The measurement of nitrogen is routinely carried out in food and environmental laboratories. There are many methods for nitrogen determination, and manufacturers of nitrogen analyzers use different methods in their instrument. All methods require an initial oxidation step where all nitrogen-containing organic compounds are converted to inorganic nitrogen species. The oxidation step can be carried out by: (1) Kjeldahl digestion, (2) ultraviolet (UV) oxidation, (3) persulfate oxidation, and (4) high-temperature oxidation (combustion). In the Kjeldahl method, the digestion step is followed by distillation and then titration. The nitrogen in the sample is converted and analyzed as ammonium. For more information on Kjeldahl method, please refer to the section dedicated to this topic. In the UV oxidation method, the organic nitrogen is converted to nitrite and nitrate, which are the measured by UV spectrophotometry. Persulfate oxidation uses potassium persulfate (potassium peroxodisulfate) in alkaline solution and digestion at temperatures of 100-120°C to convert nitrogen-containing compounds to nitrate, which can be measured spectrophotometrically. High temperature oxidation methods involve pyrsolysis at high temperatures (900-1100°C) in the presence of a catalayst. Nitrogen-containing compounds are converted ultimately to molecular nitrogen or nitrogen oxide, which can be detected using thermal conductivity (for N2) or chemiluminescence (for NO2).
Phosphorus measurement is important in environmental analysis. High levels of phosphorus and nitrogen in environmental waters lead to significant water quality problems including harmful algal blooms, hypoxia and declines in wildlife and wildlife habitat. This could become a pubic health concern since drinking water sources can be affected and there could be an increased exposure to toxic microbes such as cyanobacteria. In water, phosphorus containing compounds include inorganic phosphorus (orthophosphoric acid, condensed phosphorus, etc.) and organic phosphorous, which are collectively called total phosphorus. Phosphorus analyzers usually employ the ascorbic acid - molybdate method. The sample is first digested so that all organic phosphorus is converted to orthophosphate. Ammonium molybdate and potassium antimonyl tartrate react in acidic medium with orthophosphate to form phosphomolybdic acid, which is reduced to intensely colored molybdenum blue by ascorbic acid. The intensity of the blue color is measured spectrophotometrically.
Nitrogen and phosphorus analyzers can be stand-alone instruments, that is, a dedicated a nitrogen or phosphorus analyzer. Or they could be a combination of both nitrogen and phosphorus analyzers.
Nollet, L.M.L. (Ed). (2000) Handbook of Water Analysis. NY: Marcel. Dekker, Inc.
Liptak, B.G. (1994) Analytical Instrumentation. Pennsylvania: Chilton Book Company
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