Overview - ISO 3626:1996 (potassium thiocyanate)
ISO 3626:1996 specifies purity and test methods for photographic‑grade potassium thiocyanate (KCNS) used in photography processing chemicals. The second edition (1996) defines chemical and physical criteria, sampling and analytical procedures (some given directly, others by reference), and practical handling/storage notes. KCNS is described as colourless, deliquescent crystals with relative molecular mass 97.18. The standard is intended to assure material quality so photographic processing solutions produce consistent, defect‑free results.
Key topics and technical requirements
The standard sets mandatory limits and test procedures for critical quality parameters of potassium thiocyanate:
- Assay (KCNS content): minimum 98.0% (m/m). Assay is determined by an argentometric titration (AgNO3) with ammonium thiocyanate (NH4SCN) after acidification.
- Heavy metals (as Pb): ≤ 0.002% (m/m). Test method: ISO 10349‑5.
- Iron: ≤ 0.0002% (m/m). Test method: ISO 10349‑5.
- Halide (as Cl‑): ≤ 0.05% (m/m) [= 0.1% as KCl]. Procedure references ISO 10349‑6.
- pH: aqueous solution between 5.3 and 8.7 (measured at 20 °C).
- Sulfur compounds: for developer use, ammoniacal silver nitrate precipitate ≤ 0.001% (m/m); for non‑developer (e.g., fixers) the iodine‑consumption test must be passed.
- Sulfate (as SO4 2‑): ≤ 0.06% (m/m) [= 0.1% as K2SO4].
- Copper: ≤ 0.0001% (m/m).
- Appearance: aqueous and ethanolic solutions should be clear and essentially free from insoluble matter (allowing only slight flocculence).
The standard references general test methods compiled in ISO 10349 parts 1, 5 and 6 and allows modern instrumental methods where correlated to the specified tests.
Applications and practical value
ISO 3626:1996 is used to:
- Certify and qualify photographic‑grade potassium thiocyanate for developers, fixers and other photographic processing solutions.
- Define acceptance criteria in quality control (QC) at chemical manufacturers and distributors.
- Guide laboratory testing and batch release to prevent photographic defects due to impurities (staining, fogging, reduced activity).
- Inform safe storage and handling: store in a cool, dry place; avoid heat/humidity that can decompose KCNS and emit toxic fumes.
Who uses this standard
- Chemical manufacturers and suppliers of photographic reagents
- Photographic materials formulators and film/process labs
- Quality assurance and analytical chemistry laboratories
- Standards bodies and procurement specialists specifying photographic‑grade chemicals
Related standards
- ISO 10349‑1, ‑5 and ‑6 - Photographic‑grade chemicals: general test methods, determination of heavy metals/iron and halide content.