ISO 3423:1975

Overview

ISO 3423:1975 specifies an iodometric method for determining sulphur dioxide (SO2) content in sulphuric acid and oleums for industrial use. The procedure is applicable to products with SO2 content of 2 mg/kg or more and is intended for routine laboratory analysis and industrial quality control. The method uses a nitrogen purge to displace SO2 from the sample, absorption of the gas in a measured iodine solution, and titration of excess iodine with standard sodium thiosulfate.

Key topics and requirements

• Scope: Intended for sulphuric acid and oleums with SO2 ≥ 2 mg/kg. (Note: for much higher SO2 levels, ISO/R 912 is referenced.)
• Principle: Displace SO2 from a weighed test portion with a stream of pure nitrogen, absorb released SO2 in a known volume of iodine solution, then titrate excess iodine with standard sodium thiosulfate (iodometric/iodometric titration).
• Apparatus:
  • Three-neck glass flask (250 or 500 ml) with a stoppered funnel and a central tube fitted with a fritted glass disperser.
  • Series of three Drechsel-type absorber bottles (100–125 ml) for iodine absorption and to trap entrained iodine.
  • Glass stoppered sample bottle, flowmeter or bubble counter for gas flow measurement.
• Reagents:
  • Iodine solution (approx. 0.01 N, 0.05 N or 0.1 N depending on expected SO2).
  • Standard sodium thiosulfate titrant matching iodine strength.
  • Starch indicator (5 g/L) and hydrazine sulfate (used if nitrite/nitrate ions are present).
  • High-purity nitrogen (oxygen < 0.001%).
• Operational controls:
  • Sample handling for concentrated acid/oleum: introduce onto ice so temperature stays ≤ 10 °C.
  • Typical sample mass ~200 g (weighing by difference).
  • Nitrogen purge flow and duration (example given ≈ 20 L/h for about 3 hours) to ensure quantitative displacement.
• Calculation:
  • Results are reported as mg SO2 per kg. Calculation uses the thiosulfate titre, volumes used for blank and sample, the sample mass, and the chemical equivalence (32 mg SO2 corresponds to 1 mL of 1 N thiosulfate).

Applications and users

• Quality control laboratories in acid and oleum production plants
• Chemical manufacturers and process control teams monitoring SO2 contamination
• Inspection and testing labs for incoming/outgoing industrial acid consignments
• Regulatory or compliance testing where precise SO2 quantification in acids is required

Related standards

• ISO/R 912 - referenced for procedures or considerations when SO2 content is significantly higher than the mid-range covered by ISO 3423:1975.

Keywords: ISO 3423:1975, sulphuric acid, oleums, sulphur dioxide, SO2 determination, iodometric method, iodine absorption, sodium thiosulfate titration, nitrogen purge, industrial quality control.