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