TN Measurement of Urea Solution

Importance of the Topic

The reduction of nitrogen oxides (NOx) from diesel engine exhaust is a critical challenge in environmental and automotive industries. Selective catalytic reduction (SCR) systems utilizing urea solutions are widely adopted to convert harmful NOx into nitrogen and water. Ensuring the correct urea concentration in SCR fluids is essential for maximum conversion efficiency and avoiding ammonia slip, which can harm downstream equipment and the environment.

Study Objectives and Overview

This application note demonstrates a rapid and reagent-free approach for measuring total nitrogen (TN) in aqueous urea solutions of varying concentrations. The goal is to validate Shimadzu’s TOC analyzers with TN measurement capability as an accurate alternative to the traditional Kjeldahl method, offering significant time savings and simplified workflow.

Methodology and Instrumentation

Powdered urea was dissolved in ultra-pure water to prepare 10 %, 20 %, 30 %, 40 %, and 50 % (w/w) solutions. Due to high nitrogen content, samples were appropriately diluted before analysis. A two-point calibration curve was established using 0 and 30 mgN/L potassium nitrate standards.

• Analyzer models: TOC-LCPH laboratory analyzer and TOC-4200 on-line analyzer
• TN unit: TNM-L total nitrogen option
• Catalyst: Combined TOC/TN oxidation catalyst
• Measurement principle: Thermal decomposition followed by chemiluminescence detection

Main Results and Discussion

TN values measured for each diluted urea solution displayed excellent agreement with theoretical nitrogen concentrations. The correlation coefficient (r) between measured and logical TN values was 0.9997, indicating high linearity.

• 10 % urea: 4.63 mgN/L measured versus 4.67 mgN/L theoretical (99 % recovery)
• 20 % urea: 9.69 mgN/L versus 9.33 mgN/L (104 % recovery)
• 30 % urea: 14.2 mgN/L versus 14.0 mgN/L (101 % recovery)
• 40 % urea: 19.6 mgN/L versus 18.7 mgN/L (105 % recovery)
• 50 % urea: 24.4 mgN/L versus 23.3 mgN/L (105 % recovery)

These results confirm the method’s accuracy and robustness across a broad concentration range.

Benefits and Practical Applications

This TN measurement approach offers several advantages:

• Rapid analysis time (approximately 5 minutes per sample)
• No chemical reagents required for digestion or distillation
• High accuracy and reproducibility
• Minimal sample preparation and operational simplicity

It is well suited for quality control of SCR urea solutions in automotive manufacturing, environmental monitoring, and industrial process control.

Future Trends and Possibilities for Use

Emerging trends include integrating TN monitoring into real-time SCR dosing systems for continuous feedback control. Further development could involve coupling TN analysis with other online sensors for multiparameter assessment of exhaust treatment fluids. Automation and miniaturization of the analyzer hardware may expand field deployment capabilities.

Conclusion

Shimadzu’s TOC analyzers equipped with a TNM-L unit provide a fast, reliable, and maintenance-free solution for measuring total nitrogen in urea solutions. This method represents a practical alternative to the Kjeldahl technique, enhancing efficiency in laboratories and industrial settings.

References

• Shimadzu Corporation. Application News No. O67: TN Measurement of Urea Solution. First Edition, March 2018.