Overview
ISO 5990:2000 specifies purity criteria and test methods for photographic‑grade potassium sulfite, 650 g/l aqueous solution. It defines chemical and physical requirements that ensure the reagent’s suitability for photographic processing solutions, and it prescribes the analytical methods to determine purity. The standard is intended for laboratory personnel with basic analytical experience and references general test procedures in ISO 10349 series.
Key topics and requirements
- Assay (potassium sulfite content): 44.5% to 46.0% (mass fraction). Two titrimetric options are given: back‑titration or direct titration using iodine/thiosulfate chemistry with starch indicator.
- Heavy metals (as Pb): maximum 0.002% (mass fraction) - test method per ISO 10349‑5.
- Iron (Fe): maximum 0.002% - determined by ISO 10349‑5 procedures.
- Reaction to ammoniacal silver nitrate: must pass the identity/impurity reaction test (ISO 10349‑9).
- Specific gravity / density: specific gravity 1.445–1.460 or density 1.441–1.457 g/ml (ISO 10349‑11 / ISO 10349‑12).
- pH of a (1 + 9) dilution: 8.0 to 10.0.
- Thiosulfate (as S2O3) impurity: maximum 0.006% (mass fraction).
- Appearance: clear, colourless or almost colourless, free from insoluble matter (minor flocculence acceptable).
- Storage & hazards: stored in closed container at room temperature; avoid contact with acids.
Practical applications and users
ISO 5990:2000 is used by:
- Photographic chemical manufacturers and suppliers to certify product quality.
- Quality control and analytical laboratories performing acceptance testing of processing chemicals.
- Photographic labs, film processors and restoration facilities requiring consistent reagent performance.
- Procurement/specification writers and regulatory or standards compliance teams who need defined purity limits for processing chemicals.
Applying this standard helps prevent photographic defects caused by impurities (e.g., fogging, staining or inconsistent development), supports consistent processing performance, and improves interchangeability of reagents from different suppliers.
Related standards
- ISO 10349‑1: General test methods for photographic‑grade chemicals
- ISO 10349‑5: Determination of heavy metals and iron
- ISO 10349‑9: Reaction to ammoniacal silver nitrate
- ISO 10349‑11 / 10349‑12: Determination of specific gravity and density
Keywords: ISO 5990:2000, potassium sulfite, photographic‑grade, processing chemicals, assay, purity, heavy metals, pH, specific gravity, thiosulfate, ISO 10349.