Nitrogen-based compounds are widely distributed in the environment. They are essential growth nutrients for photosynthetic organisms (e.g., plants and algae). Therefore, it is important to monitor and control the amount of nitrogen compounds which are released into the environment.

In this Application Note, a method to determine the nitrogen content in water by Kjeldahl digestion and distillation followed by a photometric or potentiometric titration according to ASTM D3590 is presented. Nitrogen determination by Kjeldahl digestion and distillation has been performed since 1883. The universality, precision, and reproducibility of the Kjeldahl method have made it the internationally recognized method for e.g. estimating the protein content in many matrices and it is the standard method to which all other methods are judged against.

After the Kjeldahl distillation, the nitrogen content is determined by acid-base titration. This can either be a photometric or potentiometric titration depending on the sample and the preferences of the user. Both titration methods offer a reliable and inexpensive determination.

This application is demonstrated on wastewater for the photometric titration, and on spiked water for the potentiometric titration.

Each sample is transferred into the Kjeldahl tube for digestion. The digestion is performed automatically using a commercially available digester. After the digestion, a sodium hydroxide solution is added to the mixture and the resulting ammonia is automatically distilled into a collecting vessel containing boric acid using a steam distillation apparatus.

OMNIS Advanced Titrator equipped with a dEcotrode plus for the potentiometric determination of Kjeldahl nitrogen in water.
Figure 1. OMNIS Advanced Titrator equipped with a dEcotrode plus for the potentiometric determination of Kjeldahl nitrogen in water.

This analysis is carried out on an OMNIS Advanced Titrator equipped with an Optrode for the photometric titration, and with a dEcotrode plus for the potentiometric titration.

The prepared samples are titrated with sulfuric acid until after the equivalence point is reached. To ensure good recovery and reproducibility, it is important that the distillation apparatus used for this sample preparation is leak-proof and that the water, which is used for the blank, is nitrogen-free.

The recovery and standard deviation of the two different titrations could not be compared, as different samples were used. However, for both methods relative standard deviations are below 2%, which is acceptable for this application.

Table 1. Results of the nitrogen determinations in water by titration after the Kjeldahl digestion and distillation.
  Photometric titration (n = 4) Potentiometric titration (n = 3)
Mean 33.63 mg/L 19.78 mg/L
SD(abs) 0.45 mg/L 0.26 mg/L
SD(rel) 1.33% 1.34%

Titration is an easy method to determine Kjeldahl nitrogen in wastewater according to ASTM D3590. The titration can be either performed photometrically or potentiometrically. The potentiometric method provides the advantage that no indicator is needed. On the other hand, the Optrode for photometric measurements is maintenance-free. Which titration is used depends on the sample and the preferences of the user.

For both methods, an OMNIS Titrator can be used. This allows you to customize the system according to your needs and expand it for other titration applications required for the quality control of water.

Author

Metrohm USA

9250 Camden Field Pkwy
33578 Riverview, FL

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