Potassium Iodide

OtherKICAS: 7681-11-0
Potassium Iodide
Image: Benjah-bmm27 — Public domain

Physical Properties

  • Molecular Weight: 166 g/mol
  • Solubility (20°C): 1400 g/L

Also known as: KI, Iodide

Potassium iodide (KI; CAS 7681-11-0) is the strongest of the halide restrainers — iodide ion suppresses silver-halide reduction so forcefully that darkroom formulas use it only in trace amounts or for very specific purposes. Its primary roles are in emulsion manufacture (about 3% iodide in modern fast films), as a component of wet-plate collodion salting solutions, and in a small number of specialty restrainer and intensification formulas.[1]

Photographic uses

  • Wet-plate collodion salting solution: Dissolved in alcohol-water mixtures and added to the collodion, potassium iodide becomes the silver iodide that forms on contact with the silver nitrate bath.[2]
  • Mercury intensifier clearing: After mercury intensification of a thin negative, a dilute potassium iodide bath dissolves the excess mercury salts that would otherwise stain.
  • Trace restrainer in high-contrast developers: Fraction-of-a-gram additions to lithographic or push-development formulas fine-tune the Dmin/Dmax response.
  • Daguerreotype gilding solution: Historical daguerreotype conservation uses iodide solutions as part of the gold-chloride gilding chemistry.
  • Iodine-stain removal: Combined with iodine (I₂) as a desensitizer and stain clearer in historical darkroom recipes.

Practical notes

Supplied as white-to-pale-yellow crystalline powder. Yellows on exposure to light — the surface forms iodine from UV photodecomposition — so the commercial product is stocked in amber bottles and should be kept in the dark. Slight yellowing of aged crystals does not affect their photographic activity.

Highly soluble in water (140 g/100 mL at 20 °C). Solutions should be stored in amber glass; fresh solutions are colourless, aged solutions develop a yellow tint from photodecomposition.

Related compounds

Potassium bromide is the moderate-strength bromide restrainer used in most paper developers. Potassium chloride is the mild chloride form. Sodium iodide (NaI) substitutes in formulas that prefer sodium salts, though most iodide-using recipes specifically call for the potassium form.

Safety Notes

Acute hazards

Potassium iodide has low acute toxicity at darkroom concentrations. The compound has medical history as an iodine-deficiency supplement and radiation-exposure prophylactic at gram-scale doses, so trace-amount darkroom use is well below any health threshold:

  • Mild skin and eye irritant in concentrated solution.
  • Harmful if ingested in large quantity: substantial swallow causes gastrointestinal upset, iodine imbalance symptoms (burning throat, nausea), and at very high doses thyroid dysfunction.
  • Staining: solutions stain skin, clothing, and porous surfaces yellow-brown from photodecomposed iodine.

Storage & handling

  • Personal protective equipment: Gloves and safety glasses for solution work; dust mask for bulk dry handling.
  • Light protection: Store in amber glass or a dark cabinet. Photodecomposition is slow but real.
  • Storage: Cool dry cabinet; solutions in amber bottles.

First aid

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

  • Skin / eye contact: Flush with water. Staining fades over days.
  • Ingestion: Rinse mouth; seek medical attention for substantial swallow.

References

  1. BOOK Haist, Grant. Modern Photographic Processing, Volume 2 1st ed. John Wiley & Sons, 1979. ISBN 0-471-04635-X. ↩
  2. BOOK Crawford, William. The Keepers of Light: A History and Working Guide to Early Photographic Processes 1st ed. Morgan & Morgan, 1979. ISBN 0-87100-158-6. ↩
  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 2026-04-22