Titration

1
Titration
By Dr H W Winter
2
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

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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
• Techniquerotate 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

4
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
5
Preparation of the Pipette

• Before you start.
• Never pipette by mouth
• Always use a pipette filler
• Select an appropriate size pipette filler
• Check that the pipette has an intact tip
• 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

6
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
  mark
• 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

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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

8
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

9
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.

10
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
  meniscus
• Make the calibration on the burette face you
• Estimate the readings last digit by
  interpolating

11
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

12
Titration

• You repeat the following procedure for every
  titration
• Pipette a sub-sample into a clean conical flask
• Add 2-4 drops of the appropriate indicator
• Check for drops hanging off the burette (remove
  drops with tissue)
• Check that the funnel was removed from the burette

13
Titration

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

Rough Titration Titration 1 Titration 2 Titration 3 Titration 4
Final reading (mL)
Initial reading (mL) 7.35
Titre (mL)
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Titration

• You repeat the following procedure for every
  titration
• Pipette a sub-sample into a clean conical flask
• Add 2-4 drops of the appropriate indicator
• Check for drops hanging off the burette (remove
  drops with tissue)
• Check that the funnel was removed from the
  burette
• Put the conical flask with sample under the
  burette with the burette outlet just inside the
  flask
• Read and record the starting volume (white
  background)
• Hold the burette tap in your hand while your
  thumb and index finger operate the tap handle
• 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)
• Rinse down the walls of the flask using a
  wash-bottle with distilled water
• Approach the endpoint of the titration drop by
  drop while swirling the flask, until you can just
  see the indicator change
• Read and record your final reading

15
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 Titration 1 Titration 2 Titration 3 Titration 4
Final reading (mL)
Initial reading (mL)
Titre (mL)
16
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

17
Calculation
An unknown 20.0 mL sample of sodium hydroxide
was titrated with 0.120 molL-1 H2SO4

• Average titre
• Balanced equation 2 NaOH H2SO4 ? 2 H2O Na2SO4
  
• Ratios
• Amount of known substance (No of moles)
• Amount of unknown substance(No of moles)
• Mass of unknown substance
• Concentration of unknown substance

(12.11 mL 12.15 mL 11.98 mL) ? 3 12.08 mL
1.208x10-2 L
2 1
n c x Vn 0.120 x 1.208x10-2 1.45x10-3 mol
Ratio is 2 1 2 x 1.45x10-3 2.90x10-3 mol
MNaOH 40 gmol-1m n x Mm 2.90x10-3 x 40
0.116 g
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Problem

• 25.0 mL of an unknown concentration sodium
  hydroxide was titrated with 0.130 molL-1 sulfuric
  acid using Methyl red indicator

• Read the burettes
• Put the data into a table
• Calculate the concentration of the original
  solution

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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