Overview
ISO 787-19:2020 specifies a general spectrophotometric method using salicylic acid to determine water-soluble nitrates in pigment samples. The method nitrates salicylic acid in a sulphuric acid medium, producing a yellow nitro-compound that is measured at 410 nm in 10 mm cells. This procedure supports reliable quantification for laboratory quality control, product specification and regulatory reporting.
Key Topics
- Scope and principle: Extraction of water-soluble nitrates from pigments followed by nitration of salicylic acid and spectrophotometric measurement at 410 nm.
- Reagents: Analytical-grade sulphuric acid (concentrated and 5 N), ethanol (95 %), sodium salicylate (freshly prepared 5 g/l), sodium hydroxide solutions and dried potassium nitrate for standards.
- Apparatus: Spectrophotometer set to 410 nm, 10 mm cells, pH meter and calibrated laboratory glassware (one-mark volumetric flasks, pipettes) per referenced ISO glassware standards.
- Sampling and extraction: Representative sampling per ISO 15528 and preparation of clear aqueous extracts using the hot or cold extraction methods referenced in ISO 787 series.
- Calibration: Prepare standard solutions from dried potassium nitrate and construct a calibration graph of absorbance versus mass of anion NO. Standards described in Clauses 8.1–8.3 provide the basis for interpolation of sample nitrate mass.
- Procedure highlights: Mix extract and salicylate, evaporate to dryness, treat with sulphuric acid, neutralize and alkalize with sodium hydroxide, then measure absorbance at 410 nm. Special handling is specified for extracts containing chromate (reduction, pH adjustment and filtration).
- Results and reporting: Determine mass of NO from the calibration graph and calculate nitrate content as anion NO; include test details, reference to ISO 787-19:2020, extraction method used, any deviations and the test date in the report.
Applications
- Quality control (QC) of pigments and extenders to ensure material meets product specifications.
- Regulatory compliance where explicit quantification of water-soluble nitrates is required.
- Batch release and incoming inspection to detect elevated nitrate levels that may affect product performance or safety.
- Comparative testing and inter-laboratory studies using a standardized spectrophotometric approach.
Related Standards
- ISO 787 series - General methods of test for pigments
- ISO 787-13 - Determination of water-soluble nitrates (Nessler's method)
- ISO 787-3 - Hot extraction method for matter soluble in water
- ISO 787-8 - Cold extraction method for matter soluble in water
- ISO 648, ISO 835, ISO 1042 - Laboratory glassware standards
- ISO 15528 - Sampling of paints, varnishes and raw materials
For implementation, ensure use of certified reagents, properly calibrated spectrophotometer at 410 nm, and adherence to the sampling and extraction clauses referenced in the ISO document to achieve reproducible, comparable results.
Open preview (PDF)General methods of test for pigments — Part 19: Determination of water-soluble nitrates (Salicylic acid method)