Ammonium and urea nitrogen in NPK fertilizers

Significance of Thermometric Titration in Fertilizer Analysis

Modern agriculture relies on balanced supply of nutrients to optimize crop yield and minimize environmental impact. NPK fertilizers containing nitrogen in the forms of ammonium, ammonia and urea are widely used. Precise quantification of each nitrogen species is essential to control release rates, prevent phytotoxicity and ensure regulatory compliance. Thermometric titration provides a rapid, robust and simultaneous determination of ammoniacal and urea nitrogen in a single measurement, addressing limitations of conventional multi-step methods.

Objectives and Study Overview

This study demonstrates a fast, single-titration approach to quantify ammonium and urea nitrogen in two solid NPK fertilizer formulations. Key goals include:

1. Developing a thermometric titration protocol using sodium hypochlorite as titrant and bromide as catalyst
2. Evaluating method precision and accuracy on NPK 17-8-10 and NPK 15-15-15 samples
3. Reducing analysis time compared to traditional techniques

Used Instrumentation

• 859 Titrotherm with 10 mL buret, rod stirrer, Thermoprobe and tiamo™ software
• 846 Dosing Interface for automated liquid handling
• 800 Dosino modules with 5 mL, 10 mL and 50 mL dosing units

Methodology

Solid fertilizer samples were accurately weighed and dissolved in warm water to prepare stock solutions. The titration vessel was charged with a 50 mL aliquot of solution and all reagents were dosed automatically. Thermometric titration exploits the exothermic reaction of hypochlorite with ammoniacal nitrogen (fast reaction) and urea (slower reaction), producing two distinct temperature endpoints. Bromide catalyzes both reactions. The first endpoint (EP1) corresponds to ammonium consumption and the second (EP2) to urea decomposition.

Results and Discussion

Distinct dual-endpoint titration curves were recorded for both NPK samples. Results (n = 6) showed:

• NPK 17-8-10: 11.31 % ammoniacal nitrogen (s(rel) = 0.70 %), 4.51 % urea nitrogen (s(rel) = 0.69 %)
• NPK 15-15-15: 11.98 % ammoniacal nitrogen (s(rel) = 0.31 %), 2.03 % urea nitrogen (s(rel) = 2.35 %)

Precision and accuracy meet typical fertilizer quality control requirements. Sample spiking enhanced EP2 resolution when urea levels were low. Total analysis time remained under three minutes per sample.

Benefits and Practical Applications

• Simultaneous quantification of two nitrogen species in one titration
• High throughput: less than 3 min per analysis
• Automated dosing reduces manual handling and errors
• Excellent repeatability suitable for QA/QC and R&D laboratories

Future Trends and Potential Applications

Thermometric titration could be extended to other fertilizer formulations and complex matrices. Integration with robotics and advanced data analytics will further increase throughput. Combining thermal endpoints with spectroscopic detection may enable multi-analyte fertilizer profiling in a single workflow. Adoption of cloud-based titration control and LIMS connectivity will streamline large-scale agricultural quality monitoring.

Conclusion

The thermometric titration method provides a fast, accurate and automated solution for determining ammonium and urea nitrogen in NPK fertilizers. Its dual-endpoint detection enhances analytical efficiency and supports high-volume quality control in agricultural chemistry.

References

• Metrohm AG, Application Note AN-H-146: Ammonium and urea nitrogen in NPK fertilizers by thermometric titration