Nitrogen (total) in Effluents
Photometric determination following decomposition with potassium peroxodisulfate in the thermoreactor

Introduction

Excess nitrogen in effluents can be problematic because it can lead to contamination of groundwater and subsequent environmental problems. One of these issues is eutrophication—excessive growth of algae and plants—which can adversely affect aquatic ecosystems.¹ Total nitrogen in effluents consists of both organic and inorganic nitrogen, including ammonium, nitrite and nitrate. As a result, monitoring nitrogen is critical for environmental reasons. Here we describe the photometric determination of nitrate in effluents using the Spectroquant^® system and test kits.

Experimental Method

Organic and inorganic nitrogen compounds are converted into nitrates according to the Koroleff’s method by treatment with an oxidizing agent in a thermoreactor. In a solution acidified with sulfuric and phosphoric acid, these nitrates then react with 2,6-dimethylphenol (DMP) to form 4-nitro-2,6-dimethylphenol that is determined photometrically. The digestion is analogous to EN ISO 11905-1. The determination of nitrate is analogous to DIN 38405-9.

Reagents, Instruments and Material

Test Kits

One of the following test kits is necessary for this analysis:

• Nitrogen (total) Cell Test Method: photometric 0.5 - 15.0 mg/l N Spectroquant^® (1.14537)
• Nitrogen (total) Cell Test Method: photometric, DMP 0.5 - 15.0 mg/l N Spectroquant^® (1.00613*)
• Nitrogen (total) Cell Test Method: photometric, DMP 10 - 150 mg/l N Spectroquant^® (1.14763*)

*not compatible with Move 100

Instruments

One of the following instruments is necessary for this analysis:

• Spectroquant^® VIS Spectrophotometer Prove 100 (173016)
• Spectroquant^® UV/VIS Spectrophotometer Prove 300 (173017)
• Spectroquant^® UV/VIS Spectrophotometer Prove 600 (173018)
• Spectroquant^® Photometer NOVA 30 (1.09748)
• Spectroquant^® Photometer NOVA 60 (1.09751)
• Spectroquant^® Photometer NOVA 60A (1.09752)
• Spectroquant^® Colorimeter Move 100 (1.73632)
• Thermoreactor with 120 °C digestion temperature

Materials

• Rectangular cells 10 mm (1.14946) and/or
• Rectangular cells 20 mm (1.14947) and/or
• Rectangular cells 50 mm (1.14944)

Experimental Procedure

The content of total nitrogen can be rationally measured after decomposition according to the Koroleff method in the thermoreactor with alkaline peroxodisulfate solution. The method is restricted for the content of organic compounds, expressed as the COD content. For the decomposition of samples in the thermoreactor, the maximum COD concentration must not exceed 300 mg/l. Very slightly oxidizable nitrogen compounds are only partly measured.

Sample Preparation

The sample, pH 5 - 9, with a COD value smaller than 300 mg/l is treated as per the instructions of the Spectroquant^® Nitrogen Cell Test given. The sample is digested in a closed round-cell in the thermoreactor. After digestion the sample must be checked, as specified, for freedom from peroxides; any peroxides present must be destroyed with sodium sulfite.

Analysis

Determine with the above-mentioned test kits.

Calculation

Nitrogen (total) content in mg/l N = analysis value in mg/l N

References

1. Camargo JA, Alonso Á. 2006. Ecological and toxicological effects of inorganic nitrogen pollution in aquatic ecosystems: A global assessment. Environment International. 32(6):831-849. https://doi.org/10.1016/j.envint.2006.05.002