In waters and wastewaters the forms of nitrogen of greatest interest are, in order of decreasing oxidation state: nitrate, nitrite, ammonia, and organic nitrogen. All these forms of nitrogen, as well as nitrogen gas (N2), are biochemically interconvertible and are components of the nitrogen cycle. In this method all forms of nitrogen are referred to and reported as the nitrogen (N) component of each form: organic nitrogen as organic-N, nitrate nitrogen as NO3--N, nitrite nitrogen as NO2--N, and ammonia nitrogen as NH3-N.
Organic nitrogen is defined functionally as organically bound nitrogen in the trinegative oxidation state. It does not include all organic-N compounds. Analytically, organic-N and ammonia can be determined together and have been referred to as Kjeldahl nitrogen, a term that reflects the technique used in their determination. Organic-N includes such natural materials as proteins and peptides, nucleic acids and urea, and numerous synthetic organic materials. Typical organic-N concentrations vary from a few hundred micrograms per liter in some lakes to more than 20 mg/L in raw sewage.
Total oxidized nitrogen is the sum of NO3--N and NO2--N. Nitrate generally occurs in trace quantities in surface water but may attain high levels in some groundwaters. In excessive amounts, it contributes to the illness known as methemoglobinemia in infants. A limit for nitrate at 10 mg/L (as nitrogen) has been imposed on drinking water to prevent this disorder. Nitrate is found only in small amounts in fresh domestic wastewater, but in the effluent of nitrifying biological treatment plants, NO3--N may be found in concentrations of up to 30 mg/L. It is an essential nutrient for many photosynthetic autotrophs and in some cases has been identified as the growth-limiting nutrient.
Nitrite is an intermediate oxidation state of nitrogen, both in the oxidation of ammonia to nitrate and in the reduction of nitrate. Such oxidation and reduction may occur in wastewater treatment plants, water distribution systems, and natural waters. Nitrite can enter a water supply system through its use as a corrosion inhibitor in industrial process water. Nitrite is the form of nitrogen that causes methemoglobinemia. Nitrous acid, which is formed from nitrite in acidic solution, can react with secondary amines (RR’NH) to form nitrosamines (RR’N-NO), many of which are known to be carcinogens. The toxicologic significance of nitrosation reactions in vivo and in the natural environment is the subject of much current concern and research.
Ammonia is present naturally in surface and wastewaters. Its concentration generally is low in groundwaters because it adsorbs to soil particles and clays and is not leached readily from soils. It is produced largely by the deamination of organic nitrogen-containing compounds and by the hydrolysis of urea. At some water treatment plants, ammonia is added to react with chlorine to form a combined chlorine residual. Ammonia concentrations encountered in water vary from less than 10 µg/L (as nitrogen) in some natural surface and groundwaters to more than 30 mg/L (as nitrogen) in some wastewaters.
Total nitrogen can be determined through the oxidative digestion of all digestible nitrogen forms to nitrate, followed by quantitation of the nitrate. Two procedures, one using an in-line persulfate/UV digestion (4500-N B), and the other using persulfate digestion (4500-N C) are presented. The procedures give accurate results for total nitrogen, composed of organic-N (including some aromatic nitrogen-containing compounds), ammonia, nitrite, and nitrate. Molecular nitrogen is not determined and recovery of some industrial nitrogen-containing compounds is low, but reproducible.
Chloride ions do not interfere with the alkaline persulfate oxidation, but the rate of reduction of nitrate to nitrite (during subsequent nitrate analysis by cadmium reduction) is significantly decreased by chlorides. Ammonium and nitrate ions adsorbed on suspended pure clay or silt particles are quantitatively recovered. If suspended matter remains after digestion, remove it before the reduction step.
If suspended organic matter is dissolved by the persulfate digestion reagent, yields comparable to those from true solutions are obtained; if it is not dissolved, the results are unreliable and probably reflect negative bias. The persulfate method is not effective in wastes with high organic loadings. Dilute such samples and reanalyze until results from two dilutions agree.
A conductimetric method (4500-N D) for determining inorganic nitrogen also is presented.
The method for detecting total nitrogen (TN, see 4500-N E) recovers all forms of nitrogen including total oxidized nitrogen and Kjeldahl nitrogen without the need for a separate step for digestion and rehydration. The process uses a high temperature combustion catalyst (HTCC) reaction (typically 720-950 °C) with a catalyst and oxygen or air to initiate a redox reaction with all forms of nitrogen to a detectable species. Unlike other total nitrogen detection methods, this methodology is not significantly affected by high salts or particulate matter.