Overview
ISO 4221:1980 - "Air quality - Determination of mass concentration of sulphur dioxide in ambient air - Thorin spectrophotometric method" - specifies a wet-chemical spectrophotometric procedure for measuring sulphur dioxide (SO2) in ambient air. The method covers the typical ambient range of 3.5 to 150 µg/m3 (assuming a 2 m3 air sample and 50 ml absorption solution); higher levels require dilution. It refers to ISO 4219 for sampling equipment and is intended for routine environmental monitoring and laboratory analysis.
Key topics and technical requirements
- Analytical principle: SO2 is absorbed and oxidized to sulfate in an acidified hydrogen peroxide absorption solution. Sulfate is precipitated as barium sulfate using barium perchlorate, and the residual barium ions are complexed with Thorin to produce a coloured complex measured spectrophotometrically at 520 nm.
- Scope and range: Applicable mass concentration range 3.5–150 µg/m3 (2 m3 sample, 50 ml solution). Dilute samples for higher concentrations.
- Reagents: Hydrogen peroxide (27–30%), perchloric acid (various dilutions), barium perchlorate, barium perchlorate/dioxane solution, Thorin (2.5 g/l), and standard sulphate solutions for calibration.
- Apparatus & sampling:
- Use Drechsel-type borosilicate absorption bottles (125 ml for 24 h sampling; 250 ml for 48 h).
- Spectrophotometer (single- or double-beam) with 10 or 20 mm cells; adjust instrument per manufacturer instructions to 520 nm.
- Sampling equipment per ISO 4219 and particulate filters to remove solids.
- Calibration & QC: Prepare calibration curves with standard sulfate solutions and blank test solutions; run duplicates or triplicates for reliability. Detection limits reported on the method: order of 0.1 µg/ml (double-beam) and ~0.13 µg/ml (single-beam) for the solution measurement.
- Interferences & safety:
- Possible interference from very high ammonia or hydrogen sulfide concentrations.
- Particulates are excluded by filtration.
- Hazardous reagents: perchloric acid, barium perchlorate, dioxane, Thorin - handle with care.
Practical applications and users
- Environmental and air quality monitoring laboratories
- Regulatory agencies and compliance testing programs
- Research groups studying ambient SO2 concentrations and trends
- Industrial emissions monitoring where ambient ambient SO2 assessment is required
This method supports routine SO2 ambient air monitoring, networked air quality surveys, and comparative studies where standardized, reproducible spectrophotometric results are required.
Related standards and reporting
- ISO 4219 - referenced for sampling equipment and procedures.
- The standard notes an alternative, more sensitive tetrachloromercurate/pararosaniline (TCMV) spectrophotometric method under development at the time.
- Test reports should include sample identification, reference to ISO 4221 and ISO 4219, results and expression method, any deviations or unusual observations, and details of reagents and dilutions used.
Keywords: ISO 4221:1980, air quality, sulphur dioxide, SO2, Thorin spectrophotometric method, ambient air monitoring, calibration, detection limit.
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