ISO 23696-2:2023

Overview

ISO 23696-2:2023 specifies a validated method for the determination of nitrate as NO3‑N in water using small-scale sealed tubes based on the chromotropic acid colour reaction. The procedure covers natural waters (groundwater, surface water, bathing water), drinking water and wastewater in a measuring range of 0.20 mg/L to 30 mg/L NO3‑N. Results can be converted to NO3 using the stoichiometric factor (4.42681) when required by regulation.

Key topics and technical requirements

• Principle: Nitrate reacts in acidic solution with chromotropic acid inside a prefilled sealed tube; colour development is read by a photometer or spectrophotometer.
• Measuring conditions:
  • Photometer reading: 430 ± 5 nm (filter photometer)
  • Spectrophotometer reading: 410 ± 5 nm
  • Recommended reaction temperature 15–25 °C, sample pH 3–10
• Measuring range: Typical sealed-tube kits cover low, medium and high NO3‑N ranges; users must select the appropriate kit or dilute samples exceeding range.
• Reagents (mass fraction ranges):
  • Sulfuric acid: 75–85%
  • Sodium metabisulfite: 0.01–5%
  • Chromotropic acid: 0.01–1%
• Interferences: Common interfering ions/loads and example maximum tolerable concentrations:
  • K+, Na+ 500 mg/L; Cl‑ 500 mg/L; COD 200 mg/L; Ag+ 100 mg/L; Ca2+ 50 mg/L; Fe 10 mg/L; Cr(VI) 5 mg/L
  • High chloride may require precipitation (e.g., silver sulfate) or dilution
  • Turbid samples: 0.45 μm filtration recommended; coloured samples require blank-correction procedures
• Sampling & storage: Follow ISO 5667 series (parts 1, 3, 10). For long storage, freeze at or below −18 °C; on-site filtration is mandatory for wastewater and surface water.
• Quality control & calibration:
  • Use potassium nitrate reference material for calibration checks
  • At least five calibration points per ISO 8466‑1 when constructing custom curves
  • Labs should follow ISO/IEC 17025 competence requirements

Applications and users

ISO 23696-2:2023 is intended for:

• Environmental and water quality testing laboratories
• Drinking water utilities and municipal laboratories
• Wastewater treatment operators monitoring nitrate levels
• Field and portable testing through automated sealed‑tube devices
• Manufacturers of small-scale sealed tubes and photometric analyzers

Practical advantages include simplified sample handling (prefilled tubes), rapid field-compatible testing, and standardized procedures for compliance monitoring and process control.

Related standards

• ISO 5667‑1, ISO 5667‑3, ISO 5667‑10 - Sampling guidance and preservation
• ISO 8466‑1 - Calibration and evaluation of analytical methods
• ISO 8655‑2 - Pipettes
• ISO/IEC 17025 - Laboratory competence

Keywords: ISO 23696-2:2023, nitrate determination, water quality, chromotropic acid, small-scale sealed tubes, NO3‑N, photometer, spectrophotometer, water testing, calibration, interferences.