Potassium permanganate laboratory practical manual: from formulation calibration to redox titration

If you don't believe it, the most beloved and hated purple crystal in the lab is not the Internet celebrity reagent, but the century-old veteran.potassium permanganateLove it because redox titration is like cooking without salt; hate it because a standard solution can toss you for three days - boil, let stand, calibrate, avoid light, every step tests your patience

Today, the scientific food officer will talk about those things in the laboratory:Potassium permanganate reagentThe role and efficacy ofFrom the nature to the preparation to the titration of the actual combat, a clear for you.

First find out its temper: the physical and chemical ID card of potassium permanganate

Potassium permanganate (KMnO), CAS 7722-64-7, molecular weight158.03Black-purple slender orthorhombic crystals with metallic luster are not easy to mess with2.703 g/cm³Melting point240°C(Decomposition), soluble in water as a purplish-red solution, water-soluble about6.4 g/100mL（20°C）Slightly soluble in organic solvents such as methanol and acetone.

Chemical properties in one sentence: strong oxidation, strong enough to have no friends. Its reduction products vary with the pH of the medium.

• Strong acidity (H ² SO medium): MnO + 8H 🥰 + 5e → Mn ² 🥰 + 4H ² O (colorless)
• Neutral or weakly alkaline: MnO + 2H 2O O + 3e → MnO ²? + 4OH (brown precipitation)
• Strong alkalinity: MnO + e → MnO ² (green)

This guy also has a temper - he will react violently when exposed to organic substances such as ethanol, glycerin, oxalic acid, etc., and evenexplosionSo don't put it with organics, this is the "iron law" of the laboratorybrown bottleStore away from light, otherwise the concentration will be the same day by day.

Laboratory core function 1: "Master Huadan" for redox titration

The best thing about potassium permanganate in analytical chemistry isRedox titrationIt's called the potassium permanganate method.Itself is an indicatorWhen titrating colorless or light-colored samples, fuchsia is its own "signal flare", and the solution is slightly red and30 seconds without fading, Done. Save the money to buy the indicator, and the indicator error is missing, one word: incense.

Examples of application scenarios:

object to be tested	medium	reaction principle	Remarks
Fe²⁺	稀H₂SO₄	MnO和+ 5Fe² じ+ 8H じ→ Mn² じ+ 5Fe³ じ+ 4HO	Classic iron measurement
H₂O₂	稀H₂SO₄	2MnOじ+ 5HOλ + 6H じ→ 2Mn² じ+ 5Oλ ↑ + 8HO	direct titration
Oxalate (C ² O ²)	稀H₂SO₄	2MnO₄⁻ + 5C₂O₄²⁻ + 16H⁺ → 2Mn²⁺ + 10CO₂↑ + 8H₂O	calibration reaction(The point!)
NO₂⁻	稀H₂SO₄	2MnO₄⁻ + 5NO₂⁻ + 6H⁺ → 2Mn²⁺ + 5NO₃⁻ + 3H₂O	Nitrite determination

Laboratory Core Function 2: Preparation and Calibration of Potassium Permanganate Standard Solution (Key Points!)

Potassium permanganate cannot be directly mixed with standard solutions, because there are always impurities such as manganese dioxide (MnO _ 2) mixed in commercially available products, and it will slowly decompose by itself. So the indirect method has to be used - first match the approximate concentration, and then calibrate.

Preparation (operation steps):

1. Weigh approximately 3.2 g of KMnO and dissolve in 1000 mL of distilled water.
2. Heat and boil for 15-20 minutes (to remove reducing substances from the water and promote the complete decomposition of KMnOitself), cool.
3. Use a glass sand core funnel for suction filtration to remove MnO 2O deposits.
4. Transfer the filtrate to a brown reagent bottle and let it stand in the dark for 2-3 days (allowing the remaining reducing impurities to completely react).
5. Filtered again, the filtrate is the stock solution to be calibrated.

Calibration (with reference sodium oxalate):

1. Accurately weigh 0.15-0. 20g of benchmark sodium oxalate dried at 105 ° C to constant weight and dissolved in 100mL distilled water.
2. Add 10 mL of 1 + 1 H2O SOand heat to 70-85 ° C (do not boil, or oxalic acid will decompose).
3. Titration with KMnOsolution while hot, fast first and then slow (slow reaction at the beginning of the drop, add another drop after the fuchsia fades).
4. End point judgment: The solution was slightly red and did not fade for 30 seconds.
5. Parallel calibration 3-4 times, take the average value, and calculate the exact concentration.

Calibration reaction equation: 2MnO + 5C ² O ² + 16H 🥰 → 2Mn ² 🥰 + 10CO ² ↑ + 8H ² O

Why is the temperature 70-85 ° C? If it is low, the reaction is slow, and the end point is muddy; if the oxalic acid decomposes, the calibration result is too large. This temperature range is the "golden temperature" summed up by the predecessors who stepped on countless pits.

Other laboratory uses: disinfecting instruments, treating wastewater

In addition to titration, potassium permanganate also worked part-time in the laboratory on several errands:

• Cleaning utensils: Use oxidizing properties to remove organic stains and reducing residues from glassware (e.g. wash off Fe ² 🥰 residues in burettes).
• Water sample pretreatment: When measuring COD (chemical oxygen demand), it is used as an oxidant to oxidize most of the organic matter in the water sample, which is an old-fashioned way of water quality analysis.
• Deodorization and deodorization: If there is a smell of rotten eggs of hydrogen sulfide in the laboratory fume hood, spray it with a dilute potassium permanganate solution to oxidize it into sulfate, and it will be immediate.

Safety Precautions: Awe "Purple Grumpy"

• Do not come into direct contact with organic matter, reducing agents, acids (especially concentrated sulfuric acid) - it may catch fire or explode.
• Splash on the skin: Immediately rinse with a lot of water. The high-concentration solution will burn the skin and dye it brown (MnO ²). It cannot be washed off after washing, and you have to wait for the skin to metabolize it by itself.
• Waste liquid treatment: KMnO + waste liquid should not be directly poured into the sewer, first reduced to Mn ² with a reducing agent (such as sodium oxalate or sodium oxalate), and then treated as heavy metal waste liquid.

The scientific grain officer said: The reagent is reliable, and the experiment will not overturn

The preparation and calibration of potassium permanganate requires strict requirements for the purity of the reagent, water quality, and temperature at every step. Sodium oxalate is not pure enough, and the calibration concentration deviation is large; distilled water contains trace reducing impurities, and the solution is "scrapped" in one month. At this time, a reliable reagent platform is your "logistics confidence".Scientific Grain Officer- A one-stop shopping platform for scientific materials, with a database of over 1 million fine chemicals, from benchmark sodium oxalate to high-purity potassium permanganate, glass sand core funnels, brown reagent bottles, one-stop supply. "Intelligent manufacturing is only standard, experiments are not too embarrassing", let you spend your energy on titration skills, not worrying about the quality of reagents.

Frequently Asked Questions (FAQ)

• Q: When preparing potassium permanganate solution, why can't it be dissolved directly in distilled water? A: Ordinary distilled water may contain trace amounts of reducing substances (such as organic matter, nitrite), which will react with KMnO. It is recommended to use freshly boiled and cooled distilled water, and heating the boiling solution itself is also to decompose these impurities.

• Q: Why is the first drop of KMnO particularly slow to fade during calibration? A: Because the reaction itself is autocatalytic - Mn ² is the catalyst. At first there is no Mn ² in the solution, the reaction is slow; as the titration progresses, Mn ² is accumulated and the reaction accelerates. So the first drop has to wait until it is completely faded before adding the second drop.

• Q: How long can the KMnO standard solution be stored? A: Store in a dark, sealed and cool place, generally stable for 1-2 months. But it is recommended to re-calibrate once a month. If it is found that the brown MnO ² precipitation has increased significantly, it should be re-formulated.

• Q: Can potassium permanganate measure ClO? A: No. In acidic media, ClO will be oxidized to ClO, and the consumption of KMnO will cause interference. When measuring samples containing ClO, HgClO or MnSO should be masked, or other methods should be used.

"Intelligent manufacturing is only standard, and experiments are not too embarrassing",Scientific Grain OfficerI'll help you press down the "variable" first, and then your titration method will show off.