Overview
ISO 418:2001 - "Photography - Processing chemicals - Specifications for anhydrous sodium sulfite" defines purity criteria and test requirements for photographic‑grade anhydrous sodium sulfite (Na2SO3). The standard specifies chemical and physical requirements, analytical test methods (including an iodine titration assay), storage and handling notes, and acceptance limits intended to prevent undesirable photographic effects caused by impurities.
Key topics and requirements
- Assay (purity): Minimum mass fraction of 97.0% Na2SO3. Two accepted titration methods are described: back‑titration and direct titration using an iodine solution.
- Insoluble matter: Maximum 0.5% (determined as precipitate of Ca, Mg and NH4 hydroxides; ISO 10349-3).
- Heavy metals: Maximum 0.002% (as Pb) (test per ISO 10349-5).
- Iron (Fe): Maximum 0.005% (ISO 10349-5).
- Alkalinity: Maximum 0.15% (as Na2CO3) (ISO 10349-7).
- Thiosulfate: Maximum 0.03% (as Na2S2O3) - limits reducing contaminants that affect image stability.
- Reaction to ammoniacal silver nitrate: Material must pass the identity/impurity test (ISO 10349-9).
- Appearance of solution: Must be clear and free from insoluble matter, allowing only slight flocculence.
- Physical & storage notes: Anhydrous sodium sulfite is a white granular powder; normally handled safely with standard precautions and stored in closed containers at room temperature. Avoid contact with acids.
Practical applications and users
- Who uses ISO 418:2001
- Manufacturers and suppliers of photographic chemicals for product specification and quality control.
- QC/analytical chemists in labs verifying assay, impurity levels, and compliance before distribution.
- Photographic processing labs, conservation specialists and archival facilities that require consistent chemical performance.
- Procurement and standards officers requiring documented acceptance criteria for photographic‑grade Na2SO3.
- Why it matters
- Ensures consistent performance in photographic processing solutions by limiting impurities that cause fogging, staining or reduced shelf life.
- Provides standard test methods and pass/fail criteria to support traceable quality control and regulatory compliance.
Related standards
Normative references cited for test methods:
Keywords: ISO 418:2001, anhydrous sodium sulfite, photographic‑grade, purity specification, assay, photographic processing chemicals, test methods, quality control.