Potassium Permanganate

OtherKMnO4CAS: 7722-64-7
Potassium Permanganate
Image: Nucleus hydro elemon — Public domain

Physical Properties

  • Molecular Weight: 158.03 g/mol
  • Solubility (20°C): 64 g/L

Also known as: Condy's Crystals, KMnO4, Permanganate

Potassium permanganate (KMnO₄; CAS 7722-64-7) is a powerful oxidizer — deep purple crystalline solid whose aqueous solutions are so intensely coloured that fractions-of-a-gram per litre produce visible violet tint.[1] In photography, permanganate serves as a stain-removal agent for developer-stained trays and hands, as a bleach in some reversal processing workflows, and as a component of a small number of intensifier formulas.

Photographic uses

  • Developer-stain remover: A dilute permanganate bath (~1%) in sulfuric acid oxidizes and removes developer-oxidation stains from stainless-steel trays, enlarger carriers, and porcelain sinks. Follow with a clearing bath of sodium bisulfite to remove the resulting brown manganese dioxide residue.
  • Reversal bleach: Permanganate in dilute sulfuric acid converts silver image to silver manganate, then a clearing bath dissolves it — producing a cleared emulsion ready for second exposure and development in B&W reversal processing.
  • Stain removal from prints (risky): A very dilute permanganate bath can remove yellow oxidation stains from archival silver prints, but the chemistry risks over-bleaching the image itself. Not for routine use.
  • Historical intensifier: Some 19th-century intensifier formulas used permanganate to convert silver to a mixed silver-manganese species that was then redeveloped.

Practical notes

Supplied as dark purple crystalline granules. Extremely intensely coloured — 0.5 g in a litre of water produces a distinct lavender tint. Solutions turn brown-green over weeks as permanganate reduces to manganese dioxide (MnO₂); discard aged solutions.

Staining: permanganate solutions stain skin and clothing dark purple-brown, shifting to black-brown over days as the MnO₂ oxidation product deposits in the skin. Stains fade slowly over 1–2 weeks. Clothing stains are essentially permanent.

Related compounds

Sodium permanganate substitutes 1:1 for potassium permanganate in most formulas. Hydrogen peroxide is a different oxidizer used for related (but not identical) purposes.

Safety Notes

Acute hazards

  • Strong oxidizer (H272): intensifies combustion; dry crystals can ignite organic materials on direct contact.
  • Harmful if swallowed (H302).
  • Causes severe skin burns and eye damage (H314) in concentrated solution.
  • Very toxic to aquatic life with long-lasting effects (H410).

Fire & reactivity

  • Oxidizer, reactive with many organics: permanganate + glycerin catches fire spontaneously after a short delay; permanganate + acetone is even faster. Keep away from all organic solvents, alcohols, and glycerin.
  • Acid contamination: concentrated sulfuric acid on solid permanganate forms manganese heptoxide, which is explosive.
  • Do not grind with reducing agents or combustibles; mechanical shock can initiate vigorous reaction.

Storage & handling

  • Personal protective equipment: Nitrile gloves (replace promptly when stained — staining indicates solution penetration), safety goggles, long-sleeved clothing.
  • Storage: Tightly sealed glass or ceramic container (not plastic — permanganate slowly attacks some plastics) in a cool dry cabinet separated from organic materials and acids.

First aid

Consult the full Sigma-Aldrich Safety Data Sheet for authoritative first-aid instructions.[2]

  • Skin contact: Flush with water for 15 minutes. Staining is expected and fades over days.
  • Eye contact: Flush with running water for at least 20 minutes. Seek medical attention.
  • Ingestion: Rinse mouth; do NOT induce vomiting. Seek medical attention immediately.

References

  1. BOOK Haist, Grant. Modern Photographic Processing, Volume 2 1st ed. John Wiley & Sons, 1979. ISBN 0-471-04635-X. ↩
  2. WEB Sigma-Aldrich (Merck KGaA). Sigma-Aldrich Safety Data Sheets Sigma-Aldrich. https://www.sigmaaldrich.com/US/en/search/safety-data-sheets ↩

Reference databases

Last reviewed 2026-04-22