DISCRETE ANALYZER CHEMISTRY Nitrate/Nitrite (Cadmium Coil) Method
Method Description Before conducting analyses using this method, confirm that the field preservation technique does not require matrix matching. To do so, combine 5 mL of the field-preserved sample with an equal volume of working buffer ( SEALAnalyticalAQ NOx Cad-Buffer ), and repeat this procedure using the calibration standard. The pH of the two resulting samples should be within 0.25 pH units of each other and should fall within the range of pH 8.2 to 8.6. The sample is first mixed with pH buffer ( SEALAnalyticalAQ NOx Cad-Buffer ) and then transferred to a copperized cadmium coil, where nitrate is chemically reduced to nitrite. The resulting solution is subsequently mixed with color reagent ( SEALAnalyticalAQ NOx Cad-Color ). Both original nitrite and chemically reduced nitrite react with sulfanilamide, in dilute phosphoric acid, to form a diazonium compound. The reaction between the diazonium ion and NEDD results in the formation of an AZO bond (-C=N-) between the benzene ring and the naphthyl group, which is a reddish-purple AZO dye. The absorbance of this dye is measured photometrically at 520 nm. For nitrite-only measurements, omit the use of the cadmium coil to prevent the reduction of nitrate to nitrite. Note: Ensure that the cadmium column or coil used for this method is functioning correctly. It is recommended to perform a reduction efficiency check following any maintenance of the coil or column to verify its optimal performance prior to conducting nitrate/nitrite analysis.
Reduction of Nitrate to Nitrite (via cadmium coil): NO 3 - +2e - +2H + NO 2 - +H 2 O (Nitrate is reduced to nitrite in the presence of cadmium and acid.) Reaction of Nitrite with Sulfanilamide: NO 2 - +C 6 H 4 NH 2 SO 3 H Diazonium compound
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