Standardization of permanganate solution for use in potentiometric titrations

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.

0.1M potassium permanganate standardization against arsenic trioxide

Chemical characteristics of the arsenic trioxide As₂O₃ make it a good candidate for a standard substance in many potentiometric methods, however, because of its toxicity it is used less and less frequently.

Arsenic oxide is dissolved in sodium hydroxide, producing sodium arsenite, which is oxidized to arsenate:

5Na₃AsO₃ + 2KMnO₄ + 6HCl -> 2MnCl₂ + 2KCl + 5Na₃AsO₄ + 3H₂O

To speed up the reaction small amount of iodate or iodide is added.

Procedure to follow:

• Weight exactly about 0.15-0.20g of dry arsenic trioxide and transfer it to Erlenmayer flask.
• Add 20 mL of 1M sodium hydroxide solution and dissolve solid.
• Dilute with 150 mL of distilled water.
• Add 10 mL of concentrated hydrochloric acid.
• Add 2-4 drops of about 0.001M KIO₃ solution.
• Titrate until a faint pink color persists for 30 seconds.