What is potassium permanganate?
Potassium permanganate is an inorganic salt of permanganic acid and one of the oldest and most widely used oxidants in municipal and industrial water treatment. The potassium permanganate formula is KMnO₄, its CAS number is 7722-64-7 and its formula weight is approximately 158 g/mol. Those identifiers appear in regulatory instruments as well as supplier documentation, so quote them on an enquiry.
The useful part of the molecule is the permanganate anion, MnO₄⁻, in which manganese sits at the +7 oxidation state. That is as oxidised as manganese gets, which is why permanganate takes electrons from such a broad range of reduced species: ferrous iron, soluble manganese, sulfide, taste-and-odour compounds and a long list of organics. In doing so the manganese is reduced to manganese dioxide (MnO₂), which is insoluble and drops out as a brown-black precipitate. That single fact — the oxidant becomes a filterable solid — shapes almost every decision about how it is applied. Older literature calls it Condy's crystals; in trade documentation it sits under CN/HS code 2841 61 00.
| Property | Value |
|---|---|
| Formula / CAS | KMnO₄ / 7722-64-7 |
| Appearance | Dark purple to near-black crystals or granules with a metallic sheen; free-flowing grades look greyer |
| Bulk density | ~1.45–1.60 g/cm³ (roughly 90–100 lb/ft³) |
| Solubility in water | ~27.8 g/L at 0°C; ~65 g/L at 20°C; ~125 g/L at 40°C; ~230 g/L at 60°C |
| Thermal stability | Decomposes on heating — the ILO/WHO card gives 240°C; producer data notes onset near 150°C |
| Transport | Normally UN 1490, Class 5.1 (oxidising solid), PG II — confirm on the SDS |
Solubility matters more than it looks: only about 6.5 g dissolves in 100 mL of water at ambient temperature, so a day tank is far bigger than the equivalent for a liquid coagulant such as polyaluminium chloride, and cold make-up water is a classic cause of under-dosing.
Potassium permanganate crystals, powder and free-flowing grades
Permanganate is supplied only as a dry solid; there is no concentrated aqueous KMnO₄ product as there is for hypochlorite. Buyers choose mainly on particle size and flow behaviour:
- Crystals and granules. The standard technical form — coarse, dense, dark purple. Suited to batch dissolution in a mixed dissolver tank where particle size is not critical.
- Free-flowing grade. The same material with a small anti-caking additive. Pure KMnO₄ is non-hygroscopic, but technical grades pick up moisture and cake. Specify it for dry feeding, hopper storage or humid climates.
- Potassium permanganate powder. Finer material dissolves faster but generates far more dust, and permanganate dust is both a respiratory irritant and an oxidiser hazard. Finer grades need better dust control, not less.
Assay grades and specification
Commercial water-treatment and technical grades typically sit in the 97–99% KMnO₄ band. Free-flowing grades are guaranteed slightly lower than crystal grades — one major producer guarantees 98% for its technical grade and around 97.5% for its free-flowing drinking-water grade — because the anti-caking additive accounts for part of the mass. That is the additive doing its job, not a defect; reagent and pharmacopoeial grades reach 99% but buy nothing useful in a plant.
A realistic free-flowing specification reads: assay ≥97.5% KMnO₄, weight loss on drying ≤0.5%, with particle size controlled at both ends — for example no more than 20% retained on a 425 µm screen and no more than 7% passing a 75 µm screen. Work from the certificate of analysis for your batch, not a brochure figure. Three standards are worth naming in a tender: EN 12672 (potassium permanganate for water intended for human consumption), AWWA B603 (Standard for Permanganates) and NSF/ANSI 60 where a regulator or utility client requires it for potable duty.
How potassium permanganate works as an oxidant
The reduction pathway depends on pH. Under acidic conditions the dominant half-reactions are MnO₄⁻ + 4H⁺ + 3e⁻ → MnO₂ + 2H₂O (E° = 1.68 V) and MnO₄⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H₂O (E° = 1.51 V); under alkaline conditions, MnO₄⁻ + 2H₂O + 3e⁻ → MnO₂ + 4OH⁻ (E° = 0.60 V). In the pH range most plants operate in, the three-electron route to MnO₂ dominates and the reactions are exothermic and fast. Two consequences follow: alkaline conditions favour oxidation of organic matter while permanganate's weak biocidal action is better at low pH, and every dose produces a manganese dioxide solid that must be removed downstream.
One point deserves bluntness. Permanganate is a good oxidant and a poor disinfectant. US EPA guidance is explicit that it is not used as a primary or secondary disinfectant at normal treatment doses. If you need log inactivation, that is a job for a chlorine-based disinfectant such as sodium hypochlorite, or for chlorine dioxide. Permanganate's value is in removing what chlorine handles inefficiently, or only at the cost of by-products.
Potassium permanganate uses in water treatment and industry
Iron and manganese removal
Permanganate converts ferrous iron to ferric and soluble Mn²⁺ to Mn⁴⁺, both of which precipitate, via the classic reactions 3Fe²⁺ + KMnO₄ + 7H₂O → 3Fe(OH)₃ + MnO₂ + K⁺ + 5H⁺ and 3Mn²⁺ + 2KMnO₄ + 2H₂O → 5MnO₂ + 2K⁺ + 4H⁺. Both produce acid, so alkalinity is consumed — EPA guidance puts this at 1.49 mg/L as CaCO₃ per mg/L of Fe²⁺ and 1.21 mg/L per mg/L of Mn²⁺. On a soft source running alongside aluminium sulfate coagulation the two demands compete, and supplementary calcium hydroxide or soda ash may be needed to hold pH.
Taste, odour and colour control
Permanganate oxidises earthy-musty metabolites and many other taste-and-odour compounds. The published dose range for this duty is wide — EPA guidance cites 0.25 to 20 mg/L — which tells you plainly that it is site-specific, not a design constant. It is frequently paired with activated carbon, which handles the fraction oxidation leaves behind.
Hydrogen sulfide oxidation and pre-oxidation
Permanganate oxidises dissolved sulfide, the products depending on pH: elemental sulfur tends to dominate in the neutral-to-acid range, more fully oxidised sulfur species under alkaline conditions. Because electron demand differs by roughly a factor of four between those end points, published mass ratios per unit sulfide vary enormously — determine sulfide demand empirically on your own water.
Dosing at the head of the works also lets operators move the chlorine application point downstream, so chlorine meets water whose algae, colour and odour load has already been oxidised. EPA notes this can reduce trihalomethane formation, though at typical doses the reduction observed was small and pre-oxidation had no net effect on chlorine demand. Treat by-product control as a secondary benefit, not a justification.
Greensand regeneration, well rehabilitation and air scrubbing
Manganese greensand and similar catalytic media are regenerated with permanganate, in batch between service runs or by continuous feed ahead of the filter. Operator guidance from the Walkerton Clean Water Centre notes continuous regeneration suits waters with higher iron (above roughly 0.3 mg/L) and low manganese (below roughly 0.05 mg/L); follow the media maker's own figures, which are product-specific.
In wells, iron and manganese bacteria build tenacious biofilm and oxide deposits on screens and gravel packs, destroying specific capacity. Permanganate is one of the oxidants used in chemical well rehabilitation, normally within a sequenced programme combining mechanical development with chemical treatment. It must never be mixed directly with acid; where both are used they are applied as separated steps. This is specialist contractor work.
For vapour-phase duty, dry-scrubbing media for sewer and headworks odour control are typically activated alumina pellets impregnated with permanganate at a few percent by weight: the pellet adsorbs the odorous gas and the permanganate oxidises it in place, which works well on hydrogen sulfide, sulfur dioxide and some aldehydes. Manufacturers' capacity figures use differing test conditions and are not directly comparable.
Groundwater remediation and industrial uses
Permanganate is the most mature oxidant for in-situ chemical oxidation of chlorinated solvents such as TCE and PCE; its advantages are subsurface persistence and a visible purple front trackable between wells, its limitations delivery through heterogeneous formations and mobilised dissolved manganese. Beyond water, permanganate is a bulk oxidant in chemical manufacturing, metal surface treatment, flotation and hydrometallurgical circuits, and analysis — the permanganate index (ISO 8467) measures oxidisable matter in drinking and pool water. In several of these roles it competes with hydrogen peroxide, and the choice turns on whether a solid reaction product is acceptable.
Potassium permanganate dosing in water treatment
What drives permanganate demand
Stoichiometry gives a starting point: published figures put the requirement at 0.94 mg KMnO₄ per mg of iron and 1.92 mg KMnO₄ per mg of manganese. In practice EPA guidance records that plants use less than stoichiometry predicts, attributed to the catalytic influence of MnO₂ already in the water and on the media.
Total demand, though, is never just iron and manganese. Every other oxidisable species competes: sulfide, nitrite, natural organic matter, algal cells, reduced metals. Demand therefore moves with season, source blend, reservoir turnover and temperature — a dose that worked in August will not necessarily hold in February.
| Duty | Published basis | Notes for design |
|---|---|---|
| Fe²⁺ oxidation | 0.94 mg KMnO₄ per mg Fe²⁺ (stoichiometric) | Actual use usually lower; confirm by test |
| Mn²⁺ oxidation | 1.92 mg KMnO₄ per mg Mn²⁺ (stoichiometric) | As above, and pH-sensitive |
| Taste and odour | 0.25–20 mg/L in the literature | Source-specific; jar test |
| Alkalinity consumed | 1.49 mg/L as CaCO₃ per mg/L Fe²⁺; 1.21 per mg/L Mn²⁺ | Check against coagulant alkalinity demand |
| Contact time (Fe/Mn) | 5–10 minutes at pH above 7.0 | Colder water and lower pH need longer |
| Total applied dose | Determine on site | All oxidant demand, not just Fe and Mn |
Jar testing
The only defensible way to set a dose is to measure it. Run a jar test series on fresh raw water at plant temperature and pH, stepping the dose across a bracket around the calculated Fe/Mn requirement, holding each jar for the intended contact time, then filtering and analysing for residual soluble iron, manganese and permanganate. Repeat whenever the source changes, and where permanganate is dosed ahead of coagulation run the test with the ferric chloride or alum dose and polyelectrolyte you actually use, because the oxidant changes floc behaviour.
The pink residual as an endpoint indicator
Permanganate is unusual among oxidants in being self-indicating: the intense purple-pink of the permanganate ion fades as it is consumed, replaced by the brown tint of colloidal MnO₂. Operators use this directly — a faint, persistent pink at the filter inlet indicates demand has been satisfied with a small working excess, while a brown cast with no pink suggests oxidation is proceeding but the dose may be marginal.
What you must not do is let pink reach the clear well or distribution system. Operator guidance is consistent on both sides of the balance — too much permanganate gives a pink solution and customer complaints, too little fails to oxidise soluble manganese. The practical target is "just pink" at the filter inlet and colourless after it; breakthrough means the dose is too high, the contact time too short, or the media exhausted. A reducing agent such as sodium thiosulfate is the conventional quench for a residual — but a quench is a contingency, not a strategy.
Application point, contact time and downstream filtration
For iron and manganese, EPA guidance cites an oxidation time of 5 to 10 minutes provided pH is above 7.0. Solution — normally made up on site at 1 to 4% in a mixed dissolver tank and fed by metering pump — is typically injected at the raw water intake, at the rapid mix with coagulants, or ahead of the clarifiers; in direct filtration plants usually at the intake, to buy contact time upstream of the filters. In every configuration it goes in before filtration, and because it reacts quickly it needs no special mixing hardware.
Downstream filtration is the step most often underestimated. Every milligram of permanganate dosed becomes manganese dioxide, and that solid must be captured. Colloidal MnO₂ has a hydroxylated surface that adsorbs charged species — useful, since it assists removal of some organics and microorganisms — but the floc is fine and can carry through a poorly operated filter. The symptoms are black particulate in distribution, dirty-water complaints and rising filtered manganese. Expect extra solids loading and backwash volume, and verify filter run length at the design dose.
Packaging and storage
Permanganate ships in lined steel or plastic drums, kegs, pails and bags, sized around the feed system and the site's manual-handling limits. Stored cool, dry and in closed containers the solid is stable and keeps essentially indefinitely, and producer guidance prefers concrete floors to wooden decking. Keep containers closed — open drums in a humid plant room cake, and caking wrecks dry feeders.
Safety, handling and incompatibilities
This section is not optional reading. Potassium permanganate is a strong oxidiser. It is not itself combustible, but it supports and intensifies combustion and reacts dangerously with a long list of common materials. Documented incompatibilities include:
- Acids. Concentrated sulfuric acid can form manganese heptoxide, with explosions reported; hydrochloric acid liberates chlorine gas; acidified solutions are reported to explode on contact with a range of organics.
- Combustible and readily oxidisable organics, including glycerol, alcohols, ethers and petroleum products — contact with liquid combustibles can cause spontaneous ignition.
- Peroxides. Contact with concentrated hydrogen peroxide can cause explosion.
- Metal powders and other inorganic reducing agents, with which permanganate reacts violently.
- Acetic acid and acetic anhydride, which may explode if not kept cold.
- Absorbents. Cellulose-based and expanded polymeric absorbents are flagged as potentially incompatible — which rules out the sawdust-or-paper-towel reflex for a spill.
The consequences for a plant are concrete. Store permanganate in a dedicated, segregated area away from acids, peroxides, fuels, oils, organic chemicals and metal powders, with no shared secondary containment and no drain inside the bund. It is compatible with many metals and synthetics, but natural rubber and natural fibres frequently are not, so check gaskets, hoses and seals before commissioning. Use chemical goggles and a face shield, impervious gloves and coveralls, and respiratory protection suited to the dust exposure; dust and solution are irritating and can cause serious eye injury and burns.
Always obtain and read the current potassium permanganate SDS for the grade you are buying before handling it, and make it available to everyone who works with the material. Supplier SDSs commonly carry H272 (may intensify fire; oxidiser) and H302 (harmful if swallowed), some grades also skin and eye corrosion and aquatic hazard statements. The SDS, not an article like this one, is the controlling document for your risk assessment.
Regulatory documentation for procurement
Permanganate is a monitored substance in several jurisdictions. In the European Union it is a Category 2, subcategory 2B scheduled drug precursor under Regulation (EC) No 273/2004 (CN code 2841 61 00, CAS 7722-64-7), so suppliers routinely require a customer end-use declaration before despatch. Expect comparable controls elsewhere; build the paperwork into your lead time.
What drives potassium permanganate price and lead time
Euro Chemical Supplies quotes potassium permanganate on request rather than publishing a price, because landed cost depends on variables that only exist once a specification is on the table. The main ones are:
- Grade and assay — technical crystal, free-flowing water-treatment grade and pharmacopoeial grade are different products with different manufacturing routes.
- Certification — whether NSF/ANSI 60, EN 12672 or AWWA B603 conformity is needed, and what documentation must accompany the batch.
- Particle size control — tight sieve specifications and anti-caking treatment both add cost.
- Packaging and order volume — drums, kegs, pails and bags differ in unit cost and in how efficiently they palletise, and full containers behave differently from part loads.
- Dangerous-goods logistics — as a Class 5.1 oxidiser it attracts segregation requirements, declaration costs and carrier restrictions varying by route and mode.
- Regulatory paperwork — drug-precursor declarations and end-use documentation consume calendar time.
- Upstream manganese ore and energy costs, plus freight rates.
For a meaningful quotation, send the grade and minimum assay, particle-size requirement, annual and per-shipment volumes, packaging preference, destination, and any standard you must comply with.
Discuss your permanganate specification
Euro Chemical Supplies supplies potassium permanganate and the wider range of water treatment chemicals factory-direct to utilities and industrial water plants worldwide. Whether you are specifying potassium permanganate for a new duty or comparing it against alternative oxidants, send us the duty and water quality data. Contact our team with your specification and volumes.
