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.

Comparison

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).

Table for the comparison of different Total Nitrogen analysis methods
TNb measurement methods and what they measure

Thermal oxidation at 1,200°C

LAR Process Analysers 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 Process Analysers AG are applicable in a variety of applications.

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The total organic carbon (TOC) is one of the most important sum parameters in the assessment of the organic pollution of water. Since it includes all carbon compounds as one mass, it is exactly defined and an absolute quantity. Therefore, it may be determined directly.

Read more about TOC measurement methods.

Total organic carbon analyzers of LAR Process Analysers AG

The chemical oxygen demand (COD) indicates the amount of oxygen which is needed for the oxidation of all organic substances in water and is thus an important indicator for water analysis. It is considered in the planning and controlling of treatment, as well as assessing its efficiency, thus forming the basis for the calculation of waste water charges.

The COD can be determined in laboratory or online, whereby these methods significantly differ in duration and consumables used.

Read more about COD measurement methods.

COD analyzers of LAR Process Analysers AG

The biochemical oxygen demand (BOD) indicates the amount of oxygen which is needed for the biological degradation of organic substances in water.

Since the generally used BOD5 excludes the nitrification part of the process, this parameter is poorly suited for the control of waste water treatment plants. Alternatively, the total BOD is well-suited, due to the determination of the nitrogen as well as the carbonaceous part of biological degradation.

Read more about the online measurement of BOD.

Toxicity is described as the direct harmful effect of a substance on organisms. These effects can already occur at low concentraions of toxic substances and are dependent on the incubation period and the dosage.

Some test methods that are available on the market can detect toxicity by using fish, daphnia, molluscs, algae or luminous bacteria and testing whether a water sample has a toxic effect on the organisms. They do not, however, identify exactly which toxins are present.

Read more about online toxicity measurements.

BOD / toxicity analyzers of LAR Process Analysers AG

The total nitrogen bound (TNb) shows the pollution of water caused by nitrogen compounds. Nitrogen may be present as ammonia, ammonium salts, nitrites, nitrates and organic nitrogen compounds.

In contrast to single measurements of the above-mentioned components, the TNb determination contains all these substances in a single analysis process.

Read more about online TNb measurements.

The total phosphorus (TP) is a sum parameter that shows the organic and inorganic phosphorus compounds in water. Phosphorus is an essential nutrient for humans as well as flora and fauna. However, the substance – depending on its concentration– may cause serious damage.

Read more about online TP measurements.

TN TP analyzer of LAR Process Analysers AG