4500-NO3 NITROGEN (NITRATE)

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Abstract:4500-NO3 A. Introduction

1. Selection of Method

Nitrate (NO3) determination can be difficult because of the high probability that interfering constituents are present in various matrices. This section includes numerous methods that can be used to detect NO3. Consider the advantages and limitations of each method when selecting a test strategy by reviewing the sample matrix, concentration range, and data needs for a particular application.

The ultraviolet (UV) light technique (4500-NO3 B), which measures NO3 absorbance at 220 nm, is suitable for screening uncontaminated water (low in organic matter).

Screen a sample if necessary, then select a method suitable for its concentration range and probable interferences. Nitrate can be determined by ion chromatography (Section 4110), capillary ion electrophoresis (Section 4140), or the methods shown here. Applicable ranges for the methods in this section are:

• 0.2 to 11 mg nitrate-nitrogen (NO3-N)/L; Ultraviolet Spectrophotometric Method (4500-NO3 B)

• 0.5 to 2.5 mg NO3-N/L; Second Derivative Ultraviolet Screening Method (4500-NO3 C)

• 1 to 50 mg NO3-N/L; Nitrate Electrode Method (4500-NO3 D)

• 0.05 to 1.0 mg nitrate + nitrite-nitrogen (NO3 + NO2-N)/L; Cadmium Reduction Method (4500-NO3 E)

• 0.05 to 10 mg nitrate + nitrite-nitrogen (NO3 + NO2-N)/L; Automated Cadmium Reduction Method (4500-NO3 F)

• 0.05 to 10 mg nitrate + nitrite nitrogen (NO3 + NO2-N)/L; Automated Hydrazine Reduction Method (4500-NO3 H)

• 0.01 to 2.0 mg or 0.05 to 5.0 mg nitrate + nitrite (NO3 + NO2-N)/L; Cadmium Reduction Flow Injection Method (4500-NO3 I)

• 0.05 to 10 mg nitrate + nitrite-nitrogen (NO3 + NO2-N)/L; Enzymatic Reduction Manual Method (4500-NO3 J)

Other ranges may be possible for any of the above referenced methods. Refer to manufacturer’s instructions.

For higher NO3-N concentrations, dilute to the range of the selected method. See Sections 4110 B and 4110 C for the working range of ion chromatography methods.

Filter turbid samples. Test filters for NO3 contamination.

2. Collection and Storage of Samples

Collect samples in polyethylene, fluoropolymer, or glass containers. If possible, start NO3 determinations promptly after sampling. Samples can be stored un-acidified for up to 48 h below 6 ° C without freezing. Acidification converts any nitrite (NO2) to NO3. As a result, NO3 values are the sum of NO3 and NO2 levels. If samples must be stored for more than 48 h, acidify to pH <2 with sulfuric or hydrochloric acid (depending on the method) and store from 0 to 6 °C without freezing (or 2-6 °C for Safe Drinking Water Act [SDWA] compliance samples) for up to 28 d. Chlorinated samples are stable for at least 14 d without acid preservation.

3. Quality Control

Quality control (QC) practices and acceptance criteria are described in Sections 1020 and 4020. The following section applies to all NO3-N methods; however, some methods have additional QC measures. Complete the initial QC tasks, including initial demonstration of capability for each analyst, estimation of the method detection limit (MDL), and determination of the dynamic range, before analyzing any samples and at least annually thereafter. Apply the rest of these measures whenever samples are analyzed. If the acceptance criteria are not satisfied, stop and correct the problem. Regulators may specify different acceptance criteria than those given here.

Calibrate or verify the calibration of each instrument daily. Using a calculator, electronic spreadsheet, or instrument software, calculate the slope, intercept, and correlation coefficient (r) or coefficient of determination (r2) of the calibration curve. The r value must be at least 0.995 (r2 ≥ 0.99). Back-calculate the apparent concentrations of the standards. For standards more than 10 times the MDL, the measured values must be 90% to 110% of the true values. The calibration requirements are different for the method described in 4500-NO3 D; see instructions in 4500-NO3 D.4b or 4500-NO3 D.4c.

Prepare a calibration-verification standard (CVS) from a stock solution separate from that used to prepare the calibration standards. The CVS’s NO3-N concentration should be 30% to 70% of the highest calibration standard; however, some QA/QC programs may require different concentrations. Run the CVS immediately after calibration; the result must be 90% to 110% of the expected value.

Run an initial calibration blank (ICB) immediately after the CVS to check for contamination. The ICB reading must be less than one-half of the minimum reporting level (MRL).

Run a midpoint calibration standard as continuing calibration verification (CCV) and a continuing calibration blank (CCB) after every 10 samples and after the last sample. If the measured NO3-N concentration in the CCV is not 90% to 110% of the expected value, recalibrate and rerun all samples read since the last good CCV reading. The CCB must be less than one-half of the MRL.

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The content presented here represents the most current version of this section, which was printed in the 24th edition of Standard Methods for the Examination of Water and Wastewater.

CITATION

Standard Methods Committee of the American Public Health Association, American Water Works Association, and Water Environment Federation. 4500-no3− nitrogen (nitrate) In: Standard Methods For the Examination of Water and Wastewater. Lipps WC, Baxter TE, Braun-Howland E, editors. Washington DC: APHA Press.

DOI: 10.2105/SMWW.2882.089

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