experiment # 2 - redox tit rations

8/3/2019 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



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.



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.



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.



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.



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.



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.



2. Prepare 2 liters of 0.9 M H2SO4



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.



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.



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



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.



12. Calculate the normality of KMnO4 of the

solution. Calculate the average value, standard

deviation and % Relative Standard Deviation of

the results.



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.



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.



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.



8. Calculate the concentration of Fe (ppm) in the

solution. Calculate the average value, standard

deviation and % Relative Standard Deviation of the results.

experiment # 2 - redox tit rations

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