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O-1
EXPERIMENT O: REDOX TITRATIONS
OBJECTIVES
In this experiment, a standardized solution of potassium permanganate, KMnO
4,
will be prepared that
will be used to accurately determine the concentration of hydrogen peroxide,
H2O2, in an unknown solution. In the process, the stoichiometry of a number of redox reactions will be
studied. The technique of pipetting will be introduced and practiced.
THEORETICAL CONSIDERATIONS:
In strongly acidic solutions, the reduction of permanganate ion, MnO
4
-
, to the manganese(II) ion, Mn
2+
,
is essentially a quantitative reaction (1):
MnO
4
-
(aq)
+ 8 H
+
(aq)
+ 5 e
-
Mn
2+
(aq)
+ 4 H2O
(l)
E
= + 1.507 V
(1)
In solutions of higher pH, MnO
4
-
is reduced to MnO2 (brown solid) in a less useful reaction (2):
MnO
4
-
(aq)
+ 2 H2O
(l)
+ 3 e
-
MnO2
(s)
+ 4 OH
-
(aq)
E
= + 0.595 V
(2)
Sulfuric acid is used to control the pH in the permanganate titration so that the solution is strongly
acidic. The sulfate ion is a particularly good anion to use in this redox titration since it cannot be oxidized
by permanganate ion. As well, sulfuric acid cannot act as an reducing agent:
2 SO
4
2-
(aq)
S2O
8
2-
(aq)
+ 2 e
-
-E
= -2.010 V
Potassium permanganate is a powerful oxidizing agent and although it is available in high purity, it can be
reduced by organic impurities present in water. Consequently, potassium permanganate solutions of
known concentrations cannot be prepared by simply weighing out a sample of the solid and dissolving it
in a known volume of water. There is a further complication. When permanganate ions are reduced by
trace organic compounds in water
(pH
7), the insoluble brown solid manganese dioxide, MnO2, is formed. MnO2 is a weak oxidizing
agent. If MnO2 is present in the solution, it will affect the accuracy of the titration. To eliminate this
problem, solutions of KMnO
4
must be “aged” for at least a week and filtered to separate any MnO
2(s)
from the solution prior to its use in redox titrations. The solution of KMnO
4
will be prepared in advance
and may be used immediately in the experiment.
