Potassium Hydroxide

AcceleratorKOHCAS: 1310-58-3
Potassium Hydroxide
Image: Hoa112008 — Public domain

Physical Properties

  • Molecular Weight: 56.11 g/mol
  • Solubility (20°C): 1100 g/L

Also known as: Caustic Potash, KOH, Lye

Potassium hydroxide (KOH; CAS 1310-58-3), also called caustic potash, is a very strong alkali used in high-activity developer formulas where maximum ionization of the developing agent is needed. Photochemically it is essentially interchangeable with sodium hydroxide at equimolar concentration (both drive pH to 12–13); the choice between them is a practical one based on solubility and crystallization behaviour.[1] Potassium hydroxide is preferred for concentrated liquid formulas because it stays in solution at high concentrations where sodium hydroxide would crystallize, particularly in cold storage.

Photographic uses

  • Concentrated liquid developer formulas: Commercial products that use a high-pH developer base (~pH 12) often specify KOH over NaOH for solubility and shelf-life reasons.
  • High-contrast lithographic developers: Kodak D-8 and related high-contrast formulas use strong alkali at narrowly-titrated concentrations; KOH's clean solubility makes accurate preparation easier.
  • Push-processing / rapid developers: Formulas designed for maximum speed and minimum development time rely on the strong pH that hydroxide salts produce.
  • pH adjustment: Small KOH additions bring a nominally alkaline developer up to target pH when the primary activator is insufficient.[2]

Practical notes

Supplied as white pellets or flakes. Extremely hygroscopic — reacts rapidly with atmospheric CO₂ and water, forming a slurry of carbonate and moisture on the surface of fresh pellets. Store in a tightly sealed container with desiccant. Degraded (carbonated) KOH is not suitable for precision photographic use.

Dissolution is strongly exothermic: adding KOH pellets to water releases substantial heat (enough to boil a small volume). Always add the alkali to a large excess of cool water, never the reverse — a water-into-KOH reaction can splash boiling caustic solution out of the container.

Related compounds

Sodium hydroxide is the sodium analog and more commonly stocked by photographic suppliers; essentially interchangeable for dry-formula use. Potassium carbonate is the milder alternative for applications where hydroxide strength is excessive. Triethanolamine is the non-crystallizing organic alternative for liquid concentrates.

Safety Notes

Acute hazards

Potassium hydroxide is highly corrosive and the most dangerous chemical in this alkali cluster:

  • Severe skin burns on direct contact with pellets or concentrated solution. Saponification of skin fats produces a slippery feel that masks the injury until tissue damage is already significant.
  • Severe eye damage: concentrated solution causes permanent corneal damage within seconds.
  • Inhalation risk: fine dust from fresh pellets is strongly irritating to the upper respiratory tract.
  • Exothermic water reaction: splash hazard during mixing if acid-into-water-style technique is violated.

Fire & reactivity

  • Water-reactive in bulk — dissolution releases heat sufficient to boil small volumes. Always add pellets to excess water slowly.
  • Incompatibilities: Strong acids (violent neutralization), aluminum and zinc (hydrogen gas evolution), some organic solvents.

Storage & handling

  • Personal protective equipment: Butyl or neoprene gloves (NOT nitrile alone — KOH rapidly degrades nitrile); safety goggles or face shield for handling pellets; long-sleeved apron; closed-toe shoes.
  • Ventilation: Mix in well-ventilated area; use local exhaust when opening fresh containers of pellets.
  • Storage: Tightly sealed with desiccant in a cool dry cabinet, away from acids and water sources.

First aid

Consult the full Sigma-Aldrich Safety Data Sheet for authoritative first-aid instructions.[3] In brief:

  • Skin contact: Remove contaminated clothing. Flush affected skin with copious running water for at least 20 minutes. Seek medical attention for any visible burn.
  • Eye contact: Flush eyes with running water for at least 30 minutes, lifting the lids. Seek medical attention immediately; eye exposure to KOH is a medical emergency.
  • Inhalation: Move to fresh air; seek medical attention if breathing is difficult.
  • Ingestion: Do NOT induce vomiting. Rinse mouth with water. Seek medical attention immediately; KOH ingestion is a medical emergency.

References

  1. BOOK Haist, Grant. Modern Photographic Processing, Volume 2 1st ed. John Wiley & Sons, 1979. ISBN 0-471-04635-X. ↩
  2. BOOK Anchell, Steve. The Darkroom Cookbook 4th ed. Focal Press, 2016. ISBN 9781138959170. ↩
  3. 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 2026-04-21