By Scott Becker
Updated Mar 24, 2022

Potassium permanganate (KMnO₄) is a potent oxidizing agent that loses stability over time, especially when exposed to light. For accurate quantitative work such as titrations, the solution must first be standardized. A common reducing agent is sodium oxalate, which reacts cleanly with permanganate and allows precise determination of its concentration. The following protocol demonstrates how to standardize a KMnO₄ solution and subsequently reduce it in a laboratory setting.

Step 1 – Protect the KMnO₄ Solution

Keep the KMnO₄ solution in a dark, opaque container to prevent photodegradation. Fill a burette with the solution and record the initial volume.

Step 2 – Prepare the Oxalate Standard

Place a beaker beneath the burette and add the sodium oxalate solution, ensuring a magnetic stir bar is present. Record the volume and known concentration of the oxalate solution.

Step 3 – Perform the Titration

Using the hot plate’s magnetic stir function, titrate the KMnO₄ against the oxalate solution. Do not heat the beaker; simply stir until the persistent purple color disappears, indicating the endpoint. Record the volume of KMnO₄ added.

Step 4 – Calculate KMnO₄ Concentration

The stoichiometry of the reaction is:

2 MnO₄^– + 5 C₂O₄^2– → 2 Mn²⁺ + 5 CO₂ + 8 H⁺

Using this, compute the concentration of the KMnO₄ solution with:

2 × [KMnO₄] × V_KMnO4 = 5 × [Na₂C₂O₄] × V_oxalate

Solve for [KMnO₄].

Step 5 – Determine Oxalate Volume Needed for Reduction

With the calculated KMnO₄ concentration, use the same stoichiometric equation to find the exact volume of oxalate required to fully reduce the permanganate solution.

Step 6 – Mix and Observe

Measure the calculated oxalate volume and combine it with the KMnO₄ solution. The mixture should become nearly colorless once the reaction is complete.

Step 7 – Adjust pH

Test the solution’s pH with litmus paper. If the result is not between pH 7–8 (green or yellow), add small amounts of sodium bicarbonate, stirring until neutrality is achieved.

Step 8 – Eliminate Residual Color

If a faint pink hue remains, introduce a trace amount of sodium bisulfite until the color disappears completely.

Step 9 – Dispose of the Solution

Transfer the fully reduced, colorless solution to a designated chemical waste container for safe disposal.

Things Needed

• Potassium permanganate solution
• Burette
• Sodium oxalate solution (known concentration)
• Beaker
• Magnetic stir bar
• Hot plate
• Sodium bicarbonate (solid)
• Sodium bisulfite (solid)
• Calculator
• Paper
• Litmus paper

TL;DR (Too Long; Didn’t Read)

If the KMnO₄ concentration is already known, you can skip to Step 5 and proceed directly to reduction.

Warning

Direct contact with oxalate or permanganate solutions can be hazardous to the skin. Always wear appropriate personal protective equipment.