Overview
ISO 5379:2013 - "Starches and derived products - Determination of sulfur dioxide content - Acidimetric method and nephelometric method" specifies two validated analytical procedures for measuring sulfur dioxide (SO2) in starches and starch-derived products. The standard (second edition, 2013) defines the scope, required reagents and apparatus, stepwise procedures, calculation of results, quality checks and reporting rules for routine laboratory determinations.
Key topics and technical requirements
- Two methods specified
- Acidimetric (entrainment + titration): SO2 is displaced from the sample by acid and nitrogen entrainment, absorbed in hydrogen peroxide, converted to sulfuric acid and titrated with standardized NaOH (or determined by iodine titration as a check).
- Nephelometric: Used when titrant volumes are very small (practical limit thresholds specified); detects sulfate/barium sulfate turbidity using barium chloride/PVP reagent and measurement at 650 nm.
- Essential reagents and purity
- Reagents must be analytical quality and sulfate-free (water, hydrogen peroxide solutions, hydrochloric acid, indicators, standardized NaOH and iodine solutions, Na2S2O5/EDTA standard).
- Nephelometric reagents include polyvinylpyrrolidone (PVP), barium chloride and a mixed reagent with ethanol and sulfuric acid.
- Apparatus
- Entrainment apparatus (Lieb/Zaccheri type) with round-bottom flask, condenser and two bubblers; ground-glass joints recommended.
- Standard volumetric glassware, burettes, analytical balance, magnetic stirrer with heating, water bath (20 ±1 °C) and a spectrometer at 650 nm for nephelometry.
- Sampling and test portion
- Test portion mass depends on expected SO2 (table provided). For high contents adjust to contain ≤10 mg SO2.
- Quality control
- Check tests for entrainment apparatus and titration accuracy; minimize oxidation of standards (EDTA use).
- Repeatability: difference between two determinations by same analyst ≤ 5% of mean. Reproducibility between labs ≤ 10% (ISO 5725-2 referenced).
- Decision rule