Transcript You repeat the following procedure for every titration

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….
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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
25 mL
20°C
Pipetting – sub-sampling
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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
25 mL
20°C
Pipetting - Delivery
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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
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Indicators change their colour at the end-point of the
titration
Indicators are usually introducing an inaccuracy to an acidbase titration, therefore one should use as little indicator as
possible. 2-5 drops are usually sufficient
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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
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In order to see the colour change well it is important to have
a white piece of paper under the flask
Rough Titration
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The first titration is supposed to give you an
approximate “titre” only.
(Titre is the volume delivered by the burette)
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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
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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
meniscus
• Make the calibration on the burette face you
• Estimate the reading’s last digit by interpolating
mL
@20°C
0
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5
Cleaning your Glassware
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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
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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)
4. Check that the funnel was removed from the burette
Titration
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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
Final reading
(mL)
Initial reading
(mL)
Titre (mL)
7.35
Titration 2
Titration 3
Titration 4
Titration
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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
Titration Results
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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
Final reading
(mL)
Initial reading
(mL)
Titre (mL)
Titration 1
Titration 2
Titration 3
Titration 4
Tips for Successful Titrations
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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
(12.11 mL + 12.15 mL + 11.98 mL)  3 = 12.08 mL = 1.208x10-2 L
• Average titre
• Balanced equation
2 NaOH + H2SO4  2 H2O + Na2SO4
Ratios
• Amount of known
substance (No of moles)
2
:
n
c V
• Amount of unknown substance
(No of moles)
• Mass of unknown substance
• Concentration of unknown
substance
1
n=cxV
n = 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-1
m m =n x M
m = 2.90x10-3 x 40 = 0.116 g
n M
n
c V
c=nV
c = 2.90x10-3
2x10-2
c = 0.145 molL-1
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25
35
16
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36
V1
6
V3
7
17
27
V5
Problem
37
V7
8
18
28
38
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19
29
39
10
20
30
40
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41
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32
42
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34
44
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25
35
45
16
26
36
46
•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
V2
V4
17
27
37
18
28
38
48
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29
39
49
20
30
40
50
V6
•Put the data into a
table
47
V8
•Calculate the
concentration of the
original solution
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25 mL
20°C
10 mL
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