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Water Quality - Nitrogen

Nitrogen

Nitrogen levels are a key indicator for water quality. Nitrogen is often the limiting factor for photoautotrophs. A shift in the nitrogen cycle may have dramatic consequences for an ecosystem. In the environment, inorganic nitrogen occurs in a range of oxidized states as nitrate (NO3-), nitrite (NO2-), ammonium (NH4+) and dinitrogen, or nitrogen gas, (N2). Plants and microorganisms are capable of converting inorganic nitrogen to organic nitrogen. The term organic nitrogen refers to nitrogen compounds that originated from living material, for example: nitrogen in protein and urea.

Nitrite (NO2-) and nitrate (NO3-)

Nitrite ions are rapidly oxidized to nitrate. Nitrate is the primary inorganic nutrient that drives primary production in aquatic habitats. Therefore, nitrate analysis can help explain ecosystem dynamics. Natural sources of nitrate include rocks, land drainage, and plant/animal debris. The level of nitrate may be inflated by overuse of nitrogen fertilizers. The concentration of nitrate ions (NO3-) is expressed in units of mg/L NO3—N. The unit NO3—N describes nitrogen that is in the form of nitrate.

Various natural and man-made sources containing nitrogen can lead to the accumulation of nitrate (NO3-) in water. These include:

  • Agricultural runoff

  • Urban runoff

  • Animal feeds

  • Municipal and industrial wastewater

  • Automobile and industrial emissions

  • Decomposition of plants and animal

Nitrate levels in water should not exceed 1 mg/L, however human-derived nitrates can increase the level to above 3 mg/L, therefore nitrate levels in water are usually between 0.1 to 4 mg/L NO3—N. Unpolluted water, however, should contain nitrate levels below 1 mg/L.

The standard method for measuring nitrate levels in a water sample is the automated cadmium reduction method. In this method, the unstable nitrate ions are reduced to nitrite in the presence of cadmium. Then, the nitrite is diazotized with sulfanilamide and coupled with N-(1-Naphthyl)-ethylenediamine dihydrochloride to form a brightly colored azo dye. The concentration of nitrogen in the water sample correlates with the intensity of the color produced.

Six tubes with solutions of different shades of purple color. Under each tube is a number, indicating the nitrate level in milligrams per liter of nitrate. First tube is transparent and has a value of 0. Second to sixth tube has a shade of purple changing from light pink up to very dark purple, and values equal to 0.5, 5.0, 15.0, 30.0 and 45.0, respectively. Under the scale, the undergoing chemical reactions in the tubes are presented.In the upper chemical reaction, the sulphanilic acid reacts with nitrate, resulting in new chemical structure. In the second reaction, this newly formed chemical reacts with N - one naphthyl - ethylenediamine, resulting in a new structure named pink-purple azo dye. The dye is marked in dark pink colour.

Figure 1. First image - The darker the purple, the higher the nitrate levels. Second image - The chemical reaction in cadmium reduction method.

Referred from:

Article
Water Quality - Oxygen