ISO 6598:1985

Overview

ISO 6598:1985 specifies a gravimetric method for determining phosphorus content in fertilizers by precipitating orthophosphate as quinoline phosphomolybdate. The procedure - applicable to natural phosphate materials and manufactured fertilizers - includes hydrolysis if non‑orthophosphate forms are present, precipitation in an acidic/acetone medium at about 75 °C, followed by filtration, washing, drying and weighing of the precipitate. Results are reported as percentage of P2O5 (pentoxide of diphosphorus).

Key topics and technical requirements

• Principle: Convert orthophosphate to a quinoline phosphomolybdate precipitate; quantify phosphorus gravimetrically.
• Hydrolysis: Required when phosphate species other than orthophosphate are present; typically reflux about 1 hour.
• Reagents: Analytical‑grade reagents only. The standard specifies prepared solutions (A–E) including molybdate, citric acid, nitric acid and quinoline; acetone is used as solvent.
• Apparatus: Common lab equipment plus:
  • 500 ml wide‑neck conical flask
  • Fritted glass crucible (porosity P10 or P16)
  • Oven (250 ± 5 °C), desiccator, and a heating device without an open flame
• Procedure highlights:
  • Add precipitant solution and heat to near‑boiling (~75–80 °C) rapidly.
  • Cool, allow precipitate to settle, decant and filter under vacuum.
  • Wash repeatedly, transfer to crucible, dry to constant mass at 250 °C and weigh to 0.0001 g precision.
  • Add acetone to filtrate and dilute as specified when preparing reagent blends.
• Quality metrics:
  • Repeatability: absolute difference between two results (same operator/lab) should be < 2 mg (for ~350 mg precipitate).
  • Reproducibility: absolute difference between different operators/labs should be < 6 mg (for ~350 mg precipitate).

Applications and users

• Who uses it:
  • Fertilizer manufacturers and QC laboratories for batch release and specification compliance
  • Agricultural testing labs and certification bodies
  • Regulatory agencies monitoring fertilizer composition
• Practical value:
  • Provides a robust, reference gravimetric route to quantify phosphorus as P2O5
  • Useful where instrumental methods are unavailable or for method validation and inter‑laboratory comparisons
  • Supports labeling accuracy, compliance with fertilizer standards, and agronomic recommendation systems

Related standards

• ISO 5316 - Fertilizers: dissolution of water‑soluble phosphates
• ISO 7437 - Fertilizers: extraction of phosphates with mineral acids

Keywords: ISO 6598:1985, fertilizer phosphorus determination, quinoline phosphomolybdate, gravimetric method, P2O5 analysis, fertilizer quality control.