Overview
ISO 6768:1998 specifies a modified Griess–Saltzman colorimetric method for determination of the mass concentration of nitrogen dioxide (NO2) in ambient air. The method uses an azo‑dye formation reaction in an absorption solution and spectrophotometric measurement of the resulting pink colour (wavelength 540–550 nm). It is intended for ambient and confined‑space air monitoring and for analytical checking of calibration gas mixtures (see ISO 7996). The method is not suitable for personal breathing‑zone sampling.
Key topics and technical requirements
- Applicable range and sampling: Method reported for NO2 mass concentrations from 0.003 mg/m3 to 2 mg/m3, sampling periods 10 min to 2 h, and air flow ≈0.4–0.6 L/min.
- Reagents: nitrite‑free water, N‑(1‑naphthyl)ethylenediamine dihydrochloride (NED), sulfanilic acid absorption solution, and sodium nitrite calibration solutions.
- Sampling train: fritted bubbler absorbers (various types), optional cotton‑wool ozone trap, membrane filter, pump and flow metering (wet test meter, mass flowmeter, or calibrated orifice).
- Absorption efficiency: Each fritted bubbler must be tested (series bubblers) and demonstrate an absorption efficiency ≥ 0.95. Annual checks and frit porosity verification are required.
- Spectrophotometry & calibration: Measure absorbance at 540–550 nm using matched optical cells (optical path 10–50 mm). Calibration is prepared from known sodium‑nitrite solutions; the slope for a 10 mm cell should be approximately (0.992 ± 0.030) ml/mg (document value).
- Timing and stability: Allow sample solution to develop ≥15 min; measure between 15 min and 20 h after sampling.
- Interferences: Ozone can slightly increase readings (noticeable above ~0.20 mg/m3); peroxyacyl nitrates (PAN) may give ~15–35% response; nitrite/nitrous acid in the sampled air produce the same pink colour and must be considered.
- Performance: The standard gives expected detection and precision characteristics (see full text for numerical values).
Applications and users
- Environmental and air‑quality laboratories performing routine NO2 monitoring and compliance testing.
- Regulatory agencies and field monitoring teams assessing ambient NO2 concentrations.
- Laboratories preparing or checking NO2 calibration gas mixtures for chemiluminescence analysers.
- Research groups studying urban/industrial NO2 pollution where a validated colorimetric method is required.
Related standards and notes
- ISO 7996 - chemiluminescence method for nitrogen oxides (referenced for calibration checks).
- ISO 6349 - permeation method for preparation of calibration gas mixtures.
For procedural details, quality control, performance data and official test‑report requirements, consult the full ISO 6768:1998 document. Keywords: ISO 6768, Griess‑Saltzman, nitrogen dioxide, NO2 monitoring, ambient air, spectrophotometry, air quality standard.