Potassium Bromide

RestrainerKBrCAS: 7758-02-3
Potassium Bromide
Image: Benjah-bmm27 — Public domain

Physical Properties

  • Molecular Weight: 119.01 g/mol
  • Solubility (20°C): 650 g/L
  • Solubility (50°C): 800 g/L

Also known as: KBr, Bromide

Potassium bromide (KBr; CAS 7758-02-3) is the classic photographic restrainer — the ion source that prevents chemical fog in every silver halide developer.[1] A fog-free developer needs bromide ion in solution to shift the silver-halide-to-silver reduction equilibrium so that unexposed grains stay undeveloped while exposed grains reduce normally. This is especially critical in paper developers, where even small amounts of fog destroy the paper-white highlight the print depends on.

Photographic uses

  • Paper developer restrainer: 1–3 g/L potassium bromide in D-72 / Dektol and essentially every silver-gelatin paper developer. The exact concentration controls both fog level and working contrast — more bromide suppresses fog but also slows development and slightly warms image tone.[2]
  • Film developer restrainer: Trace amounts (0.5–2 g/L) in film developers reduce base fog at higher development temperatures or with old films. D-76 uses no bromide at all; higher-activity formulas (D-19, DK-50) include small amounts.
  • Reducer / intensifier component: Potassium bromide appears in Farmer's reducer variants that need a specific halide balance, and in some intensifier formulas for local control.
  • Test-strip bromide reduction for darker whites: A dilute bromide bath between exposure and development suppresses fog specifically where development conditions are marginal (replenished high-volume processors, warm-weather work).

Practical notes

Supplied as colourless or white crystalline powder. Highly soluble in water (65 g/100 mL at 20 °C). Solutions are shelf-stable indefinitely. Most darkroom workers mix a 10% stock solution (1 g/10 mL) for precise dispensing in gram-and-fraction quantities.

pH-neutral in solution, so bromide can be added to an existing developer without shifting the pH balance.

Related compounds

Potassium chloride is the weaker-restraining chloride analog; potassium iodide is the much stronger iodide form (used in trace amounts only). Benzotriazole is the organic alternative with higher potency per gram. Sodium bromide substitutes 1:1 for potassium bromide in all formulas.

Safety Notes

Acute hazards

Potassium bromide is essentially non-toxic at darkroom exposure levels. The compound has medical history as a sedative (pre-1970s neurology) at gram-scale doses; darkroom concentrations (a few grams in a working tank) pose no risk.

  • Mild skin and eye irritant in concentrated solution. Rinse with water.
  • Not a respiratory hazard. Fine dust is an inert nuisance.

Storage & handling

  • Personal protective equipment: Optional at working strength; gloves and safety glasses when weighing bulk dry powder as basic good practice.
  • Storage: Tightly closed in a dry cabinet. Indefinitely stable.

First aid

Consult the full Sigma-Aldrich Safety Data Sheet for authoritative first-aid instructions.[3] In brief: rinse skin with water; flush eye for 10 minutes; ingestion rinse mouth, no other action typically needed.

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 2026-04-22