Preparation and Standardization of Potassium Permanganate Standard Solution Purposes

Topics: Titration, Potassium permanganate, Hydrogen peroxide Pages: 7 (1417 words) Published: January 19, 2014
Experiment 5

Preparation and Standardization of

Potassium Permanganate Standard Solution
Purposes
1.To know the method of preparation and standardization of potassium permanganate standard solution.
2. To grasp the principle, the conditions and the method of permanganate titration. 3. To grasp the determination of hydrogen peroxide in hydrogen peroxide solution with permanganate method.
Principle
Permanganate titration with potassium permanganate solution (KMnO4) as standard solution is one of the oxidation-reduction titration methods. KMnO4 is a vigorous oxidant in an acidic solution, and its electrode reaction and standard electrode potential φ are as follows,

MnO4- + 8H+
Mn2+ + 4H2O
φ = + l.51V
The contents of many reducible substances can be determined directly or indirectly by the permanganate method.
The titration reaction of permanganate method should be carried out in sulfuric acid (H2SO4) solution. Because nitric acid (HNO3) is of oxidization, and because hydrochloric acid (HCl) is of reduction, they are unavailable to adjust the acidic concentration of solution. In general, the available concentration of H2SO4 is 1

c( H2SO4)=1~2molL-l. If the acidic concentration is too high, it will result in 2
decomposition of KMnO4:
4MnO4- + 12H+
4Mn2+ + 5O2
+ 6H2O
If it is too low, the reduction product of KMnO4 is MnO2 which is brown deposit, and this kind of reaction can not be used in titration reaction, either. KMnO4 on sale usually contains impurities such as manganese dioxide, chloride, sulfate, nitrate and so on. Therefore, it can not be used directly in the preparation of standard solution. Moreover, because the oxidization ability of KMnO4 is strong, and it readily reacts with reductive substances such as organic impurities in water, ashes in air and so on, it easily decomposes when exposed to light. When it is prepared, its solution must be boiled or be dissolved with cold distilled water and then kept in brown reagent bottle in dark.

The primary standard substances used in standardizing KMnO4 solution include sodium oxalate(Na2C2O4), oxalic acid (H2C2O4 2H2O), arsenic trioxide (As2O3) etc because it has no crystalline water or water absorption, Na2C2O4 is often used. In H2SO4 solution the reaction between KMnO4 and Na2C2O4 is

2MnO4- + 5C2O42- + l6H+
CO2
+ 2Mn2+ + 8H2O
The reaction carries on under the condition of 75~85ºC, so that the presence of

Mn2+ functions as catalyst. At the beginning of titration, the KMnO4 solution must be titrated drop by drop for the speed of reaction is very slow; during the course of titration, Mn2+ is gradually produced in the solution, and it increases the reaction speed, so titration speed may be increased slightly, but don't drop KMnO4 solution with a flow. Otherwise, KMnO4 solution will decompose without reacting with C2O42in time in hot acidic solution as follows: 4MnO4- + l2H+

4Mn2+ + 5O2 + 6H2O
Because KMnO4 solution itself has color, and when the reaction reaches its stoichiometric point in the course of titration, slightly excessive amount of KMnO4 solution is added to make the solution pink, and no additional indicator is needed. In acidic solution, KMnO4 can oxidize H2O2, and the reaction is: 2MnO4- + 5H2O2 + 6H+

2Mn2+ + 5O2
+ 8H2O
Because H2O2 readily decomposes when heated, the titration must carry on under room temperature.
H2O2 with mass concentration ρ(H2O2) ==0.025~0.035kg L-l is a common medical sterilizing reagent. The mass concentration of H2O2 on sale is ρ(H2O2)=0.03kg L-1or ρ(H2O2))=0.3kg L-1, which is extremely instable, and before titration it is necessarily diluted with distilled water to a certain mass concentration to reduce sampling error.

Apparatus and reagents
Apparatus Analytical balance; wash bottle; acid buret of 50mL; pipet of 25mL; beaker of 50mL;, beaker of 500mL;measuring pipet of 5mL; volumetric flask of 250mL; Erlenmeyer flask of 250mL; measuring cylinder of l0mL; 1

Reagents c( H2SO4)=6molL-1 ; solid...
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