Overview - EN ISO 787-19:2020 (Salicylic acid method)
EN ISO 787-19:2020 specifies a spectrophotometric method for the determination of water‑soluble nitrates in pigment samples using the salicylic acid nitration reaction. The nitrate in an aqueous extract nitrates salicylic (or sodium salicylate) in a sulphuric acid medium to form a yellow nitro‑compound. After alkalinization the colour intensity is measured at 410 nm and related to nitrate mass via a calibration graph. The standard is intended for routine pigment quality control and laboratory testing.
Key topics and technical requirements
- Scope: Determination of water‑soluble nitrates in pigments by the salicylic acid method (spectrophotometry).
- Principle: Nitration of salicylate in sulphuric acid → yellow nitro‑compound → absorbance at 410 nm in alkaline solution.
- Reagents & calibration:
- Use analytical‑grade reagents and purified water.
- Potassium nitrate is used to prepare standard solutions for the calibration graph.
- Sodium salicylate, sulphuric acid, ethanol and sodium hydroxide are specified reagents.
- Apparatus: Spectrophotometer (410 nm), 10 mm cells, pH meter, one‑mark volumetric flasks and calibrated pipettes.
- Sample extraction: The method uses a clear aqueous extract obtained by either the hot extraction (ISO 787-3) or cold extraction (ISO 787-8) procedures.
- Interference control: If chromate is present in the extract, a prescribed reduction step (sulphuric acid + ethanol, heat, neutralize and adjust pH) is required prior to the reaction.
- Calibration & measurement: Construct an absorbance vs. nitrate mass calibration graph using measured standards; determine sample nitrate mass from sample absorbance.
- Expression of results: Report water‑soluble nitrates as mass percent expressed as the anion NO3‑ (from the calibrated mass and original sample mass). If a smaller aliquot is used (e.g., 5 ml), adjust calculations accordingly.
- Test report: Must include product identification, reference to ISO 787‑19:2020, extraction method used, test result, deviations and test date.
Applications and users
- Quality control and R&D laboratories in the pigment, paint, coatings and printing ink industries.
- Raw material acceptance testing for pigments and extenders.
- Regulatory and compliance testing where water‑soluble nitrate content affects product performance or safety.
- Analytical service labs offering standardized nitrate determinations.
Related standards
- ISO 787‑3 (hot extraction of water‑soluble matter)
- ISO 787‑8 (cold extraction of water‑soluble matter)
- ISO 787‑13 (nitrate determination by Nessler’s method)
- ISO 648, 835, 1042 (laboratory glassware); ISO 15528 (sampling)
Keywords: EN ISO 787-19:2020, water‑soluble nitrates, salicylic acid method, spectrophotometric method, pigments, nitrate determination, pigment quality control.
