Potassium Carbonate

AcceleratorK2CO3CAS: 584-08-7
Potassium Carbonate
Image: Edgar181 — Public domain

Physical Properties

  • Molecular Weight: 138.21 g/mol
  • Solubility (20°C): 1120 g/L

Also known as: Pearl Ash, Potash

Potassium carbonate (K₂CO₃; CAS 584-08-7) is a highly soluble alkali accelerator, essentially the potassium analog of sodium carbonate. It raises developer pH to the same 10–11 range as its sodium sibling but dissolves more readily and — more importantly — does not crystallize out of concentrated solutions. This is the reason commercial liquid developer concentrates almost universally use potassium carbonate rather than sodium carbonate: a sodium-based concentrate would precipitate at refrigerator temperatures or settle out in storage.[1]

Photographic uses

  • Commercial liquid developer concentrates: Kodak HC-110, Ilford DD-X, and most other 1L-bottle pre-mixed developers use potassium carbonate as the alkali.
  • Cold-weather processing: Field photographers working in cold conditions benefit from potassium carbonate's lower crystallization point.
  • DIY powder developers: Formulas that specify sodium carbonate can substitute potassium carbonate at roughly 1.4× the sodium carbonate weight (MW ratio 138/106) for equivalent alkali content, if avoiding sodium-carbonate precipitation matters.[2]

Practical notes

Supplied as white hygroscopic granules. Absorbs atmospheric moisture readily — store in a tightly sealed container. A partially hydrated stock may weigh 10–20% more than pure K₂CO₃ for the same dry-weight content; if precision matters, dry the salt at 120 °C for an hour before weighing.

Solutions are stable indefinitely in closed containers.

Related compounds

Sodium carbonate is the direct substitute — same chemistry, lower solubility, the traditional choice for dry-formula developers. Potassium hydroxide is the stronger potassium alkali used in high-activity formulas. Potassium carbonate solution is stocked separately for recipes that specify the liquid form.

Safety Notes

Acute hazards

Potassium carbonate is a moderate alkali — more irritating than borax or bicarbonate, less corrosive than hydroxide salts:

  • Skin irritant: Concentrated solution causes redness and stinging. Extended contact can cause chemical burns in sensitive individuals.
  • Serious eye irritant: Splashes cause painful irritation; extended flushing needed.
  • Respiratory irritant: Fine dust irritates the upper airway. Handle dry salt with ventilation.

Storage & handling

  • Personal protective equipment: Nitrile gloves, safety glasses (splash-rated for dilution work). A dust mask when handling bulk dry powder.
  • Storage: Tightly sealed — hygroscopicity is real. Cool dry cabinet.
  • Segregation: Store away from acids; neutralization is vigorous and exothermic.

First aid

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

  • Skin contact: Flush with water for 15 minutes; seek medical attention for any burn-like reaction.
  • Eye contact: Flush with running water for at least 15 minutes; seek medical attention.
  • Inhalation: Move to fresh air; seek medical attention if respiratory irritation persists.
  • Ingestion: Rinse mouth; do NOT induce vomiting; seek medical attention.

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