Overview
ISO 6777:1984 specifies a molecular absorption spectrometric method for the determination of nitrite in potable, raw and wastewater. The method is based on diazotization at pH 1.9 using amino‑benzene sulfonamide in the presence of orthophosphoric acid, followed by coupling with a naphthyl‑diamine reagent to form a pink azo dye. Absorbance is measured at about 540 nm. The standard covers sample handling, reagents, calibration, detection limits, interference assessment and quality checks for standard nitrite solutions.
Key topics and technical requirements
- Principle: nitrite → diazo salt → colored azo complex; measure molecular absorption at ~540 nm.
- Sample volume: maximum 40 mL test portion (smaller volumes allowed for higher concentrations).
- Reagents: orthophosphoric acid solutions (high and ~1.5 mol/L), amino‑benzene sulfonamide and naphthyl‑diamine coupling reagent; only analytical‑grade reagents and distilled or equivalent water.
- Calibration: prepare standard nitrite solutions (e.g., 100 mg/L and 1 mg/L as described) and establish an absorbance vs. nitrite mass calibration curve (use matching cuvette thickness).
- Measuring cells: cuvettes of 10 mm and 40 mm pathlengths are cited for sensitivity adjustment.
- Detection and sensitivity: interlaboratory trials reported detection limits in the order of 0.0001–0.002 mg/L (as N) depending on cell thickness, and a sensitivity example of ~0.062 mg/L (as N) giving ~0.66 absorbance units with a 40 mm cell.
- pH control: final test pH must be 1.9 ± 0.1; high bicarbonate alkalinity may require extra orthophosphoric acid.
- Interferences: Annex lists matrix effects - chlorine, chloramine, thiosulfate, sodium hexametaphosphate and Fe(III) can cause significant interference; detailed concentration‑effect data are provided.
- Quality assurance: blank and colour corrections, repeatability/reproducibility ranges from interlaboratory tests, and a procedure to verify standard nitrite solution concentration (permanganate titration) because standard solutions may be unstable.
- Safety: the colored reagent is hazardous; avoid skin contact and ingestion.
Applications and users
- Routine nitrite monitoring in drinking water, raw water, wastewater and treated effluents.
- Environmental and municipal water testing laboratories, wastewater treatment operators, regulatory compliance labs and research groups requiring low‑level nitrite quantification by spectrophotometry.
- Useful where spectrophotometric instrumentation is available and controlled reagent preparation and calibration are feasible.
Related standards
ISO 6777 was developed by ISO/TC 147 (Water quality). For comprehensive water nitrogen analysis, users typically consult other ISO water‑quality methods and national regulations covering sampling, nitrate/nitrogen determinations and laboratory QA/QC.
Keywords: ISO 6777, nitrite determination, water quality, molecular absorption spectrometric method, spectrophotometry, nitrite analysis, potable water, wastewater, nitrite interferences, 540 nm.
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