Overview - ISO 6767:1990 (Ambient air - SO2, TCM/pararosaniline method)
ISO 6767:1990 specifies a spectrophotometric procedure (the tetrachloromercurate / pararosaniline or TCM/pararosaniline method) for determining the mass concentration of sulfur dioxide (SO2) in ambient air. The method is intended for typical ambient ranges of about 20 µg/m3 to 500 µg/m3 (sampling periods of 30–60 minutes). It is based on absorption of SO2 into a sodium tetrachloromercurate solution, chemical conversion to an intensely violet pararosaniline derivative, and measurement of absorbance near 550 nm.
Key technical topics and requirements
- Sampling and equipment
- Sampling train and absorbers as per ISO 4219; air intake materials and absorber geometry influence absorption efficiency (should be ≥ 0.95).
- Recommended sampling periods: 30 or 60 min; adjust flow or period if concentrations exceed the basic range.
- Gas meter or critical orifice and pressure measurement (two manometers) for flow control.
- Chemical procedure
- Absorption in sodium tetrachloromercurate (TCM) solution (c ≈ 0.04 mol/L).
- Destruction of nitrite with sulfamic acid, conversion with formaldehyde and pararosaniline hydrochloride to form the colored species.
- Read absorbance between 540–550 nm (548 nm recommended for spectrophotometers).
- Calibration and quality control
- Calibration gas mixtures prepared by the permeation method (ISO 6349); at least four concentration levels for calibration graph.
- Zero and reagent blank procedures; routine check graphs can be produced using sodium disulfite solutions (with calibration via permeation).
- Annexes: PRA purity test (A), sodium disulfite calibration (B), mercury recovery (C, informative).
- Performance
- Lower detection limit in a 10 ml TCM sample: 0.2–1.0 µg SO2, corresponding to ~7–33 µg/m3 for a 30 L air sample.
- EDTA in reagent mix reduces heavy-metal interferences; sulfamic acid reduces NO2 interference up to specified levels.
- Method suitable for orientating field measurements; for higher accuracy, use specially tested/calibrated instrumentation.
Practical applications and users
- Environmental and air-quality monitoring laboratories performing routine SO2 surveillance.
- Regulatory agencies and consultants needing a standardized, field-capable colorimetric method.
- Research groups or compliance labs requiring cross-checks or validation of instrumented SO2 monitors.
- Useful for site surveys, emissions impact studies, and situations where portable or simpler wet-chemistry methods are needed.
Related standards
- ISO 4219:1979 - Air quality: sampling equipment for gaseous sulphur compounds.
- ISO 6349:1979 - Gas analysis: preparation of calibration gas mixtures (permeation method).
Keywords: ISO 6767:1990, sulfur dioxide, SO2 monitoring, TCM/pararosaniline method, tetrachloromercurate, pararosaniline, ambient air, spectrophotometric method, air quality, ISO 4219, ISO 6349.
