Overview
ISO 5522:1981 specifies a standardized laboratory method for the determination of total sulphur dioxide (SO2) in fruits, vegetables and derived products. The procedure is an acidimetric entrainment method in which SO2 is liberated by acidification and heat, swept out of the test portion with oxygen‑free nitrogen, absorbed and oxidized to sulphuric acid, then quantified by titration. Verification procedures (Annex A and B) provide gravimetric and nephelometric options to confirm results.
Key topics and technical requirements
- Scope: Applicable to fruits, vegetables and derived products for any SO2 content.
- Principle: Acidify and heat test portion → entrain liberated SO2 with nitrogen → absorb and oxidize in dilute hydrogen peroxide → titrate resulting sulphuric acid with standard NaOH.
- Reagents: oxygen‑free nitrogen; hydrogen peroxide (~9.1 g/L, sulphate‑free); hydrochloric acid (100 g/L); bromophenol blue indicator; standard NaOH (0.01 mol/L or 0.1 mol/L); iodine standard (for checks); barium chloride/polyvinylpyrrolidone for verification.
- Apparatus: Entrainment assembly (round‑bottom flask, high‑efficiency reflux condenser, dropping funnel, nitrogen inlet and two bubblers), burettes/semi‑microburette, balance, spectrophotometer for nephelometry.
- Verification options:
- Annex A - Gravimetric precipitation and weighing of barium sulphate.
- Annex B - Nephelometric measurement against a calibration curve (650 nm).
- Operational limits & rules:
- Two determinations per sample; repeatability requirement: difference between two results by the same analyst ≤ ±5% of mean.
- If titration volume (V) of 0.01 mol/L NaOH is <5 mL, use nephelometric verification; if >10 mL, use gravimetric verification; 5–10 mL range allows nephelometry as an option.
- Specific checks for apparatus performance (blank reflux, sucrose heating test) and choice of bromophenol blue indicator to avoid nephelometric interference.
Applications and users
- Who uses it:
- Food testing laboratories and QA/QC teams in fruit and vegetable processing.
- Regulatory authorities and inspection labs verifying legal limits or labeling claims.
- Research labs studying preservative residues or processing effects.
- Import/export control and commodity grading services.
- Practical applications:
- Quantifying residual sulphur dioxide from sulphiting or preservation processes.
- Compliance testing for food safety and labeling.
- Quality control during storage, processing, and formulation of fruit/vegetable products.
Related standards
- Developed by ISO/TC 34 (Agricultural food products). For comprehensive food chemical analysis, consult other ISO methods and national laboratory accreditation guidelines relevant to sulphite/sulphur dioxide testing.
Keywords: ISO 5522:1981, total sulphur dioxide, SO2 determination, fruits and vegetables, acidimetric entrainment, hydrogen peroxide oxidation, gravimetric verification, nephelometric verification, food testing, laboratory method.