ISO 15681-1:2003

Overview

ISO 15681-1:2003 - Water quality: Determination of orthophosphate and total phosphorus by flow injection analysis (FIA) - Part 1 describes a standardized, automatable flow injection analysis (FIA) method for measuring orthophosphate (PO4‑P) and total phosphorus (P) in water. 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 (ISO 6878) in the range 0.1 mg/L to 10 mg/L (P). It is applicable to ground, drinking, surface, leachate, waste waters and-with salinity-adapted carriers and calibrations-to seawater.

Key topics and technical requirements

• Analytical principle: Sample injection into a carrier stream forms molybdophosphoric acid which is reduced to molybdenum blue (measured e.g. by a flow photometer).
• Total phosphorus: Organic and condensed phosphates are converted to orthophosphate by manual digestion (potassium peroxodisulfate) in accordance with ISO 6878 before FIA measurement.
• Calibration & ranges: At least five calibration points evenly distributed over the chosen working range; examples provided for common ranges (0.01–0.10, 0.10–1.00, 1–10 mg/L P).
• Reagents & preparation: Specifies analytical-grade chemicals (e.g., ammonium heptamolybdate, tin(II) chloride, hydrazine sulfate or DEHA), degassing of carrier/reagent solutions, and stability/storage notes.
• Interferences: Guidance on common interferences-arsenate, high silicate, fluoride (>50 mg/L), nitrite (>5 mg/L), oxidizing agents and high COD-and approaches to mitigate them (acidification, dilution, removal).
• Quality controls: Instrument performance checks, reagent blank checks, digestion-efficiency verification using standards (e.g., potassium pyrophosphate), and system shutdown procedures.
• Apparatus: Flow injection manifold, detectors (flow photometer), and additional equipment for digestion and sample handling.

Applications and users

ISO 15681-1 is intended for:

• Environmental and regulatory laboratories monitoring water quality and nutrient loads.
• Drinking water and wastewater treatment facilities measuring phosphate and total phosphorus for process control.
• Research laboratories and field service providers performing high-throughput, automated phosphorus analyses. Benefits include rapid throughput, reproducibility, and compatibility with automated sampling and data systems.

Related standards

• ISO 15681-2 (Part 2: continuous flow analysis, CFA)
• ISO 6878 (phosphorus determination - ammonium molybdate spectrometric method; digestion)
• ISO 3696 (water for analytical laboratory use)
• ISO 5667 series (sampling guidance)
• ISO 8466-1 (calibration and evaluation of analytical methods)

Keywords: ISO 15681-1:2003, flow injection analysis (FIA), orthophosphate, total phosphorus, molybdenum blue, water quality, phosphate determination, ISO 6878, seawater analysis, analytical calibration.