Overview
ISO 3622:1996 specifies purity, analytical test methods and acceptance criteria for ammonium thiocyanate (NH4SCN) used as a photography processing chemical. The standard defines chemical assays, contaminant limits, pH and visual acceptability for photographic-grade ammonium thiocyanate to ensure consistent performance in developers, fixers and other photographic solutions.
Key topics and technical requirements
- Assay (content): quantitative determination by argentimetric titration. Reagents include nitric acid, ammonium iron(III) sulfate, silver nitrate (0.100 mol/L) and a standard ammonium thiocyanate solution (0.100 mol/L). Apparatus: 50 ml burette and 50 ml pipette. Endpoint: permanent red. The assay is expressed using the conversion factor 7.612 and the titration result (see standard for exact formula and variable definitions).
- Residue after ignition: maximum 0.10% (m/m); sample ignited at ~600 °C and weighed per procedure in the standard.
- Heavy metals: maximum 0.002% (m/m); determined according to referenced ISO 10349 methods.
- Iron: maximum 0.0005% (m/m); measured using ISO 10349 procedures with specified test portion sizes and standards.
- Halide (as Cl–): maximum 0.06% (m/m); includes a peroxide/NaOH digestion followed by comparison/turbidity procedures.
- pH: solution pH shall be between 4.5 and 6.0 (measured at 20 °C using a calibrated pH meter).
- Sulfur compounds: sulfide precipitated by ammoniacal silver nitrate - maximum sulfur 0.005% (m/m); also an iodine-consumption titration where not more than 1.0 ml of 0.100 mol/L iodine solution is required.
- Sulfate (as SO4 2–): maximum 0.04% (m/m); turbidity comparison against a sulfate standard.
- Appearance: clear and free from insoluble matter, allowing only slight flocculence.
The standard includes reagent preparations, apparatus lists and step-by-step procedures; it also highlights safety/hazard warnings and disposal cautions for reactive reagents.
Applications and who uses it
- Chemical manufacturers and suppliers of photographic-grade ammonium thiocyanate for quality control and specification compliance.
- Analytical and QC laboratories performing acceptance testing for photographic chemicals.
- Photographic processors and industrial users who require consistent developer and fixer performance and need assurance of chemical purity.
- Standards and regulatory bodies referencing photographic chemical specifications.
Related standards
ISO 3622:1996 references test and analytical methods from the ISO 10349 series (e.g., ISO 10349-4, -5, -6 and related parts) for residue, heavy metals, halide and other determinations. Consult those parts for full procedural details.
Keywords: ISO 3622, ammonium thiocyanate, photography processing chemicals, specifications, purity criteria, photographic-grade chemicals, analytical tests.
