Overview
EN ISO 15681-1:2004 (identical to ISO 15681-1:2003) specifies a standardized laboratory procedure for measuring orthophosphate and total phosphorus in water using flow injection analysis (FIA). The method covers orthophosphate in the mass concentration range 0.01 mg/L to 1.0 mg/L (P) and total phosphorus after manual digestion (per ISO 6878) in the range 0.1 mg/L to 10 mg/L (P). Applicable to a wide variety of waters - ground, drinking, surface, leachate, wastewaters and, with salinity-adjusted reagents, seawater - this standard is adopted by CEN for consistent water quality monitoring.
Key Topics
- Analytical principle: sample injection into a carrier stream merged with an acidic ammonium molybdate reagent to form molybdophosphoric acid, then reduction (molybdenum blue) and detection by a flow detector (e.g., flow photometer).
- Total phosphorus: requires manual oxidation (potassium peroxodisulfate digestion) in accordance with ISO 6878 to convert bound phosphorus to orthophosphate prior to FIA measurement.
- Concentration ranges & flexibility: routine ranges stated above; working range can be altered by varying FIA operating conditions (carrier, reagent flows, sensitivity).
- Interferences and mitigation:
- Arsenate causes significant positive bias (100 µg/L As ≈ 30 µg/L P).
- Silicate interference negligible if <60× phosphorus concentration.
- Fluoride (>50 mg/L) and nitrite (>5 mg/L) can interfere; acidification or sample pretreatment may be required.
- Strong oxidizers and high COD may affect digestion - dilution or removal recommended.
- Self-absorption can be corrected by measuring sample signal without reagents and subtracting.
- Reagents & quality: use analytical grade chemicals, degassed carrier/reagent solutions and phosphate-checked blanks; reagents named include ammonium molybdate, tin(II) chloride (reducer) and digestion reagents per ISO 6878.
- Annexes include example FIA system diagrams, precision/accuracy data and alternative chemistries (e.g., ascorbic acid reduction, DEHA substitution).
Applications
- Routine monitoring of water quality in environmental laboratories and utilities.
- Regulatory compliance testing for drinking water, surface water, groundwater and wastewater effluents.
- Process control at wastewater treatment plants to monitor phosphorus removal.
- Research studies on nutrient cycling, eutrophication risk assessment and marine/coastal monitoring (with salinity-adapted calibrations).
Who uses it: environmental labs, municipal and industrial water laboratories, regulatory bodies, consultants and academic researchers seeking reproducible, high-throughput phosphorus determinations.
Related Standards
- ISO 6878 - Determination of phosphorus (ammonium molybdate spectrometric method) / digestion guidance
- ISO 3696 - Water for analytical laboratory use
- ISO 5667‑1 / -2 / -3 - Water sampling guidance
- ISO 8466‑1 - Calibration and evaluation of analytical methods
Keywords: EN ISO 15681-1:2004, ISO 15681-1:2003, flow injection analysis, FIA, orthophosphate, total phosphorus, water quality, phosphorus determination, ISO 6878.