Article written by Carson Bennett, Chemist, Analytical Products Group, Inc.
Complete article from Edition 30 APG eNewsletter
Nitrogen compounds leached from fertilizer, raw sewage, and cattle feedlots can easily accumulate to excess in waste water (WP) and drinking water (WS). Through metabolic processes, nitrate is then readily reduced to nitrite by microorganisms and the human alimentary tract. When ingested, nitrite and the nitrate that is subsequently converted to nitrite, react with hemoglobin in the blood to form methemoglobin, which in turn undermines the body's ability to transport oxygen. As a result, the US EPA has set the highest acceptable concentration of nitrate in the drinking water at 10 ppm, the highest concentration of nitrite at 1 ppm, and the highest concentration of the two together at 10ppm. (1,2)
One common method of analysis of nitrate and nitrite in environmental labs is by a reduction reaction followed by a colorimetric assay. A solution of nitrate and nitrite is run through a copperized cadmium column, reducing all nitrate to nitrite. The resulting nitrite solution is reacted with sulfanilamide to form a diazo compound, which then combines with N-1-napthylethylenediamine dihydrochloride creating a purple-red color allowing for colorimetric determination. This method establishes total initial concentration of nitrate and nitrite together. The same colorimetric method can be run without the initial reduction reaction to ascertain the concentration of only nitrite in the initial solution. The concentration of nitrate can then be inferred from the difference between the two.(3) Common interferences include Sb 3+, Au 3+, Bi 3+, Fe 3+, Pb 2+, Hg 2+, Ag +, chloroplatinate (PtCl 6 2-) and metavanadate (VO 3 2-) because of precipitation under test conditions; NCl 3 which reacts to impart a false red color; cupric ion which catalyzes decomposition of the diazonium salt; and any colored ions. (4)
In some environmental labs, nitrate and nitrite levels in nutrient samples are analyzed by suppressed Ion Chromatography (IC) in which individual and integratable peaks are obtained for each anion in the sample. Using calibration plots, the Nitrate and Nitrite values are calculated independently. This method is more versatile than the reduction-colorimetric method and allows for the identification of other common anions in solution. Despite this, IC does have one potential drawback. Depending on the column and mobile phase used, potential interferences are anions that co-elute with either nitrate or nitrite. For example, chlorate has the potential to co-elute with nitrate. Therefore, any chlorate present in solution may interfere with nitrate to create a single peak of both anions or a shouldering peak which would yield inaccurate results. Because of this possibility, it is necessary to investigate potential interferences of non-target analytes as well as other target analytes during method development.
Figure 1. Ion Chromatogram of 10.00 mg/L nitrite as N and 10.00 mg/L nitrate as N in Ultra Pure De-Ionized (UPDI) Water using a 150 mm anion column, 20 uL sample loop and Carbonate-Bicarbonate mobile phase.
While a few states differ in their PT reporting criteria, NELAC currently (as of June 1, 2005) defines WP Nutrient parameters as Ammonia as N, Nitrate as N, Nitrite as N, Nitrate-Nitrite as N, Orthophosphate as P, Total Kjeldahl-Nitrogen (TKN), and Total Phosphorous and WS Nutrient parameters as Chloride, Fluoride, Nitrate as N, Nitrite as N, Nitrate+Nitrite as N and Ortho-Phosphate. Table 1 below details the concentration ranges.
Table 1. Current NELAC WS and WP Nutrient PT requirements. (5)
WS Nutrients |
mg/L |
WP Nutrients |
mg/L |
Fluoride |
1 to 8 |
Ammonia as N |
0.25 to 19 |
Nitrate as N |
3 to 10 |
Nitrate as N |
0.25 to 40 |
Nitrite as N |
0.4 to 2 |
Orthophosphate as P |
0.5 to 5.5 |
Ortho-Phosphate |
0.5 to 1.6 |
Total Kjeldahl-Nitrogen |
0.35 to 35 |
Nitrate+Nitrite as N |
3.5 to 9.0 |
Total Phosphorus |
0.15 to 10 |
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Nitrate-Nitrite as N |
0.25 to 40 |
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Nitrite as N |
0.4 to 4.0 |
Due the numerous ways of expressing Nitrate and Nitrite results, be sure to contact your State Coordinators to determine which analyte name is appropriate for your accreditation.
References
- http://www.doh.state.fl.us/environment/water/manual/nitrate.htm - Florida Department of Health website
- http://www.chemetrics.com/analytes/nitrate.html - Nitrate Method
- http://ral.coafes.umn.edu/water.htm - Typical Nitrate-Nitrite colorimetric method
- Method 4500-NO 2 - Standard Methods for the Examination of Waste Water. 20th ed.
- http://www.epa.gov/nerlesd1/land-sci/nelac/pttables.html - Tables of NELAC proficiency testing analytes and ranges
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