Experiment 7 Potentiometric Titration

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					             Experiment 7
         Potentiometric Titration




Valdosta State University
Purpose
 To accurately determine the molar mass and pKa of an
 unknown weak acid.




 Valdosta State University
Background
 • In this experiment, the pH of a solution will be
   measured directly as the sodium hydroxide is added.
 • The pH will be measured using a glass combination
   electrode.
 • This device measures pH (or [H3O+]) as voltage.




 Valdosta State University
Background
pH Combination Electrode
   • combination refers to the ion sensing cell and the
     reference cell.




  Valdosta State University
Background
pH Combination Electrode
   • combination refers to the ion sensing cell and the
     reference cell.




  Valdosta State University
 Background
Consider the following graph:
            13


            12


            11


            10


            9
       pH




            8


            7


            6


            5


            4


            3
                 0   5          10   15              20   25   30   35
                                     Volum e NaOH (m L)

    Valdosta State University
 Background

Consider the following graph:
                         13


                         12


                         11


                         10


                         9
                    pH




                         8


                         7


                         6


                         5


                         4


                         3
                              0   5   10   15              20   25   30   35
                                           Volum e NaOH (m L)




 In this region H+ dominates, the small change in pH is the
 result of relatively small changes in H+ concentration.


   Valdosta State University
 Background

Consider the following graph:
                            13


                            12


                            11


                            10


                            9
                       pH




                            8


                            7


                            6


                            5


                            4


                            3
                                 0   5   10   15              20   25   30   35
                                              Volum e NaOH (m L)




 In this region, relatively small changes in H+ concentration
 cause large changes in pH, The midpoint of the vertical
 region is the equivalence point.
   Valdosta State University
 Background

Consider the following graph:
                          13


                          12


                          11


                          10


                          9
                     pH




                          8


                          7


                          6


                          5


                          4


                          3
                               0   5   10   15              20   25   30   35
                                            Volum e NaOH (m L)




 In this region OH- dominates, the small change in pH is the
 result of relatively small changes in OH- concentration.


   Valdosta State University
Background
 HA(aq) + NaOH(aq)  NaA(aq) + H2O(l)

 • This experiment will use monoprotic acids.
 • Therefore, at the equivalence point,
              moles HA = moles NaOH
 • Using the definition of moles, the molar mass can be
   determined:




 Valdosta State University
Background
 HA(aq) + NaOH(aq)  NaA(aq) + H2O(l)

 • This experiment will use monoprotic acids.
 • Therefore, at the equivalence point,
              moles HA = moles NaOH
 • Using the definition of moles, the molar mass can be
   determined:

                           sample weight
              molar mass 
                             moles HA

 Valdosta State University
Background
 HA(aq) + NaOH(aq)  NaA(aq) + H2O(l)

 • pKa can be determined by using the Henderson-
   Hasselbach equation.




 Valdosta State University
Background
 HA(aq) + NaOH(aq)  NaA(aq) + H2O(l)

 • pKa can be determined by using the Henderson-
   Hasselbach equation.
                                  conjugatebase 
                                  weak acid  
                 pH  pK a  log                  
                                                  




 Valdosta State University
Background
 HA(aq) + NaOH(aq)  NaA(aq) + H2O(l)

 • pKa can be determined by using the Henderson-
   Hasselbach equation.
                                  conjugatebase 
                                  weak acid  
                 pH  pK a  log                  
                                                  
 • If the pH is measured at the point where
                [conjugate base] = [weak acid]




 Valdosta State University
Background
 HA(aq) + NaOH(aq)  NaA(aq) + H2O(l)

 • pKa can be determined by using the Henderson-
   Hasselbach equation.
                                  conjugatebase 
                                  weak acid  
                 pH  pK a  log                  
                                                  
 • If the pH is measured at the point where
         [conjugate base] = [weak acid]
                             pH  pKa  log1
                             pH  pKa

 Valdosta State University
Background
HA(aq) + NaOH(aq)  NaA(aq) + H2O(l)

• This assumption is valid at the half-titration point (i.e.
  halfway to the equivalence point).
• If the equivalence point is 20.00mL, the half-titration
  point is at 10.00mL.
• The pH is then obtained directly from the graph.




 Valdosta State University
Background
      13


      12


      11


      10


      9
 pH




      8


      7


      6


      5


      4


      3
           0   5         10   15              20   25   30   35
                              Volum e NaOH (m L)




 Valdosta State University
Background
      13


      12


      11


      10


      9
 pH




      8


      7


      6


      5


      4


      3
           0   5         10   15              20   25   30   35
                              Volum e NaOH (m L)




 Valdosta State University
Background
      13


      12


      11


      10


      9
 pH




      8


      7


      6


      5


      4


      3
           0   5         10   15              20   25   30   35
                              Volum e NaOH (m L)




 Valdosta State University
Procedure – Experiment 7

- For this experiment, work in pairs (you also can work in
  groups of two if you wish).




  Valdosta State University
Technique - Titration
Clean The Buret
1. Rinse a buret twice with distilled water.
2. Rinse the buret twice with 10-mL portions of your
   standardized NaOH solution.
3. Run some of the NaOH solution through the tip of the
   buret.
4. Drain the buret, then fill it with the standardized NaOH
   solution.
5. Let some NaOH solution run through the tip of the
   buret so that all air bubbles are removed.




  Valdosta State University
Technique - Titration
Clean The Buret




  Valdosta State University
Technique - Titration
Prepare the Unknown
1. Weigh 0.300 – 0.400g of solid unknown or 0.120 –
   0.180g of liquid unknown to the nearest 0.001 g into a
   250mL beaker.
2. Add approximately 50 mL of distilled water to the
   beaker and stir until all the unknown is dissolved.




  Valdosta State University
Technique - Titration
Set-up the pH meter - Computer
•    Hook-up the interface equipment as you have in previous
     experiments.
•    Turn on the computer. When the computer has booted-up,
     double click on the “Data Studio” icon. The program should start.
•    On the opening screen, select “Create Experiment”.
•    Click and drag the “pH Sensor” icon to Channel B of the interface
     box shown in the experimental setup window.




    Valdosta State University
Technique - Titration
Set-up the pH meter - Calibration
•    Double click on the “pH” icon underneath the Science Workshop
     display on the screen. Click on the “Calibration” tab.
•    Under the heading “High Point”, change the value to “10”. Put
     the electrode in the pH 10 buffer solution. When the number
     under “Voltage” has stabilized, click on “Take Reading” under the
     “High Point” heading.




    Valdosta State University
Technique - Titration
Set-up the pH meter - Calibration
•    Get a 400 mL beaker of distilled water to rinse the electrode
     between readings. Rinse the electrode with distilled water.




    Valdosta State University
Technique - Titration
Set-up the pH meter - Calibration
•    Under the heading “Low Point”, change the value to “4”. Put the
     electrode in the pH 4 buffer solution. When the number under
     “Voltage” has stabilized, click on “Take Reading” under the “Low
     Point” heading.




    Valdosta State University
Technique - Titration
Set-up the pH meter - Calibration
•    Click on “Ok”. The pH electrode is now calibrated. Place the
     electrode in the distilled water and leave it there until needed.




    Valdosta State University
Technique - Titration
Set-up the pH meter - Display
Click and drag the digits icon (under Displays) to the icon marked
“pH, Ch B (pH)” under Data. Click on the “down arrow” immediately
to the right of the 3.14 icon on the upper left corner of the Digits 1
box. Select “Increase Precision” and increase the precision to two
decimal places.




  Valdosta State University
Technique - Titration
Performing The Experiment
1. Put the electrode tip in the unknown acid solution. Click on the
   “Start” button on the screen to display the pH.
2. Fill the buret to 0.00mL and record this initial pH and buret
   reading.
3. Slowly add NaOH with stirring until either the pH increases by
   0.20 units or 2.0 mL of NaOH have been added, whichever
   occurs first. Stop and record both the pH and the buret reading.
4. Repeat step 17 until the pH reaches 12, or until the titration has
   gone 15 mL past the equivalence point. The equivalence point is
   the point at which single drops of NaOH cause the pH to
   increase dramatically (i.e. 0.4 units for a single drop).




  Valdosta State University
Technique - Titration
Performing The Experiment




  Valdosta State University
Data
Mass of unknown: 0.449g
               mL NaOH          pH   mL NaOH      pH
              0.00            4.02   18.80     10.08
              2.30            4.36   19.00     10.60
              3.00            4.47   19.40     10.98
              4.80            4.67   19.92     11.24
              6.10            4.81   21.25     11.46
              8.48            5.03   23.95     11.69
              10.61           5.21   25.10     11.84
              12.80           5.42   27.28     11.94
              15.08           5.72   28.12     11.94
              16.90           6.09   29.80     12.00
              17.82           6.47
  Valdosta State University
Data                                 Experiment 7, trial 1


         13.00


         12.00


         11.00


         10.00


          9.00


          8.00
    pH




          7.00


          6.00


          5.00


          4.00


          3.00


          2.00
              0.00   5.00    10.00       15.00          20.00   25.00   30.00   35.00
                                          Volum e NaOH (m L)




 Valdosta State University
Data                                 Experiment 7, trial 1


         13.00


         12.00


         11.00


         10.00


          9.00


          8.00
    pH




          7.00


          6.00


          5.00


          4.00


          3.00


          2.00
              0.00   5.00    10.00       15.00          20.00   25.00   30.00   35.00
                                          Volum e NaOH (m L)




 Valdosta State University
 Calculation – Molar Mass
                      1 L NaOH     0.116 mol NaOH
18 .50 mL NaOH                                   0.00215 mol NaOH
                    1000 mL NaOH      1 L NaOH




    Valdosta State University
 Calculation – Molar Mass
                      1 L NaOH     0.116 mol NaOH
18 .50 mL NaOH                                   0.00215 mol NaOH
                    1000 mL NaOH      1 L NaOH


                        1 mol unknown
                        1 mol NaOH   0.00215 mol unknown
    0.00215 mol NaOH                
                                     




    Valdosta State University
 Calculation – Molar Mass
                      1 L NaOH     0.116 mol NaOH
18 .50 mL NaOH                                   0.00215 mol NaOH
                    1000 mL NaOH      1 L NaOH


                        1 mol unknown
                        1 mol NaOH   0.00215 mol unknown
    0.00215 mol NaOH                
                                     



                  0.449g unknown 
                  0.00215 mol unknown   209g / mol unknown
                                      
                                      




    Valdosta State University
Calculation - pKa                     Experiment 7, trial 1


          13.00


          12.00


          11.00


          10.00


           9.00


           8.00
     pH




           7.00


           6.00


           5.00


           4.00


           3.00


           2.00
               0.00   5.00    10.00       15.00          20.00   25.00   30.00   35.00
                                           Volum e NaOH (m L)




  Valdosta State University
Calculation - pKa                     Experiment 7, trial 1


          13.00


          12.00


          11.00


          10.00


           9.00


           8.00
     pH




           7.00


           6.00


           5.00


           4.00


           3.00


           2.00
               0.00   5.00    10.00       15.00          20.00   25.00   30.00   35.00
                                           Volum e NaOH (m L)




  Valdosta State University
IMPORTANT REMINDER

Save the unused standardized NaOH solution in a location
identified by your laboratory instructor.




  Valdosta State University
Safety

The NaOH solutions are corrosive. If you get some on your
skin, wash with water for at least five minutes. If any NaOH
solution gets in the eyes IMMEDIATELY wash with water
for at least fifteen minutes. Call for medical assistance!




  Valdosta State University
Waste Disposal

All wastes from this experiment can be flushed down the
drain with plenty of running water.




  Valdosta State University
Important Reminders

• Midterm (i.e. last day to withdraw from classes) is March 2, 2007.
• If you decide to drop the lecture, you do NOT need to drop the lab.
  However, if you wish to drop one but not the other, you CANNOT
  withdraw on the web; you need to actually get the form from the
  registrar and get the signature of the instructor. You only need the
  signature of the laboratory instructor, you do not need my signature
  unless you are in section A.
• Also, registration begins on March 26, 2007.
• Be sure to see your advisor in order to have your “flag” lifted so you
  are eligible to register
• If you do not know who your advisor is, please see the department
  of your major



   Valdosta State University

				
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