USGS Test Method I-2545

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Method Name:
Nitrogen, nitrite plus nitrate, dissolved, colorimetric, ASF. Official Name: Nitrogen, nitrite plus nitrate, colorimetry, cadmium reduction-diazotization, automated-segmented flow

An acidified sodium chloride extraction procedure is used to extract nitrate and nitrite from samples of bottom material for this determination (Jackson, 1958).Nitrate is reduced to nitrite by cadmium metal. Imidazole is used to buffer the analytical stream. The sample stream then is treated with sulfanilamide to yield a diazo compound, which couples with N-1naphthylethylenediamine dihydrochloride to form an azo dye, the absorbance of which is measured colorimetrically. The result is the sum of the nitrite originally present plus that formed by the reduction of the nitrate (Morris and Riley, 1963; Brewer and Riley, 1965; Wood and others, 1967; Strickland and Parsons, 1972; Nydahl, 1976; Sherwood and Johnson, 1981; Patton, 1982; U.S. Environmental Protection Agency, 1983).

This method is used to analyze samples of surface, domestic, and industrial water and brines containing from 0.05 to 5.0 mg/L of nitrite- plus nitrate-nitrogen. Samples containing greater concentrations must be diluted. Originally started in 1973, this modified method was implemented in the National Water Quality Laboratory in March 1988.

M.J. Fishman, 1993, Methods of analysis by the U.S. Geological Survey National Water Quality Laboratory--Determination of inorganic and organic constituents in water and fluvial sediments: U.S. Geological Survey Open-File Report 93-125

Concentrations of potentially interfering substances generally are negligible in unpolluted surface and ground water. For specific details of inorganic and organic compounds that interfere, see Norwitz and Keliher (1985, 1986) as well as more general information from the American Society for Testing and Materials (1991).Sulfides, often present in anoxic water, rapidly deactivate cadmium reactors by forming an insoluble layer of cadmium sulfide on the active metal surface (Strickland and Parsons, 1972). The buffer capacity of the imidazole solution and the approximate 10:1 volume ratio of buffer to sample eliminate the possibility of erroneous results for moderately acidic (pH > 1) samples.Mercury (II) does not interfere. It rapidly forms an amalgam on the inlet end of the cadmium reactor without detriment to the reduction reaction.

QC Requirements:
Calibrate instrument using calibration standards (CAL); quality control samples (QCS); and laboratory blanks (LB) analyzed at a minimum of 1 for every 10 samples.

Maximum Holding Time:
30 days



0.05 to 5.0 mg/L

Precision and average recovery based on determination of 24 replicates over several days.

Reporting Level defined as smallest measured concentration of a constituent that may be reliably reported using a given analytical method.

Revision Number:

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