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Titration

VIEWS: 4 PAGES: 31

									Titration

By Dr H W Winter
             Sampling Glassware
• If you collect solutions (standard or sample solutions)
  from a stock solution container, the beaker must be
  absolutely clean (rinsed with distilled water) and dry
  before you transfer anything into it; alternatively it is
  rinsed twice with small amounts of the stock solution

• Make sure you label your receptacles! You must never
  mix up any of these containers

• Keep the solutions in a safe place at the back of your
  work station such that they cannot be contaminated

• It is good practice to have the standard solution to the
  left and sample preparation to the right of the burette
                Rinsing the Burette
Before filling the burette:
• Rinse it with water
• Then rinse it with distilled water
   using a wash-bottle
• Then rinse it twice with small
   quantities of the standard solution

•   Technique:
    rotate the burette in your hands
    while holding it horizontally in
    order to wet all sides, and ….
    … while pouring some of the liquid
    out the top;
    now drain the rest of the liquid
    through the bottom of the burette
                   Filling the Burette
Close the stop cock
Then fill the burette with the standard
   solution using a funnel
Now remove the funnel
Release the air below the stop cock
   by opening the tap fully for a very
  short time
Read and record the bottom of the
  meniscus as accurately as possible
  (the meniscus does not have to be on
  the 0.00 mL mark).
Tips: avoid a drop of standard hanging off
   the burette outlet;
   hold a white piece of paper behind the
   burette for easier reading.
   Clamp your burette absolutely vertical
         Preparation of the Pipette
Before you start….
•   Never pipette by mouth
•   Always use a pipette filler
•   Select an appropriate size pipette filler
                                                      25 mL
•   Check that the pipette has an intact tip           20°C


•   Rinse the pipette with distilled water first, …
•   …then with a small volume of the standard or
    sample solution you will pipette
•   Make sure you rinse all inside walls of the
    pipette right up to above the mark in this
    process
•   Discard and blow out the washings
•   Wipe the tip of the pipette with a tissue
    Pipetting – sub-sampling
•   Make sure you have enough sample for at
    least 4 titrations
•   Put a clean 100 mL conical flask next to the
    sample container (rinsed with distilled water, it
    does not have to be dry)
•   Re-set your pipette filler and then suck up your
    sample material to just above the calibration       25 mL

    mark                                                 20°C



•   Fine-adjust the level such that the bottom of
    the meniscus is level with the mark
•   Withdraw the pipette gently from the sample
    container while keeping contact between the
    pipette tip and the beaker wall. Make sure you
    do not lose any solution from the pipette
    through hasty vertical movement
•   You may tilt the pipette and allow an air bubble
    to get sucked in; when you move the pipette to
    the conical flask make sure you do not lose
    any liquid
Pipetting - Delivery
•   After inserting the pipette to the conical flask
    deliver the measured amount by pressing the
    fast release lever of your pipette filler. Your
    pipette tip should be in contact with the conical
    flask wall
•   It is important to leave the tip in contact with
    the wall for 3 more seconds after the solution
    has been drained from the pipette. This allows
    for the exact delivery of the volume
•   Any residual liquid in the tip of the
    pipette must stay in the pipette.
    The pipette has been calibrated
    to have delivered the exact
    volume without draining the
    last drop in its tip
                              Indicators
•   Indicators change their colour at the end-point of the
    titration
•   Indicators are usually introducing an inaccuracy to an acid-
    base titration, therefore one should use as little indicator as
    possible. 2-5 drops are usually sufficient

•   Make sure you know what the expected colour change is
    before you start the titration. Try it out in a test-tube using
    some other chemicals

•   In order to see the colour change well it is important to have
    a white piece of paper under the flask
                      Rough Titration
•   The first titration is supposed to give you an
    approximate “titre” only.
    (Titre is the volume delivered by the burette)

•   You therefore allow the standard from the burette to
    flow into the conical flask while swirling the flask
    without any ambition to get an exact result. The
    endpoint is indicative of the expected titre. Record
    this value as the “rough” titration

•   Subsequent titrations are then allowing you to
    discharge the solution from the burette with the tap
    fully open to about 1 mL before the anticipated
    endpoint of the titration.
           Reading the meniscus
• Make sure you are eye-level with the bottom of the
  meniscus; always read the bottom of the meniscus!
• Have your burette dead vertical
• Hold a white piece of paper behind the burette to
  improve the recognition of the bottom of the          mL
  meniscus                                             @20°C
                                                        0
• Make the calibration on the burette face you
• Estimate the reading’s last digit by interpolating    1



                                                        2



                                                        3



                                                        4



                                                        5
          Cleaning your Glassware
•   After each titration you should clean your conical flask immediately by
    emptying it first, then rinsing it twice with tap water and twice with distilled
    water. It is not important that the flask is dry before it is re-used
•   Your pipette is not cleaned between pipetting processes of the same
    sample
•   Pipettes, volumetric flasks and burettes are rinsed with tap water first and
    then twice with distilled water
•   None of the glassware needs to get dried after rinsing with distilled water




                                                             10 mL
                                                             20°C
                                 25 mL
                                  20°C
                                  Titration
•  You repeat the following procedure for every titration:
1. Pipette a sub-sample into a clean conical flask
2. Add 2-4 drops of the appropriate indicator
3. Check for drops hanging off the burette
   (remove drops with tissue)
4. Check that the funnel was removed from the burette
                                                 Titration
•     You repeat the following procedure for every titration:
1.    Pipette a sub-sample into a clean conical flask
2.    Add 2-4 drops of the appropriate indicator
3.    Check for drops hanging off the burette
      (remove drops with tissue)
4.    Check that the funnel was removed from the burette
5.    Put the conical flask with sample under the burette with
      the burette outlet just inside the flask
6.    Read and record the starting volume (white background)
7.    Hold the burette tap in your hand while your thumb and
      index finger operate the tap handle

                       Rough       Titration 1    Titration 2   Titration 3   Titration 4
                       Titration

     Final reading
     (mL)
     Initial reading
     (mL)
                                   7.35
     Titre (mL)
                                 Titration
•     You repeat the following procedure for every titration:
1.    Pipette a sub-sample into a clean conical flask
2.    Add 2-4 drops of the appropriate indicator
3.    Check for drops hanging off the burette
      (remove drops with tissue)
4.    Check that the funnel was removed from the burette
5.    Put the conical flask with sample under the burette with
      the burette outlet just inside the flask
6.    Read and record the starting volume (white background)
7.    Hold the burette tap in your hand while your thumb and
      index finger operate the tap handle
8.    Allow the standard from the burette to flow into the
      conical flask while swirling the flask to about 1 mL before
      the anticipated endpoint (based on the “rough titration”)
9.    Rinse down the walls of the flask using a wash-bottle with
      distilled water
10.   Approach the endpoint of the titration drop by drop while
      swirling the flask, until you can just see the indicator
      change
11.   Read and record your final reading
                              Titration Results
•   It is important to process the data collected straight after each titration
•   As soon as you have concordant results (3 titres within 0.1 mL of each other)
    you can stop titrating and start calculating
•   It might be easier to write the final reading above the initial reading for quick
    manual subtraction

                  Rough         Titration 1   Titration 2   Titration 3   Titration 4
                  Titration
Final reading
(mL)
Initial reading
(mL)
Titre (mL)
     Tips for Successful Titrations
•   Close to the endpoint of the titration you might like to
    add only part of a drop; this is possible by allowing
    half of a drop to come out of the burette and with the
    help of your wash-bottle you can rinse it into your
    flask
•   It is also good practice to wash down the inside walls
    of your conical flask again once you have reached
    the end-point
•   The indicator change should be visible for 10 to 15
    seconds after the addition of the last drop from the
    burette. If this is not the case, add another ½ drop.
•   Some indicator changes are fading after 30 seconds
    or so, rely on your earlier observations
•   Have a test tube with the changed indicator next to
    your titration flask to remind you of the expected
    colour change
                           Calculation
An unknown 20.0 mL sample of sodium hydroxide was titrated with 0.120 molL-1 H2SO4

 • Average titre  (12.11 mL + 12.15 mL + 11.98 mL)  3 = 12.08 mL = 1.208x10-2 L

 • Balanced equation          2 NaOH + H2SO4  2 H2O + Na2SO4
                      Ratios           2    :     1
 • Amount of known                   n          n=cxV
   substance (No of moles)                      n = 0.120 x 1.208x10-2 = 1.45x10-3 mol
                                    c V
 • Amount of unknown substance                         Ratio is 2 : 1
                                                       2 x 1.45x10-3 = 2.90x10-3 mol
   (No of moles)                                           MNaOH= 40 gmol-1
 • Mass of unknown substance                          m m =n x M
                                                           m = 2.90x10-3 x 40 = 0.116 g
                                                   n M
 • Concentration of unknown                                 c=nV
   substance                           n                    c = 2.90x10-3
                                      c V                        2x10-2
                                                            c = 0.145 molL-1
 5        15        25        35
     V1
 6        16        26        36


 7        17
               V3
                    27   V5   37
                                        Problem
                                   V7
 8        18        28        38


 9        19        29        39
                                        •25.0 mL of an unknown
10        20        30        40        concentration sodium
                                        hydroxide was titrated
11        21        31        41
                                        with 0.130 molL-1 sulfuric
12        22        32        42        acid using Methyl red
                                        indicator
13        23        33        43

                                        •Read the burettes
14        24        34        44

                                        •Put the data into a
15        25        35        45
                                        table
16        26        36        46
     V2                                 •Calculate the
17        27
               V4   37        47        concentration of the
                         V6
                                   V8   original solution
18        28        38        48


19        29        39        49


20        30        40        50
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         @20°C

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               10 mL
       25 mL   20°C
        20°C
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                           Tips
• In the process of sample preparation it is advisable to
  prepare 2 or 3 samples simultaneously.
• Filling the burette can happen while you hold it, it does
  not have to be clamped to a stand.
• If you have to add an acid to a redox-titration aliquot it
  does not have to be measured by pipette, measuring
  cylinder accuracy is sufficient.
• Make sure your burette does not leak.
• Swirling the conical flask is important; particles have to
  meet in order to react with each other.
• Tidy work place and tidy table of results mitigate
  mistakes

								
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