Modern Water: A00243: Nitrate in water by colorimetric assay
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Official Method Name
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Nitrate FS RaPID assay A00243 (SDI 1998) http://www.sdix.com/ |
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Current Revision
| 1998 |
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Media
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WATER |
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Instrumentation
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Immunoassay |
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Method Subcategory
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Inorganic |
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Method Source
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Citation
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Brief Method Summary
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Colorimetric assay based on reduction of nitrate to nitrite by zinc, followed by azo-dye formation. Colorimetric response is directly proportional to the concentration of analyte in the sample or standard. Calibrated for sum of nitrate and nitrite (equivalent response). Reagents are purchased in kits with manufacturers' instructions for sample preparation and analysis. Requirements: a clean water supply to prepare dilutions of water samples; filtration equipment to remove particulates from water samples; standards for calibration from 0.5 to 10 mg/L NO3-N, or an analytical balance to prepare standards; a spectrometer which accepts sample tubes (SDI), for analyses at visible wavelengths (550 nm); equipment to automate sample preparation for large numbers of samples (prefiltration, measurement of 0.5 mL volumes), so that contact times with reagents are controlled as required; computerized data acquisition and processing. Expendable supplies - $3 per water sample. |
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Scope and Application
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Screening for compliance and ambient monitoring. |
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Applicable Concentration Range
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0.5 - 10 mg/L NO3-N (sum of nitrate and nitrite) |
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Interferences
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Sensitivity > nitrate: 0.25 mg/L copper; 0.25 mg/L mercury. Sensitivity = nitrate: nitrite. Sensitivity < nitrate: sulfite, calcium and peroxide. Sensitivity << nitrate: iron, nickel, silicates, sulfate, magnesium, zinc |
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Quality Control Requirements
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Recommended for a kit allowing 50 semiquantitative determinations: a. calibration with 4 standards and 1 blank b. precision: 3 samples analyzed in duplicate (repeats) c. accuracy: 2 matrix samples, spiked with the target analyte at different levels in the range for quantitative analysis. d. validation: analysis of 2 positive and 2 negative samples by an independent method, for confirmation |
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Sample Handling
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Samples are collected in glass tubes with reagents from manufacturer (SDI). Instructions are written for analysis in the field at the time of sampling. An alternate procedure is mentioned for analysis using a bench-top spectrometer. Water samples generally need to be centrifuged or prefiltered to 0.2 mm. Aliquots of samples and reagents are accurately measured (0.5 mL) and mixed for required times, following manufacturer's instructions for use of the materials purchased. |
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Maximum Holding Time
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See EPA method for allowable holding time. |
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Relative Cost
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Less than $50 |
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Sample Preparation Methods
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