USGS Test Method I-2546-91

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Method Name:
Nitrate and Nitrite in Low Ionic-Strength Water by Colorimetry. Official Name: Nitrogen, Nitrate Plus Nitrite, Low-Ionic Strength Water, Colorimetry, Cadmium Reduction - Diazotization, Automated-Segmented Flow

Nitrate is reduced to nitrite by cadmium metal. The sample stream then is treated with sulfanilamide under acidic conditions to yield a diazo compound, which couples with N-1-naphthylethylenediamine 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 precipitation or natural water, containing from 0.005 to 1.0 mg/L of nitrite- plus nitrate-nitrogen. Concentrations greater than 1.0 mg/L must be diluted. This method was implemented in the National Water Quality Laboratory in March 1986 and modified in May 1989. See OFR 93-125 for additional modifications to the method.

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 greater than or equal to 1).

QC Requirements:
Quality-control samples area analyzed at a minimum of one in every ten samples. These QC samples include at least one of each of the following: blanks, quality control samples, third party check solutions, replicates, and spikes. Correlation coefficients for calibration curves must be at least 0.99. QC samples must fall within 1.5 standard deviations of the mean value. If all of the data-acceptance criteria in the SOPs are met, then the analytical data are acceptable.

Maximum Holding Time:
30 days from sampling



0.005 - 1.0 (undiluted)

Single operator precision for nitrite- plus nitrate-nitrogen, as determined for natural-water samples, is as follows: a series of 11, 21, 22 and 22 determinations were made with mean recoveries of 0.009, 0.105, 0.465 and 0.717 mg/L with standard deviations of 0.001, 0.006, 0.015, and 0.011 mg/L and 11.1, 5.7, 3.2 and 1.5 %-RSD, respectively.

Method detection limits (MDLs) were determined for this method using the procedures of the U.S. Environmental Protection Agency (1997).

Revision Number:

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