of the Pumping Unit by setting the Set PointDevice output to 0.00 V.

G 55. Open valve HV1 of the Pumping Unit completely and let the water in theColumn drain back to the reservoir.

G 56. Turn off the Pumping Unit and the 24-V DC Power Supply by setting theirPOWER switch at O.

G 57. Disconnect the hoses of the Pumping Unit from the system and safelydispose 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.


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G 58. Disconnect the system. Return all leads, hoses, and components to theirstorage 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 toAppendix 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 theclassroom. Carefully wash your hands.

CONCLUSION

In this exercise, you learned how to titrate a strong base solution with a strong acidsolution using the Process Control Training System. You analyzed the titration curveobtained and observed that the process takes more time to react when the tankcontent is improperly mixed. You also learned how to calculate the pH of a strongacid 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 pHof a strong base solution an approximation has been made. What is thisapproximation?

2. What is the pH of a solution of 10 mol/l of hydrochloric acid, HCl.


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3. What is a diprotic acid?

4. When titrating a strong base with a strong acid, which chemical species arepresent in solution at the equivalence point?

5. What is the pH of a solution of 0.2 mol/l of potassium hydroxide, KOH.

Exercise Titration of a Strong Acid - Lab- · PDF file · 2016-04-18Titration of a...

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