Total bound nitrogen

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Total bound nitrogen or TNb (engl. Total Nitrogen bound ) is a sum of parameters in the water and wastewater analysis and reflects the impact of the water with nitrogen compounds resist, for example in the form of the fact ammonium , nitrites or nitrates may occur. For this purpose, the amount of total nitrogen that is bound in the various chemical compounds contained in water is determined and shown as a concentration in mg / L. For orientation: The nitrogen content of unpolluted surface water is between 3 and 7 mg / L, in polluted water it can rise to 200 mg / L.

The most common method is to measure the amount of nitric oxide with chemiluminescence detectors. To do this, all of the nitrogen compounds contained in the water sample must first be converted into nitrogen oxide. Alternatively, there are, for example, IR detection, potentiometry , amperometry or NO / NO 2 gas sensors. Dissolved nitrogen from the air is not also determined.

In water chemistry , the determination of TNb competes with the Kjeldahl nitrogen determination . The advantages of the TNb determination are the increased accuracy, the short preparation and measuring time and the better automation. Another advantage is that there are no harmful reagents. The disadvantage is the much greater expenditure on equipment; However, many TOC analyzers ( Total Organic Carbon ) for measuring the total content of carbon enable the joint determination of TOC and TNb by adding a TN detector.

Analytical method

The standard literature (for example DIN EN 12260) describes the conversion of nitrogen compounds by catalytic combustion in an oxygen atmosphere at> 700 ° C to nitrogen monoxide with subsequent chemiluminescence detection. Not all nitrogen compounds can be determined with this method (especially those in which the nitrogen is double or triple bound). High concentrations of organic carbon (TOC) can also influence the determination.

Chemiluminescence detection

The detection of nitrogen oxide takes place by means of chemiluminescence. It arises in the second analysis stage when nitrogen monoxide reacts with ozone . The ozone is generated in the required small amount by an ozone generator. Unnecessary ozone is removed from the carrier gas. The concentration measured by the chemiluminescence detector is recorded as a function of time. The resulting integral (often referred to as the peak area) is a measure of the nitrogen from the sample.

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