experiment # 2 - redox tit rations
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E XPERIMENT # 2:REDOX TITRIMETRY
Dates of Experiment: Nov. 22, 24 & 29
Quantitative Analysis of Fe2+
in an Unknown Solution
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The oxidation number of an element
indicates the number of electrons
lost, gained, or shared as a result of
chemical bonding. The change in the
oxidation state of a species lets youknow if it has undergone oxidation
or reduction.
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Oxidation can be defined as "anincrease in oxidation number".
In other words, if a species starts outat one oxidation state and ends up at
a higher oxidation state it has
undergone oxidation.
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Conversely,
Reduction can be defined as "a
decrease in oxidation number".
A ny species whose oxidation number
is lowered during the course of a
reaction has undergone reduction.
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A. Preparation and Standardization of KMnO4
1. Prepare a 500 mL of 0.1000 N KMnO4. Store this
solution in an amber bottle and keep it under a
shade.
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KMnO4 is not a primary standard. It is difficult
to obtain the substance perfectly pure and
completely free from manganese (IV) oxide.Ordinary distilled water is likely to contain
reducing substances (traces of organic matter,
etc) which will react with KMnO4 to form MnO2:
4 MnO4 - + 2 H2O 4 MnO2 + 3 O2 + 4 OH ²
Permanganate is inherently unstable in the
presence of Mn 2+ ions:
2 MnO4 ² + 3 Mn 2+ + 2 H2O 5 MnO2 + 4 H+
This reaction is slow in acid solution, but is very
rapid in neutral solution.
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For these reasons, KMnO4 is rarely made up by
dissolving amounts of the purified solid in water;
it is more usual to heat a freshly prepared
solution to boiling and keep it on the steam bath
for an hour or so, and then filter the solution
through a non-reducing filtering medium, such as
purified glass wool or a sintered-glass filteringcrucible (porosity No. 4). A lternatively, the
solution may be allowed to stand for 2 ² 3 days at
room temperature before filtration.
Solutions of KMnO4
should be protected from
unnecessary exposure to light: a dark-colored
bottle is recommended. Diffuse daylight causes
no appreciable decomposition, but bright sunlight
slowly decomposes the solution.
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2. Prepare 2 liters of 0.9 M H2SO4
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Sulfuric acid is used to acidify the solution since
it has no action upon permanganate in dilute
solution. With hydrochloric acid, there is alikelihood of this reaction taking place:
2 MnO4 ² + 10 Cl ² + 16 H + 2Mn 2+ + 5Cl2 + 8
H2O
and some MnO4 may be consumed in the
formation of Cl2. This reaction is particularly
liable to occur with iron salts unless special
precautions are adopted.
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3. Dry sodium oxalate (Na2C2O4) at 105 °C for 1
hour and allow it cool in a desiccator.
4. Weigh 0.25 gram samples of dried Na2C2O4 to
the nearest milligram. Transfer the solids in a
500 mL or 400 mL Erlenmeyer flask.
5. Dissolve the Na2C2O4 into a 250 mL of 0.9 M
H2SO4 solution.
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6. A dd 95 % of the theoretical amount of KMnO4
solution needed for the titration. (This can be
calculated from the mass of KMnO4 used toprepare the KMnO4 solution.)
MnO4 ² + 8 H + + 5 e ² Mn 2+ + 4 H2O
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7. Stir the mixture properly at room temperature
until it is colorless.
8. Heat the solution to 55 ° ² 60 ° C for at least 10
minutes. Use a thermometer to monitor the
temperature of the solution.
9. Without delay, complete the titration until the
first pale pink color persists.
10. A
s a blank, titrate 250 mL of 0.9 M H2SO4 tothe same pale pink color.
11. Perform at least three (3) trials.
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12. Calculate the normality of KMnO4 of the
solution. Calculate the average value, standard
deviation and % Relative Standard Deviation of
the results.
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B. Analysis of Fe2+ in an Unknown Solution
1. You will be assigned to a particular volume of unknown Fe2+ solution to be used throughout the
experiment.
2. Pipet out the assigned volume of unknown of unknown Fe2+ solution into a 100 mL volumetric
flask.
3. Dilute the solution to the mark and homogenize the
mixture. This solution would serve as your unknown.
4. Pipet 20 mL of the unknown solution and mix it into
a 250 mL of 0.9 M H2SO4 solution.
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5. A dd 25 mL of Zimmermann and Reinhardt·s
solution.
The Zimmermann and Reinhardt·s solution
is prepared by dissolving 50 grams of crystallized
manganese (II) sulfate (MnSO4 · 4 H2O) in
250 mL water, adding a cooled mixture of 100 mLconcentrated H2SO4 and 300 mL water, followed
by 100 mL orthophosphoric acid, H3PO4. The
MnSO4 lowers the reduction potential of the
MnO4 ² ² Mn 2+ couple and thereby makes it a
weaker oxidizing agent; the tendency of thepermanganate ion to oxidize Cl ² is thus reduced.
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5. Without delay, complete the titration until the
first pale pink color persists.
6. A s a blank, titrate 250 mL of 0.9 M H2SO4
added with 25 mL of Zimmermann and
Reinhardt·s solution to the same pale pink
color.
7. Perform at least three (3) trials.