Nitrogen: Nitrate (NO3–) and Nitrite (NO2–)
What is Nitrate?
Nitrate or NO3– is a measurement of the amount of nitrate, the most stable form of nitrogen in water.
How is Nitrate measured?
A common method for nitrate measurement is Method 353.2. Samples are run through a cadmium column to reduce nitrate to nitrite. Nitrites react with an added reagent and generate a red coloured solution whose absorbance is then measured spectrophotometrically at 543 nm. The absorbance of the solution is directly proportional to the concentration of nitrite in the sample. Another method used for the detection of nitrate in water is the nitrate electrode method. This method operates in a similar fashion to a dissolved oxygen meter, where an electric potential of a solution is correlated to the concentration of nitrates in mg/L. Nitrate concentrations can be reported as mg/L NO3 or as mg/L nitrate-nitrogen (NO3-N).
Why is Nitrate important?
Nitrogen is a major nutrient for microbial life and is therefore very important with regards to its effects on the environment. Among nitrogen containing compounds, nitrate is the most common form in water. All other dissolved forms of nitrogen (nitrite, ammonia and organic nitrogen) get oxidized to nitrate over time. Nitrates are found in both ground and surface water, originating from the natural decaying process of biological matter. Many anthropogenic sources also contribute to nitrate levels in the environment such as industrial and municipal wastewater discharge and agricultural runoff containing nitrogen based fertilizers and livestock manure. Nitrate has a high solubility in water and will not be filtered out like other contaminants as it seeps through the soil layers to groundwater level. Measurement of nitrate is important for several applications.
In the drinking water industry, maximum contaminant levels of 50 mg/L NO3 in the EU and 10 mg/L NO3-N in the USA have been established for public safety. When nitrate enters the bloodstream of humans they reduce the ability of red blood cells to carry oxygen, which has serious health effects. This is particularly important for infants under the age of 6 months where excessive levels of nitrates in their bloodstream leads to a health condition known as methemoglobinemia or “blue baby syndrome”. Thus, measuring and removing nitrate from drinking water are important.
Wastewater contains high amounts of nitrates from human sewage and industrial process waste. Monitoring nitrate in wastewater is important to ensure proper removal prior to discharging into the environment.
From an environmental perspective, monitoring nitrate levels in surface water can indicate the potential for eutrophication or “nutrient pollution”. The influx of nutrients to surface waters can cause accelerated algal growth. When the large algae blooms die out, bacterial action increases expending oxygen levels. Waters may become hypoxic or anoxic, causing stress and possible death of aquatic life. Monitoring nitrate input to surface waters not only protects the health of the water body, but the safety of those who rely on the source.
Measuring Nitrate with UV-VIS technology
Measuring nitrates and or/nitrites with UV-VIS technology is a practical and reagentless solution for real-time monitoring. Nitrate ions have natural absorbance peaks in the 200-220 nm wavelength range in the UV spectrum. As the concentration of nitrates in water or wastewater increases, the absorbance of light in this distinct wavelength range will also increase.
Common interferences with nitrate measurement at 220 nm, such as organic compounds, nitrite, iron (II), hexavalent chromium and turbidity or suspended solids are compensated for by measuring absorbance at additional reference wavelengths in the UV-VIS spectrum where the interfering compounds absorb, but nitrates do not. This multiple wavelength approach ensures a high degree of precision and accuracy.
Where is Nitrate measured?
- Influent of a drinking water plant from a ground or surface water source
- After blending of multiple sources to determine reduction
- Effluent drinking water for compliance assurance
- Pre and post ion exchange or reverse osmosis for removal efficiency
- In process for nitrogen removal from wastewater
- Environmental monitoring stations for contamination events or locating source pollution