TNb - Total Nitrogen Bound
The total nitrogen bound (TNb) defines the total pollution of water by nitrogen compounds. It is an analytical parameter for water and is specified in mg/l.
The pollution of nitrogen can appear in form of ammonia, ammonium salts, nitrites, nitrates and organic nitrogen compounds. In contrast to its individual determination, the TNb covers all components in one analytical run. The determination of the TNb value is standardized by the German Institute of Standardisation (DIN EN 12260:2003).
Need of monitoring
Nitrogen is an essential nutrient for humans and nature. Today the nutrient contents in water are very high. A further increase would lead to eutrophication (overfertilization). Therefore, it is required to monitor this parameter by online measurement systems and thus enabling the regulation of nutrient concentrations in public water.
An overview of methods for measuring nitrogen compounds in water
There are standard methods for determination of nitrogen in water analysis:
- Kjeldahl nitrogen determination (EN 25663) (TKN = Total Kjeldahl Nitrogen) This method only contains ammonium nitrogen and organic nitrogen compounds.
- The determination of total nitrogen, according to the German Federal Water Act (WHG/ AbwAG) involves the measurement of the sum of all inorganic nitrogen, such as nitrate, nitrite and ammonia.
- The determination of total nitrogen according to Koroleff determines all nitrogen compounds available through a persulphate digestion. Here, a reduction of the resulting nitrate occurs with a copper/cadmium alloy to nitrite, followed by a quantitative determination of nitrite.
- The analysis of the total bound nitrogen is standardized by the German Institute of Standardisation according to DIN EN 12260:2003. This method detects all kinds of nitrogen except molecular nitrogen (N2). It uses the high temperature oxidation with the support of hydrogen or oxygen. With the reduction with H2 all nitrogen compounds are transformed into ammonia. Upon oxidation of nitrogen with O2, nitrogen is converted to nitrogen oxygen (NO). These reactions usually take place at high temperatures of about 700°C and are supported by a catalyst. However, the higher the temperature, the more reliably the nitrogen compounds are dissolved.
Kjeldahl nitrogen compounds (TKN) and persulphate digestion
On the market, the determination of the TNb competes with the Kjeldahl nitrogen determination (TKN) and with the persulphate digestion according to Koroleff. In contrast to the TKN, the persulphate digestion and the thermal determination of TNb determines inorganic components such as nitrite and nitrate, too. The methods according to Kjeldahl and Koroleff are time-consuming, labor-intensive and require high amounts of chemicals. Hence, these methods are not suitable for fast and accurate online determination of the nitrogen content.
Thermal TNb determination
The thermal determination of TNb is characterized by a high degree of automation, increased accuracy as well as by short measuring cycles. Additionally, the user benefits from the fact that hazardous reagents are not necessary at thermal determination.
Commonly two detections of TNb in water samples are used. The detection of the concentration is made with a chemiluminescent- or an electrochemical detector. For chemoluminescence detection ozone is required for the reaction with NO, disadvantaging the approach due to the use of hazardous reagents and the high cost. The electrochemical method is low maintenance, includes lower aquisition costs and the accuracy of its measurement is comparable with the chemoluminescence detection.
Due to the different oxidation power and the unstable character of the types of measurement (NO) it is impossible to compare the TN-results of the various methods. In general, the method of the total bound nitrogen achieves the highest measurement results.
Furthermore, different results may occur within a method. Variations in the reagent composition can lead to different results in chemical processes (Koroleff), just as the different catalysts can affect the results as well (EN 12260). As a general rule: The higher the temperature, the better the results are. (see: K. Nagel, O. Primm : Research Report No. 20,022,231, Federal Environment Agency).
Thermal oxidation at 1,200°C
LAR AG guarantees the complete oxidation of all organic and inorganic compounds with the ultra-high temperature method at 1,200°C. After the oxidation the TNb is detected by an electrochemical sensor . This is an environmentally friendly method that provides very accurate measurement results. The chemoluminescence detector is available as an option of the QuickTONultra. Operators also have the option to measure the TNb in combination with TOC and COD. The high temperature devices of LAR are applicable in a variety of applications.