Determination of Nitrite, Nitrate and Phosphate


Summary of Method. Water samples are filtered and analyzed by ion chromatography (IC). Each analytical batch consists of a blank, 5 external calibration standards and a calibration check for every 8 samples. The method detection limits (MDL) are as follows: 0.1 mg/L for phosphate, and 0.05 mg/L for nitrite and nitrate.

Preparation of IC samples. Water samples are pre-filtered before analysis in the Dionex ICS-90 ion chromatography system. Fresh water samples (5 mL) are filtered through 0.20 mm IC Millex syringe filters (for freshwater samples) while saltwater samples are filtered through the Dionex Ag/H cartridges and diluted 50:50 using deionized distilled water.


Preparation of IC calibration standards. The following calibration standards are prepared.
(1) 0.1 mg/L of phosphate, 0.05 mg/L of nitrite and 0.05 mg/L nitrate
(2) 0.2 mg/L phosphate and 0.1 mg/L nitrite and 0.1 mg/L nitrate
(3) 1.0 mg/L phosphate and 0.5 mg/L nitrite and 0.5 mg/L nitrate
(4) 2.0 mg/L phosphate and 1.0 mg/L nitrite and 1.0 mg/L nitrate
(5) 10.0 mg/L phosphate and 5.0 mg/L nitrite and 5.0 mg/L nitrate


Calibration. The calibration curve must consist of at least three data points. The response of each calibrator must be within 10%, otherwise, it does not meet method criteria and must be removed from the working calibration curve.


Final data are summarized in Excel worksheets. The nitrite as N (mg/L) is calculated by multiplying the nitrite (mg/L) with 0.304 (factor). Likewise, the nitrate as N (mg/L) content is calculated by multiplying the nitrate (mg/L) content with 0.226 (factor). The phosphate (mg/L) content is converted to phosphorus (mg/L) by using the factor of 0.326.

Determination of Total Nitrogen and Total Phosphorus

Summary of Method. To analyze the total N and total P content, all phosphorus-and nitrogen-containing substances in the water sample are first converted to inorganic nitrates and phosphates by photo-oxidation. After photo-oxidation, the samples are analyzed as described above.

Photo-oxidation. Water samples (100 mL) added with 2 drops of 30% H2O2, are photo-oxidized using Ace-Hanovia photooxidation apparatus (Department of Health State Laboratory) for four (4) hours to hydrolyze all phosphorus-containing substances into phosphates, and all nitrogen-containing substances into nitrates.

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Last Update: January 18, 2006