Sulatanat of Oman
Sultan Qaboos university
Collage of science
Department of Chemistry
CHEM4441
Experiment 3:
Redox Titration
Done by: lora
ID #:
Date:5/3/2011
1
Objectives:
* To standardize potassium permanganate solution using primary standard
sodium oxalate.
* To determine the amount of calcium in limestone.
Introduction:
A redox titration is based on oxidation-reduction reaction between analyte
(Na2C2O4) and titrant (KMnO4). Potassium permanganate is a powerful
oxidizing agent and serves as a self indicator in the titration with reducing
agent.
Sodium oxalate is used for standardization of potassium permanganate as
the following reaction show
5C2 O 2 4( aq)  2MnO  4( aq)  10CO2( g )  8H 2 O(l )  2Mn 2 ( aq)
And this is the reaction which used in this experiment.
Methods:
1- Standardization of Potassium Permanganate.
Two samples of Na2C2O4 were weighed and transferred to 250 ml conical
flasks. Then 50 ml of H2SO4 was added to each of the flasks and then heated
to 80-90 0C.After that titrated against KMnO4 immediately when it is
reached that temperature.
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2- Analysis of the unknown Limestone.
A sample of dry limestone were weighted and transferred to 250 ml beaker.
10ml of water was added and covered with watch glass. Then 10ml of
concentrated HCl was added. After that 5 drops of saturated bromine was
added followed by boiling in the hood for 5 minutes to remove the excess Br2.
Then it was diluted to about 50 ml with water and heated to boiling. 100 ml
of hot 6% ammonium oxalate solution were added in the hood 3 or 4 drops of
methyl red indicator were added and precipitate CaC2O4 by slowly adding
6M NH3 and continue adding 1 drop every 3 to 4 second until the solution
turn to yellow orange color. The solution was allowed to cool fro 30 minutes
then it was filtered. Then the precipitate was transferred to 250 ml conical
flask. Finally titrated it against KMnO4.
Apparatus:
Two 250 ml conical flasks, 250 ml beaker, 100 ml graduated cylinder,
burette, filter flask, funnel.
Materials:
KMnO4 solution, Na2C2O4, Limestone, Bromine H2O, Concentration HCL,
(NH4)2C2O4 solution, Methyl red indicator and dilute H2SO4.
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Results:
A
sample 1
sample 2
m(NaC2O4) -(g)
0.233 ± 0.0001
0.2345 ± 0.0001
Vi(KMnO4) -(ml)
0.10 ± 0.05
11.91 ± 0.05
Vf(KMnO4) -(ml)
11.91 ± 0.05
23.69 ± 0.05
V(KMnO4) -add-(ml)
11.81 ± 0.07
11.78 ± 0.05
M(KMnO4) -(mol/l )
0.05889 ± 0.0001
0.05916 ± 0.0001
Table 1: Data taken from the experiment.
B
sample 1
sample 2
m-(g)
0.2590 ± 0.0001
0.2593 ± 0.0001
Vi(KMnO4) -(ml)
32.45 ± 0.05
23.69 ± 0.05
Vf(KMnO4) -(ml)
42.32 ± 0.05
32.23 ± 0.05
V(KMnO4) -add-(ml)
9.87 ± 0.05
8.45 ± 0.05
% mass CaO
31.6
27.03
% mass Ca
22.58
22.55
Table 2: Data taken from the experiment.
Calculations:
See the appendix.
Discussion:
 We must get rid of other cations such as Fe2+ & Ni2+ to avoid forming
of insoluble oxalates or co-precipitate of the unknown with calcium
oxalate making it impure.
 Alternative methods to determine Ca2+ is substitution titration in
which the calcium oxalate titrated to get color changing from red to
blue.
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 The advantage in using my method is in the detection of color change
from colorless to pink at the end point is easy, but in substitution
method the color changes from red to blue and that difficult to detect.
 The advantage of using substitution method is the product solubility
so it is easily to be filtered. The disadvantage of this method cannot be
used to any sample except which contain magnesium or alkali metals.
Conclusions:
 Redox titration is important in determination of the mass of calcium
in a sample and then determination of the percent of mass in limestone
is by titrating it with KMnO4. The result is 22.57 % and it shows
that there are many sources of errors in this experiment:

 Co-precipitate of the other cations with the calcium oxalate and
incomplete precipitation of CaC2O4 when adding NH3.
 Standardization process of KMnO4, impurities of the chemicals.
 Wrong determination of the end point.
 Errors in reading the burette.
 Errors in the analytical balance and the glassware used.
 Incomplete heating at a specific temperature and doing the titration
when the media is cool.
 Loss of precipitate during filtration and loss of precipitate during
transferring
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Referances:
Handout.
D. C. Harris, Quantitative Chemical analysis, 6th ed, Freeman: New York,
1999.
Index:
Index
Page no.
Objectives
2
Introduction
2
Method
2
Apparatus
3
Materials
3
Results
4
Calculations
In appendix
Discussion
4
Conclusions
5
References
6
6