Overview
ISO 22018:2021 specifies a standardized laboratory method for determining EDTA soluble phosphorus in inorganic fertilizers. The procedure targets fertilizers made from or blended with phosphate sources such as superphosphate, ammonium phosphate, triple superphosphate and nitrophosphate. It is explicitly not suitable for fertilizers containing calcium magnesium phosphates. The method uses EDTA extraction followed by a gravimetric determination of phosphorus as quinolinium molybdophosphate and reports results as a mass fraction percentage of P2O5.
Key topics and requirements
- Extraction principle: EDTA disodium salt (0.1 mol/L, 37.5 g/L) is used to extract EDTA-soluble phosphorus; orthophosphate in the extract reacts with quimociac reagent to form a yellow quinolinium molybdophosphate precipitate.
- Gravimetric determination: Precipitate is isolated on a glass filter crucible, dried (oven ~180 °C), cooled and weighed to calculate EDTA soluble P (expressed as P2O5).
- Representative test steps:
- Sample preparation per ISO 8358 and ISO 14820-2.
- Extraction at controlled temperature (thermostatic water bath oscillator ~60 °C for 1 h) with a defined EDTA volume.
- Acidification and reaction with quimociac reagent, precipitation, filtration, washing, drying and weighing.
- Blank test and parallel determinations for accuracy.
- Precision metrics: repeatability limit r = 0.25 (mass fraction percentage); reproducibility limit R = 0.50 (mass fraction percentage). A ring test (interlaboratory study) supports precision data.
- Reporting: Test reports must identify the sample, reference ISO 22018:2021, include results, sampling/analysis dates, and note whether repeatability requirements are met.
Applications and practical value
- Quality control in fertilizer production to verify the soluble phosphorus fraction accessible via EDTA extraction.
- Product specification and compliance testing for inorganic phosphate fertilizer blends (excluding calcium magnesium phosphate products).
- Independent testing laboratories and accreditation bodies use the method to provide traceable, comparable results across labs.
- R&D and formulation teams assessing how different phosphate sources contribute to EDTA-soluble phosphorus in compound fertilizers.
- Helps agronomists and regulatory agencies evaluate phosphorus availability characteristics relevant to nutrient management and labeling.
Who should use this standard
- Fertilizer manufacturers and formulation chemists
- Agricultural testing laboratories and contract labs
- Regulatory and conformity assessment bodies
- Research institutions studying fertilizer composition and phosphorus availability
Related standards
- ISO 8157 - Fertilizers and soil conditioners: Vocabulary
- ISO 8358 - Solid fertilizers: Preparation of samples for analysis
- ISO 14820-2 - Fertilizers and liming materials: Sampling and sample preparation (Part 2)
- ISO 5725-2 - Accuracy (trueness and precision): Repeatability and reproducibility methods
Keywords: ISO 22018:2021, EDTA soluble phosphorus, inorganic fertilizers, determination method, quinolinium molybdophosphate, fertilizer testing, sample preparation, repeatability, reproducibility.
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