ISO 420:1994

Overview

ISO 420:1994 - Photography - Processing chemicals - Specifications for potassium bromide - defines minimum purity criteria and required test methods for photographic-grade potassium bromide (KBr). This second edition (1994) cancels and replaces ISO 420:1976. The standard establishes chemical and physical requirements that correlate with undesirable photographic effects and sets acceptance limits to ensure KBr is suitable for photographic processing solutions.

Key topics and technical requirements

• Assay (KBr content): 99.0% (m/m) minimum to 100.3% (m/m) maximum.
  • Assay example method: titration with silver nitrate and back-titration with ammonium thiocyanate (specified in ISO 420).
• Impurities and limits:
  • Insoluble matter (as precipitate of Ca and Mg): ≤ 0.3% (m/m)
  • Heavy metals (as Pb): ≤ 0.002% (m/m)
  • Iron (Fe): ≤ 0.002% (m/m)
  • Halides: ≤ 0.50% (m/m) as KCl (equivalent to 0.25% as Cl–)
  • Alkalinity (as KOH): ≤ 0.015% (m/m)
  • Acidity (as HBr): ≤ 0.010% (m/m)
  • Volatile matter: ≤ 0.3% (m/m)
  • Sulfide (as S2–): ≤ 0.00024% (m/m) (≈ 0.0008% as K2S)
  • Iodide and oxidizing substances: qualitative “to pass test” requirements
• Appearance: aqueous solution of KBr shall be clear and free from insoluble matter (allowing slight flocculence).
• Storage: store in a closed container at room temperature.
• Test method guidance: many required tests reference ISO 10349 series parts; instrumental or alternative validated methods may be used but the specified methods prevail in case of dispute.
• Safety note: some analytical reagents cited are hazardous; ISO 420 highlights use of appropriate laboratory PPE and safe handling.

Applications and who uses it

• Quality control and acceptance testing for photographic chemical manufacturers and raw material suppliers.
• Analytical and QC laboratories verifying KBr batches for photographic processing.
• Photographic film and paper processors, specialty chemistry formulators, and procurement/specification teams who require traceable purity criteria.
• Useful for ensuring consistent photographic processing performance and minimizing artifacts related to impurities.

Related standards

• ISO 10349 series (Parts 1, 3, 5, 6, 7, 8, 10) - referenced test methods and general laboratory procedures for photographic-grade chemicals.

Keywords: ISO 420, potassium bromide, photographic chemicals, purity specification, KBr assay, photography processing chemicals, ISO 10349.