Potassium Permanganate
Properties
| State | Solid (dark purple, almost black, needle-like crystals) |
| Color | Dark purple (deep purple in solution) |
| Solubility | Soluble in water (64 g/L at 20°C) |
| Melting Point | 240°C (decomposes) |
| Boiling Point | Decomposes before boiling |
About Potassium Permanganate
Potassium permanganate, KMnO4 at 158.034 g/mol, is the dark purple crystal that gives chemistry its most theatrical color change. Manganese sits in the +7 oxidation state, which is as high as Mn ever goes, and that thermodynamic restlessness is what makes the salt such a strong oxidizer (E° = +1.51 V vs SHE in acidic solution). Solutions are intensely violet — about 2400 L/(mol·cm) molar absorptivity at 525 nm — which means even a millimolar solution looks deeply colored, and that visual sensitivity makes KMnO4 a self-indicating titrant. Drop it from a buret into Fe2+ or oxalate or H2O2, and the purple disappears as fast as it falls until the equivalence point, when the next drop persists as a faint pink that never decolorizes. Permanganometry is one of the oldest analytical titration methods still in routine use — the Reineckert-Stamm BOD analysis dropped permanganate in favor of dichromate decades ago, but iron and oxalate determinations still default to KMnO4. In organic synthesis, hot concentrated KMnO4 cleaves alkenes oxidatively to carboxylic acids, while cold dilute KMnO4 (the Baeyer test) syn-dihydroxylates alkenes to vicinal diols and tells you visually that an unknown is unsaturated. In water treatment, KMnO4 oxidizes dissolved Fe2+ and Mn2+ to insoluble oxides that precipitate out at the filter bed, and it knocks down hydrogen sulfide odor in well water.
Where you'll encounter it
If you've ever watched the classic 'volcano' chemistry demonstration where glycerin sets fire spontaneously when dropped on KMnO4, treated a tropical fungal infection with a dilute purple soak, or tested an alkene with the Baeyer reagent, you've worked with permanganate. In a working water-treatment plant, KMnO4 is dosed at the head of the plant to pre-oxidize iron and manganese in raw groundwater — a typical dose runs 1-5 mg/L, and the pink color must completely disappear before the water reaches the contact tank, otherwise the operator overfed and downstream filters will load up with MnO2. In an undergraduate organic lab, the Baeyer test is still the cheapest unsaturation screen: a few drops of cold 1% KMnO4 onto a sample of cyclohexene loses its purple color and drops a brown MnO2 floc within seconds; cyclohexane stays purple.
Common Uses
- Self-indicating titrant for iron, oxalate, and hydrogen peroxide quantification
- Pre-oxidation of dissolved iron, manganese, and H2S in municipal water treatment
- Topical antiseptic at 1:10000 dilution for fungal skin infections and dermatitis
- Oxidative cleavage of alkenes to carboxylic acids in organic synthesis
- Baeyer qualitative test for carbon-carbon double bonds in unknown samples
- Etching agent for plastic preparation prior to electroless metal plating
- Snake-bite first aid antiseptic in remote and military field medicine
Safety Information
GHS: H272 oxidizer Cat 2, H302 harmful if swallowed, H314 causes severe skin burns, H410 chronic aquatic toxicity Cat 1. Solid KMnO4 mixed with glycerin, ethylene glycol, or other organic combustibles can self-ignite within seconds — never store in the same cabinet as solvents or paper. Occupational exposure limits for the dust are set on its manganese content and written as a ceiling. Solutions stain skin and clothing brown-purple as the MnO4- gets reduced to MnO2 on contact with organic matter; the stain wears off skin in a few days but is permanent on cotton. Concentrated solutions cause chemical burns and eye damage that look superficial at first and worsen over hours. Aquatic discharge is restricted in most jurisdictions because of toxicity to fish and invertebrates at sub-mg/L levels. Dispose by reducing with a stoichiometric quench of sodium thiosulfate or bisulfite before drain disposal.
This safety summary is for educational reference only and may not be complete. It is not a substitute for Safety Data Sheets (SDS), medical advice, or professional chemical safety guidance. Always consult appropriate SDS and qualified professionals before handling chemicals. We deliberately do not publish occupational exposure limits or other regulatory thresholds: those values are revised over time and differ between jurisdictions, so the only correct source is the current SDS and the regulations that apply where you work.