Fertilizers and soil conditioners. Determination of total nitrogen by combustion
1Key Takeaways
This document specifies the method for the determination of total nitrogen content in all nitrogen-containing fertilizers by the combustion method. NOTE 1: The presence of non-nutritive nitrogen sources (e.g. chelating agents) can produce a positive bias in samples analysed for nutritive nitrogen content. The non-nutri…
2Expert Interpretation
An in-depth interpretation of the combustion determination method for total nitrogen content in fertilizers and soil conditioners in the BS ISO 20620:2021 standard, covering key technical points such as instrument configuration, chemical reagent requirements, operating procedures and precision control, which is suitable for quality testing of various types of nitrogen-containing fertilizers.
Analysis of the core content of the standard
This standard specifies the method and principle for determining the total nitrogen content in nitrogen-containing fertilizers and soil conditioners by combustion method: the sample is burned at a high temperature of more than 900°C in an automatic nitrogen analyzer, and the nitrogen oxides are reduced to elemental nitrogen, and then quantitatively analyzed by a thermal conductivity detector.
Key Equipment and Technical Parameters
| Equipment Type | Technical Requirements | Accuracy Requirements |
|---|---|---|
| Analytical Balance | Resolution≤0.1% of Sample Weight | ±0.1mg |
| Combustion System | Temperature≥900℃ | Temperature Control Accuracy±5℃ |
| Thermal Conductivity Detector | Linear Range0.1-100mgN | RSD≤1.5% |
Standard Material and Reagent Requirements
Standard Material Selection:It is recommended to use materials with certified nitrogen content such as EDTA, nicotinamide, and ammonium nitrate. Liquid urea solution cannot be used for calibration.
Gas Purity:Oxygen purity ≥99.999%, helium (if necessary) purity ≥99.999%.
Key points of the operation process
- Sample preparation must comply with the homogenization requirements of ISO 14820-2:2016
- More than 3 standard substances must be used to establish the calibration curve
- At least 3 parallel determinations are performed for each measurement
- Quality control charts monitor instrument performance trends
Method validation data
Ring studies showed that the average recovery of urea samples (n=22) was 99.6%, with a repeatability standard deviation of 0.120%; the reproducibility standard deviation of ammonium sulfate samples (n=20) was 0.087%.
Background of Standard Evolution
This method is based on the research results of AOAC 993.13 and ISO 22241-2:2019 Appendix B. For the first time, the combustion method is extended to the detection of non-mineral fertilizers. Compared with the traditional Kjeldahl method, it has the advantages of fast analysis speed (5-10 minutes/sample) and no need for strong acid digestion.
Implementation Recommendations
- For samples containing chelating agents, the non-nutritional nitrogen content needs to be additionally determined
- It is recommended to insert a quality control sample every 50 samples
- The combustion tube needs to be replaced every 1000 measurements
- The laboratory temperature should be controlled at 23±2℃