Kjeldahl nitrogen in waste water

Significance of the Topic

A reliable assessment of nitrogen content in wastewater is critical for environmental monitoring and regulatory compliance. Nitrogen compounds support the growth of photosynthetic organisms in aquatic systems, but their uncontrolled release can lead to eutrophication and ecosystem imbalance. The Kjeldahl method remains a gold standard for measuring total nitrogen in various matrices, offering universality, precision, and reproducibility.

Objectives and Overview of the Study

This application note demonstrates a streamlined procedure for determining Kjeldahl nitrogen in wastewater according to ASTM D3590. It compares two endpoint titration approaches—photometric and potentiometric—after automatic Kjeldahl digestion and steam distillation.

Methodology and Instrumentation

Sample Preparation:

• Transfer aliquots of wastewater or spiked water into Kjeldahl digestion tubes.
• Perform acid digestion automatically in a commercial block digester.
• Add sodium hydroxide to convert ammonium to ammonia.
• Distill ammonia steam into boric acid collection solution.

Titration:

• Photometric titration: use an Optrode sensor to detect endpoint color change.
• Potentiometric titration: use a dEcotrode plus pH electrode to measure the equivalence point without indicators.

Instrumentation:

• OMNIS Advanced Titrator with magnetic stirrer
• Optrode for photometric detection (8 selectable wavelengths)
• dEcotrode plus combined pH electrode

Results and Discussion

Four replicate photometric titrations yielded a mean nitrogen concentration of 33.63 mg/L with an absolute standard deviation of 0.45 mg/L (1.33 % RSD). Three replicate potentiometric titrations measured 19.78 mg/L nitrogen with a 0.26 mg/L standard deviation (1.34 % RSD). Both methods demonstrated good precision (RSD below 2 %) and are suitable for routine wastewater analysis.

Benefits and Practical Applications

• High reproducibility and compliance with ASTM D3590.
• Choice of photometric or potentiometric endpoint detection based on sample matrix and user preference.
• Potentiometric mode eliminates the need for chemical indicators.
• Modular OMNIS platform allows seamless expansion for additional titration methods in water quality control laboratories.

Future Trends and Opportunities

Advancements may include enhanced automation of digestion and distillation steps, integration with data management and LIMS, miniaturized flow-through systems, and coupling with spectroscopic or mass spectrometric detectors for multi-analyte screening. Development of robust, maintenance-free sensors will further streamline routine nitrogen monitoring in environmental and industrial applications.

Conclusion

The described Kjeldahl workflow provides a straightforward, cost-effective approach for total nitrogen determination in wastewater. Both photometric and potentiometric titrations deliver precise, reproducible results in compliance with ASTM D3590. The adaptable OMNIS titration platform supports diverse analytical requirements in water quality laboratories.

Reference

• ASTM D3590: Standard Test Method for Determination of Kjeldahl Nitrogen in Water
• Metrohm Internal Reference AW TI CH1-1301-032020