Homework answers / question archive / Laboratory 2 Potentiometric Acid - Base Titrations Objective To obtain titration curves by titrating a solution of phosphoric acid with a standardized strong base and using these titration curves to determine of the concentration of the acid

Laboratory 2 Potentiometric Acid - Base Titrations Objective To obtain titration curves by titrating a solution of phosphoric acid with a standardized strong base and using these titration curves to determine of the concentration of the acid

Chemistry

Share With

---

Laboratory 2

Potentiometric Acid - Base Titrations

Objective

To obtain titration curves by titrating a solution of phosphoric acid with a standardized strong base and using these titration curves to determine of the concentration of the acid.  The curves will be obtained in two ways, manually, and with an automated titrator.

Background 

This experiment will illustrate the titration curve obtained when titrating a triprotic weak acid with a strong base. Data obtained from these types of titrations can be used for standardizing a solution, analyzing an unknown acid or base sample, and for determining the dissociation constants (pK_as and pK_bs) of weak acids and bases. They are called potentiometric titrations. 

Each pair of partners will receive a solution containing an unknown concentration of phosphoric acid.  This is a two week experiment and it will be done in two parts, one part each week.  In one part of the experiment, the students will titrate the unknown solution manually using a regular burette and a pH meter.  In the other part of the experiment, the students will titrate the same unknown solution using an automated titrator.  Titrations on the automated titrator will be run in triplicate.  

The titration curve that you get in this experiment will look something like this.  The horizontal axis shows volume of titrant added and the vertical axis represents pH.

You can see that the pH rises slowly at first, then increases more steeply as it approaches the first end point.  It then increases slowly again after the first end point.  There will be a second end point at a volume of titrant which is twice the volume at the first end point.

When you are doing the manual titration, you will collect data points which are (volume, pH).  When you are measuring pH in a part of the curve that is rising slowly, you need to measure

__________________________________________________________________________

CHEM25770 10 Fall 2020 the pH only every 1 mL or so.  When you get close to an endpoint, you must measure the pH at volumes that are closer together, because if you don’t, you will miss the endpoint.

Chemicals Required

Standardized 0.1 M NaOH (prepared for you)       Phosphoric Acid unknown solution

RO water

Buffers (pH 4, 7, and 10)

 

Equipment Required

Manual Titration:                                    Automated Titration:

25 mL volumetric pipette       10 mL volumetric pipette

20 mL volumetric pipette                 Mettler-Toledo G20 automated titrator

250 mL volumetric flask        Laptop computer

pH meter and pH combination electrode    Wash bottle 

50 mL burette           400 mL beaker

400 mL beaker

Wash bottle

Magnetic stir bar

Hotplate/Stirrer

           

Procedure

Week 1

 

Dilution of Unknown Solution

 

1. Obtain an unknown sample of phosphoric acid.  In your lab notebook, record which unknown you received.  Pipet a 25.00 mL aliquot of the unknown acid sample into a 250.0 mL volumetric flask and dilute to the mark with RO water.  Mix well.  Transfer the diluted unknown to a clean, dry storage bottle.  Label it and keep it in your drawer at the end of the first week of this experiment.

You will use this diluted unknown solution for both parts of this experiment (manual and automated).

 

Calibration of pH Meter 

 

1. pH combination electrodes should be conditioned for at least 8 hours in pH 7.00 buffer solution (soaking buffer).  This has been done for you.
2. The electrode must be filled to the plug hole with saturated KCI. Treat electrodes with extreme care - do not scratch the bottom
3. Plug the pH meter into the power outlet and attach the combination electrode to the meter.
4. The electrode is normally kept sitting in a pH 7 buffer.  Buffers having pH 4 and 10 are also available in the lab.  Calibrate the pH meter/electrode using each of the three buffers (pH 4, 7, and 10).  Rinse off the electrode with your wash bottle and a waste beaker when changing from one buffer to another and before immersing the electrode in your solution.
5. For each buffer, use the following steps to calibrate the pH meter.
   1. Press mode until you see pH (in the upper left corner).
   2. Press setup until you see clear buffer.
   3. Press enter to accept.
   4. Press std; it will say Standardize.
   5. When it shows stable, press std again.

           

Manual Potentiometric Titration 

 

1. Fill and cap a burette with standardized 0.1 M NaOH.  Record the exact concentration of the NaOH in your lab notebook. Adjust the level of NaOH in the burette so that it is exactly 0.00 mL at the start of the titration.
2. Pipet a 20.00 mL aliquot of the diluted unknown sample into a 400 mL beaker.  Place the beaker in the centre of a hotplate/stirrer.  Make sure that the heat is turned off.
3. Rinse the electrode thoroughly by spraying it from a wash bottle containing RO water while holding a waste beaker underneath. Immerse the electrode in the solution containing the sample.
4. Add a magnetic stir bar to the solution and stir gently (about 200 – 300 rpm). Be careful not to allow the magnetic stir bar to hit the electrode. 
5. If the bottom of the electrode is not fully immersed in the sample, you can add RO water to the beaker to bring the solution level up until the electrode is properly immersed. It is all right to add RO water as long as you do it before you start the titration. Do not add RO water to the beaker once the titration has begun.
6. Position the burette so that the reagent can be delivered without splashing. 
7. Measure the pH. Record the initial pH as well as the burette volume (0.00 mL). 

NOTE:  Allow time between additions of titrant for the pH to become constant within 0.05 pH units. Record the pH once it becomes stable.  The solution should be stirred continuously. 

8. Make sure your burette is at 0.00 mL at the start of the titration.  Begin the titration by adding about 1 mL of titrant.  Close the stopcock on the burette.  Record the exact volume on the burette.  Wait for the pH reading on the pH meter to stabilize.  Once it has stabilized, record the pH reading.
9. Add another increment of titrant that is about 1 mL, record the exact volume on the burette, and record the pH once it has stabilized.
10. Keep adding 1 mL increments and recording volume and pH after each increment until you see that there is a larger jump in the pH.
11. Then add smaller increments (0.50 then 0.10 mL) as the first end point is approached.
12. Finally add one-drop portions between each reading in the immediate vicinity of the first end point.   

NOTE: As the end point is approached, the change in pH becomes large even when only small amounts of base are added. 

13. After the first end point is reached (indicated by a large change in pH) continue to add titrant, in portions that are the reverse of those added when approaching the end point, until little change in pH is observed. 
14. When you start to come close to a volume that is twice the volume of the first end point, you should again start adding increments of titrant that are smaller.
15. Once you are past the second end point, you can increase the size of the increments.
16. Continue to measure volume and pH until you have gone about 3 mL past the second end point. 
17. If you did not get enough data points around the end points, it may be necessary to repeat the titration. More data points close to the end points of the reaction will produce more accurate results.  As in most other labs, part of your grade on this lab will depend on accuracy. 
18. Set the pH meter to Standby. Rinse and the electrode with RO water. Place the electrode in soaking buffer.

           

Week 2

Automated Titration

You will use a Mettler-Toledo G20 automated titrator to automatically run the same titration that you did manually in the other part of the experiment.  The automated titrator is connected to a laptop computer, which will receive all of the data (volume and pH readings) for each titration.  In this part of the experiment, instead of recording data in your lab notebook, you will e-mail the data files from the laptop computer to yourself and also to your lab instructor.

Start the Computer and Autotitrator

 

1. There is bottle on the left side of the automated titrator which contains the titrant solution.  The exact concentration of the titrant is written on a label on the bottle.  Record this exact concentration in your lab notebook.  Make sure that there is enough titrant in the bottle before you begin.  The bottle should be at least 1/3 full before you begin.
2. Turn on the laptop computer.  After it has finished booting, find the icon for LabX software and double click on that to start the LabX software.  (LabX is the name of the software on the laptop that receives data from the G20 automated titrator.)
3. The LabX software on the computer will show PredefinedUser

       Press OK

4. The software will show a menu with choices such as Activation, New Instrument, etc.

Ignore these and press Close.

5. Turn on the Mettler-Toledo G20 automated titrator by pressing the button on the right side at the front.
6. After booting, the G20 will show

User Name    PredefinedUser

7. Press Login
8. It may show  PnP sensor DGi115-SC detected at input SENSOR Press OK

 

Calibrate the Electrode

 

9. Press Methods
10. Press   Calib.  Calibration pH-Sensor Calibration
11. Open the rubber stopper on the side of the electrode.
12. Hold a 400 mL waste beaker under the electrode and rinse the electrode using RO water from a wash bottle.  Wipe the electrode with a Kimwipe.
13. Immerse the electrode in pH 4 buffer.
14. Press Start
15. It will show

Number of samples 3

Add sample 1/3 Press OK.

16. It will show

Add Sample 2/3.  Do not press OK yet.

17. Remove the electrode from the pH 4 buffer.  Hold a 400 mL waste beaker under the electrode and rinse the electrode using RO water from a wash bottle.  Wipe the electrode with a Kimwipe.
18. Immerse the electrode in pH 7 buffer.
19. Press OK.
20. It will show

Add Sample 3/3.  Do not press OK yet.

21. Remove the electrode from the pH 7 buffer.  Hold a 400 mL waste beaker under the electrode and rinse the electrode using RO water from a wash bottle.  Wipe the electrode with a Kimwipe.
22. Immerse the electrode in pH 10 buffer. 
23. Press OK
24. It will show

SLOPECal    -58.93 mv/pH   (or a similar number)

ZEROCal      6.953   (or a similar number)

       Record these two numbers in your lab notebook.

25. Press OK
26. When the yellow rectangle in the top right corner changes to blue, press OK.
27. The electrode is now calibrated.
28. Remove the electrode from the pH 10 buffer.  Hold a waste beaker under the electrode and rinse the electrode using RO water from a wash bottle.  Wipe the electrode with a Kimwipe.
29. Attach the titration cup to the titration head by tightening the blue ring to hold it in place.
30. Put the electrode into a hole in the top of the titration head, so that the electrode is in the titration cup.

           

Rinse the Burette

 

To “rinse” the burette means to push the liquid titrant from the burette into the titration cup (before doing a titration).  After the burette is rinsed, it automatically refills itself.  You would normally rinse the burette 3 or 4 times when you put a new titrant into the bottle.  In this procedure you will rinse the burette only once.

31. Press the Home button (a picture of a house) on the G20.
32. On the G20, press Manual
33. Press Burette
34. Press Rinse
35. Press Start
36. Loosen the blue ring holding the titration cup and discard its contents in the sink.  Rinse out the titration cup with RO water before putting a sample in it.  Hold a waste beaker under the electrode, stirrer, and dispensing tip and use an RO wash bottle to rinse them off and remove all traces of the previous liquid.

 

Titrate Unknown Samples

 

37. Press the Home button (a picture of a house) on the G20.
38. Press Methods
39. Press PHOS  Phosphoric Acid
40. Press Start
41. In the ID 1 field, type in your name followed by 01 for your first titration, 02 for your second titration, etc.  For example, if your name is Ravi Shankar and you are now running your second titration on the G20, you would enter 

Ravi Shankar 02  in the ID 1 field

42. Press OK
43. Press Start
44. The G20 will tell you to Add Sample 1/1.  Do not press OK yet.  You must press OK only after you have added the sample.
45. Pipette 10.00 ml of your diluted unknown into the titration cup.  Add enough RO water to bring the level up to about 40 mL in the titration cup.  (There are volume markings on the titration cup.)
46. Attach the titration cup to the titration head by tightening the blue ring to hold it in place. 47. Press OK

48. The titration will proceed automatically and it will stop once 30 mL of titrant have been added.  The data from the titration (volume and pH readings) will be sent automatically to the laptop computer.
49. After the titration has ended, loosen the blue ring holding the titration cup and discard its contents in the sink.  Rinse out the titration cup with RO water before putting the next aliquot of unknown into it.
50. Hold a large waste beaker under the exposed electrode, stirrer, and dispensing tip and use an RO wash bottle to rinse them off and remove all traces of the titration mixture.
51. Repeat steps 40 to 50 to do a total of three automated titrations.  Remember to give each one a unique name, which should be

Your Name 01

Your Name 02

Your Name 03  in the ID 1 field

52. Remove the electrode from the titration head and put it into the test tube holding water.

       

       E-mail Data to Yourself and your Instructor

      

After you have done the three automated titrations on the G20, the data for each titration will be in the LabX software on the laptop computer.  From the LabX software, you will export the data for each titration to an Excel file.  You will then e-mail the three files to yourself and also to your lab instructor.

53. In the LabX software on the laptop, in Data, find and double-click on one of your samples.
54. Press the Measured Values tab.
55. Press the Print Data tab.
56. Press the little down arrow next to Export to.
57. Select XLSX File
58. Press OK
59. For Filename, type in the name that you gave to the sample, e.g., Ravi Shankar 02.
60. Select Downloads.  Select AT method.  This is where your file will be saved.
61. Press Save. 
62. Repeat steps 53 to 61 for your two other titrations.
63. Click on the WiFi icon in the taskbar at the bottom right corner of the screen.
64. Click Sheridan Secure Access.
65. Turn off Connect automatically.
66. Press Connect.
67. Log in to the Sheridan network with your Sheridan username and password.
68. Open Internet Explorer.
69. Enter the following URL:  https:/mail.sheridancollege.ca/owa/auth.owa
70. Sign in to Outlook Web App.
71. Press new mail
72. Click on Insert.  Click on Attachment.
73. Attach the three files that contain your data.
74. Send the e-mail to yourself and also to your instructor.
75. Sign out from Outlook.
76. Click on the WiFi icon in the taskbar.
77. Click on Sheridan Secure Access.
78. Press Disconnect.

 

 

 

 

 

 

 

         

Report Format

 

Note:  For this lab report, the two partners will together submit one joint lab report.  That is, the two partners will work together to write only one lab report.  Both partners will get the same grade, based on the one lab report.  This is different from other experiments, in which the two partners write separate, independent lab reports.

 

Reference:  Textbook, pages 624 – 625 and 388

This report will include both the manual and automated titrations done in the two weeks of this experiment.

The lab report will include:

COVER SHEET

TITLE

PURPOSE

OBSERVATIONS

CALCULATIONS

RESULTS

QUESTION

CONCLUSION

SOURCES OF ERROR

REFERENCES

PHOTOCOPY OF ALL DATA FROM LAB NOTEBOOK

 

The cover sheet, title, purpose and references are the same as in the previous reports.

 

OBSERVATIONS

Manual Titration:

State which unknown you received (A, B, C, or D).

State the standardized concentration of the titrant used in the manual titration to four significant figures.  

The numerical observations for the manual titration must be entered into a spreadsheet (shown below) which will be used to plot the manual titration curve and the first derivative of the manual titration curve.

           

You will be plotting:  

• Titration curve
• First derivative of titration curve

Make up a spreadsheet in Excel using your volume and pH readings from the manual titration as the first and third columns (these are your observations).  Six columns are required.

 

The column headings for the spreadsheet are: 

• V (volume of titrant (mL)) observation
• ?V
• pH                                    observation
• ? pH
• ? pH / ?V
• Average volume   

 

Set up the spreadsheet to calculate and fill in the remaining 4 columns.  

For each row in the spreadsheet, starting in the second row, these are the meanings of the variables:

• ?V – the difference in volume between this row and the row above, i.e., the volume in this row minus the volume in the row above
• ? pH – the difference in pH between this row and the row above, i.e., the pH in this row minus the pH in the row above
• ? pH / ?V – the ? pH in this row divided by the ?V in this row
• Average volume – the average of the volume in this row and the volume in the row above

An example of a spreadsheet with typical data might look like this near the beginning.

V Titrant vol (mL)	?V mL	pH	? pH	? pH ?V	Average volume, mL
0.00	--	2.06	--	--	--
1.02	1.02	2.13	0.07	0.069	(1.02+0.00)/2  = 0.51
2.03	1.01	2.19	0.06	0.059	(2.03+1.02)/2 = 1.52

 

Fill in the Excel spreadsheet shown above using your volume and pH data in the first and third columns.  Have Excel calculate the changes, the ratio of changes, and the average volume needed in the other columns.  Include the spreadsheet in the Observations section of your lab report.

From the above spreadsheet, plot the following 2 graphs using volume as the horizontal axis.  Include the plots in the Observations section of your report.  Each plot must show the individual data points, not just a smooth line.

Graph 1 will plot pH vs titrant volume (titration curve).

Graph 2 will plot ?pH/?V (column 5) vs average volume (first derivative curve).

Notice that in Graph 1, the actual volume readings (from the first column of your spreadsheet) are used for the horizontal axis, but for Graph 2, the average volume (from the sixth column of your spreadsheet) is used for the horizontal axis. 

Each of these graphs should fill a page.  If a graph is too small, the end point volumes cannot be accurately read.

Your 2 plotted curves should resemble those in your textbook on page 625, Figure 21 – 21, with the difference being that your plotted curves will have two end points instead of one.  You will find the two end points from the titration curve and you will also find them from the first derivative curve. 

 

End points:

TITRATION CURVE

The titration curve should have two steeply – rising parts.  The end points are halfway up each steeply – rising part.  From the graph, estimate the volumes at the two end points.  Label these two volumes on your titration curve.

 

FIRST DERIVATIVE CURVE

The end points occur where the first derivative curve has its maxima (two highest points). 

The volume at the second maximum should be about twice the volume at the first maximum.  Indicate these volumes on your plot.

 

Automated Titration:

State the standardized concentration of the titrant used in the automated titration to four significant figures.  

List the names of the three files containing the automated titration data that you emailed to yourself and to your instructor.  

Copy and paste the three Excel files into the Observations section of the report.  The Excel files indicate where the autotitrator found the two end points.  They are listed as EQP1 and EQP2.

List all of the endpoints found in both types of titration in a single table like the one shown below, where V₁ is the volume at the first end point and V₂ is the volume at the second end point.

	V1, mL	V2, mL	V2/V1
Manual titration curve
Manual first derivative curve
Automated titration 1
Automated titration 2
Automated titration 3

 

CALCULATIONS

Molarity of unknown:

Calculate the molarity of your phosphoric acid unknown using the end point volumes V₁ determined by the manual titration curve, manual first derivative curve, and each of the automated titrations.  

At the first end point, the phosphoric acid molarity can be calculated from

 

 

 

where M_B  is the molarity of the NaOH titrant, V_B is the volume of NaOH titrant needed to reach the end point, and V_A is the volume of diluted unknown acid that was titrated.  

Remember that V_A was 20.00 mL for the manual titration, and it was 10.00 mL for the automated titrations.  It is also possible that the molarity of the base that you used in the manual titration was different from the molarity of the base used in the automated titrations.  The value of M_A that you calculate in this equation is the molarity of your unknown after you diluted it.

Recall that you diluted your unknown sample by pipetting 25.00 mL of your original unknown into a volumetric flask and making it up to 250.0 mL.  Therefore the dilution factor is

 

 

 

In the Results section, you must report the concentration that your unknown had as you received it, not after you diluted it.  The molarity that you must report is

????????_{????????????????????????????????}???????????????????????? = ????????_????????  × ????????????????

For the three automated titrations, calculate the mean, standard deviation, and percent relative standard deviation for the molarity of your unknown acid.

 

Determination of Dissociation Constant:

Using only the data from the first automated titration, find the values of pK_a1 and pK_a2 for phosphoric acid as follows.  pK_a1 is equal to the pH of the titration curve at a volume which is exactly half way from the start to the volume at the first end point.  pK_a2 is equal to the pH of the titration curve at a volume exactly half way between the first end point and the second end point.

Find the pH reading corresponding to the volume reading which is closest to half way between zero volume and the volume of the first end point.  Take this pH reading to be your value for pK_a1.

Find the pH reading corresponding to the volume reading which is closest to half way between the volume of the first end point and the volume of the second end point.  Take this pH reading to be your value for pK_a2.

 

RESULTS

In a single table, list the following molarities for your unknown phosphoric acid:

• Molarity from manual titration curve
• Molarity from manual first derivative curve
• Molarity from each of the three automated titrations
• Average molarity for the three automated titrations
• Standard deviation of the molarities for the three automated titrations
• Percent relative standard deviation of the molarities for the three automated titrations

In a second table, list the two pK_a values that you found for phosphoric acid.  Also, in this table, list the known values for pK_a1 and pK_a2 for phosphoric acid.  Find the known values in your textbook or another reliable source.  Cite your source in the References.

 

QUESTION

Phosphoric acid is triprotic.  We should see three “jumps” in the titration curve, one for each ionization.  However, in your automated titrations, you observed only two jumps in the curve, not three.  Explain why you did not see a third jump in the titration curve.

 

CONCLUSION

The Conclusion is two sentences which state “From the manual titration curve, the molarity of the unknown phosphoric acid was found to be ____ M.  The mean molarity found in the automated titrations was ____ M.”

SOURCES OF ERROR

List at least 3 sources of error are to be discussed here with their effect on the molarity of the unknown acid.  Remember to state the type of error.

 

REFERENCES

List all of your references.