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Determination of Nitrate Nitrogen (N03-N) by Specific Ion Electrode

Reagents

  1. EXTRACTION SOLUTION - In a 4 L beaker, add 173.2 g Al2(S04)3, 12.8 g H 3B0 3, and 0.7218 g KNO3 and 25.2 g sulfamicacid (NH2S03H) to 3 L deionized water. Stir to dissolve. Pour solution in carboy and dilute to 10 L with deionized water. Adjust to pH 3.0 adding at least 15 mL conc. NaOH (may need more).
  2. N03-N STANDARDS - To make 1000 ppm N stock solution, dissolve 0.7218 g KNO3 (dry and stored in desiccator) in deionized water in a 100 mL volumetric flask. Add 1 mL preservation solution then dilute to 100 mL. (Make up blank, 10 ppm, and 100 ppm N03-N standards using stock solution and diluting with extracting solution.)
  3. OUTER FILLING SOLUTION - 0.04 M ammonium sulfate - Add 0.52 g (NH4)2S04 to 100 mL vol. flask. Dilute with H20.
  4. INNER FILLING SOLUTION - Orion cat. 900002
  5. PRESERVATION SOLUTION - Dissolve 6.2 g Boric Acid in 100 mL hot deionized water. Allow to cool.

Apparatus

  1. ORION ION ANALYZER
  2. NITRATE ION ELECTRODE (ORION 93-07)
  3. REFERENCE ELECTRODE (ORION 90-02)
  4. MAGNETIC STIRRER

Procedure

  1. Weigh 2 g plant or feed sample into 100 mL beaker. (When testing cotton petioles, weigh 0.25 g sample, follow steps 2-11, then multiply final answer by 8).
  2. Add 50 mL extraction solution.
  3. Let stand for 30 minutes.
  4. Check levels of inner and outer filling solutions in reference electrode (see reference electrode manual for specifications).
  5. To set up instrument, press 2nd function - 2 to select appropriate electrode.
  6. Press speed-0 to calibrate N03-N.
  7. Follow prompts - select 2 standards - Do blank correction. (Rinse electrodes and blot dry before switching solutions.)
  8. Set 10 ppm standard to read 25.
  9. Set 100 ppm standard to read 250.
  10. Measure N03-N in sample and multiply reading by 10. Record as ppm with no decimal.
  11. When finished place meter in standby mode by pressing speed-8. Submerge electrodes in proper solutions.

References

  1. Baker, A. S., and R. Smith. 1969. Extracting Solution for Potentiometric Determination of Nitrate in Plant Tissue. J. Agr. Food Chem. 17:1284-1287.
  2. Barker, A. V. 1974. Nitrate Determination in Soil, Water, and Plants. Massachusetts Agricultural Experiment Station Bulletin 611, p.35.
  3. Cantliffe, D. J., G. E. MacDonald, and N. H. Peck, 1970. The Potentiometric Determinations of Nitrate and Chloride in Plant Tissue. New York's Food and Life Sciences Bulletin 3, p.7.