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Standardization of redox solutions
EXAMPLE; 0.1M potassium permanganate standardization against sodium oxalate
Sodium oxalate is probably the most important standard substance used in permanganometry. Oxalate is easily oxidized to carbon dioxide, according to the reaction equation:
5Na2C2O4 + 2KMnO4 + 8H2SO4 → 10CO2 + 8H2O + 5Na2SO4 + 2MnSO4 + K2SO4
First portions of titrant added are decolorizing slowly. To speed up reaction at the beginning solution can be heated to about 70°C. However, reaction is catalyzed by the presence of Mn2+ cations, so it gains speed later during titration
Procedure to follow:
- Weight exactly about 0.25-0.30g of dry sodium oxalate and transfer it to Erlenmayer flask.
- Add 100 mL of distilled water and dissolve solid.
- Add 60 mL of 1M sulfuric acid solution.
- Heat up the solution to about 70°C.
- Titrate adding small volumes of titrant and waiting each time till the solution decolorizes.
- Close to titration end point add titrant dropwise, until a faint pink color persists for 30 seconds.
Titration of Potassium Permanganate against Oxalic Acid
Prepare a standard Oxalic acid solution of about 250 ml.
The molecular mass of oxalic acid is calculated by adding the atomic mass of each constituent atom
The molecular mass of H2C2O4.2H2O = 126
Since the weight of oxalic acid that is required to make 1000 ml of 1M solution is 126 g. Hence, the weight of oxalic acid needed to prepare 250 ml of 0.1 M solution = 126/1000 x 250 x 0.1 = 3.15 g
Determining the Strength of KMnO4 using Standard Oxalic Acid
In this titration, the analyte is oxalic acid and the titrant is potassium permanganate. The oxalic acid acts as a reducing agent, and the KMnO4 acts as an oxidizing agent. Since the reaction takes place in an acidic medium, the oxidizing power of the permanganate ion is increased. This acidic medium is created by the addition of dilute sulfuric acid.
MnO−4+8H++5e−→Mn2++ 4H2O
KMnO4 acts as an indicator of where the permanganate ions are a deep purple colour. In this redox titration, MnO4– is reduced to colourless manganous ions (Mn2+) in the acidic medium. The last drop of permanganate gives a light pink colour on reaching the endpoint. The following chemical equation can represent the reaction that occurs.
Molecular equation
2KMnO4 + 3H2SO4→K2SO4 + 2MnSO4 +3H2O+5[O]
H2C2O4.2H2O+[O]→2CO2+3[H2O]×5
Complete Reaction
2KMnO4+3H2SO4+5H2C2O4.2H2O→K2SO4+2MnSO4+18H2O+10CO2
Ionic equation
MnO−4+8H++5e−→Mn2++4H2O]×2
C2O2−4→2CO2 + 2e−]×5
Complete Reaction
2MnO−4+16H++5C2O2−4→2Mn2++8H2O + 10CO2
From the above-balanced chemical reaction, it can be observed that 2 moles of KMnO4 reacts with 5 moles of oxalic acid