E-book – Potassium permanganate test

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Section 4: Potassium permanganate test I. Method summary: Methanol contains reducing impurities, which react with potassium permanganate in a neutral solution, reducing potassium permanganate to manganese dioxide. Determine the time required for the pink color of potassium permanganate to gradually fade to match the color standard. II. Reagents and Solutions (1) Preparation of water: Add an appropriate amount of water to sufficient dilute potassium permanganate solution until a stable pink color is achieved; boil for 30 minutes. If the pink color fades, add more potassium permanganate solution until the pink color reappears. Allow it to cool before use; this solution is prepared just before it is needed. (2) Preparation of potassium permanganate solution: Accurately weigh 0.200 g of potassium permanganate (KMnO4) and place it in a 1000 mL brown volumetric flask; dissolve it in water and dilute to the mark, then mix well. Store sealed in a dark place; shelf life is one week. (3) Preparation of color standards: Weigh 2.5000 g of cobalt chloride (COCl2•6H2O) and 2.8000 g of uranyl nitrate [2(NO3)2•6H2O], dissolve them in water, transfer the solution quantitatively to a 1000 mL volumetric flask, add 10 mL of nitric acid solution (concentration = 2 mol/L), dilute to the mark with water, and mix well for later use. This solution is valid for three months. III. Instruments and Equipment (1) Water bath: Temperature to be controlled at (15±0.5)℃; KF-4 type low-temperature constant temperature water bath or an instrument with equivalent precision ； (2) Colorimetric tube: 50 mL, colorless glass container with a glass stopper. (3) Pipette: 2 mL. Analysis procedure: Before measurement, all instruments should be pre-rinsed with 1+1 hydrochloric acid, then washed with tap water, followed by washing with distilled water and drying. Using a pipette, take 50 mL of the methanol sample at approximately 15°C, pour it into a colorimetric tube, and place it in a water bath at (15±0.5)°C. The water level in the water bath should be above the level of the sample in the colorimetric tube. After 15 minutes, remove the colorimetric tube from the water bath, add 2 mL of potassium permanganate solution using a pipette; record the time when the first drop is added, then seal the tube, shake it well, and place it back in the water bath. After a certain period of time, the sample tube was taken out of the water bath and an equal volume of colorimetric tube was added to it (this tube did not need to be placed in the water bath); then, against a white background, the change in color of the methanol sample was observed from the side. Be careful to avoid exposing the sample solution directly to sunlight to prevent the decomposition of potassium permanganate. Record the time at which the color change of the methanol sample matches the color of the color chart. This time range is the measurement time for the potassium permanganate test. 5. The difference between the results of parallel measurements is allowed to be no more than 3 min. The average value is taken as the measurement result.