Overview
ISO 6878:2004 - "Water quality - Determination of phosphorus - Ammonium molybdate spectrometric method" specifies validated colorimetric procedures for measuring different phosphorus fractions in all types of water, including seawater and effluents. The standard covers determination of orthophosphate, orthophosphate after solvent extraction, hydrolysable phosphate plus orthophosphate, and total phosphorus after decomposition. Typical undiluted measurement range is 0.005 mg/L to 0.8 mg/L; a solvent-extraction variant extends detection down to about 0.0005 mg/L.
Key topics and technical requirements
- Analytical principle: formation of an antimony‑phosphomolybdate complex, reduction with ascorbic acid to produce a molybdenum‑blue color measured spectrometrically (most sensitive at 880 nm, alternative 700 nm).
- Scope of determinations: orthophosphate, solvent‑extracted orthophosphate (low-level), hydrolysable phosphate (acid hydrolysis), and total phosphorus (peroxodisulfate oxidation or nitric‑sulfuric acid digestion).
- Reagents & reagents handling: acid molybdate/antimony reagents, sulfuric acid solutions, ascorbic acid, thiosulfate for arsenate reduction; several reagents include recommended storage times and handling notes.
- Sampling & sample prep: filter samples (nominal pore size 0.45 µm) within 4 hours, avoid phosphate‑containing detergents and cap liners, adjust sample pH to 3–10 as required.
- Interferences & controls: Annex A lists known interferences; arsenate is reduced with thiosulfate, turbid/coloured samples require a turbidity‑colour compensation reagent; users should verify and eliminate other interferences.
- Instrumentation & cell geometry: spectrometers capable of 10–50 mm (or 100 mm for greater sensitivity) optical cells; guidance on sample volumes versus cell thickness for linear calibration.
- Quality practice: calibration using orthophosphate standards, blank tests, linearity checks, and glassware cleaning procedures to avoid contamination.
Applications and who uses it
ISO 6878:2004 is targeted at laboratories and organizations performing routine and regulatory water quality monitoring:
- Environmental and public‑health laboratories monitoring surface water, groundwater and coastal waters
- Wastewater treatment plants and effluent compliance testing
- Research institutions studying nutrient dynamics and eutrophication
- Commercial testing laboratories and instrument manufacturers validating spectrometric phosphorus assays
The method supports regulatory compliance, process control, and scientific studies where reliable, traceable phosphorus quantification is needed.
Related standards
- Prepared by ISO/TC 147 (Water quality).
- Often used alongside other national/international nutrient analysis protocols (e.g., APHA, EPA methods) and laboratory quality standards for sample handling and method validation.
Keywords: ISO 6878:2004, phosphorus determination, ammonium molybdate spectrometric method, orthophosphate, total phosphorus, water quality, solvent extraction, molybdenum blue, spectrometric analysis.