Nitrate-Nitrogen ISE

Nitrate-Nitrogen (NO3-N) is determined routinely on saturated extracts and mineral soils when requested.

Nitrate Extracting Solution

Dissolve 173.2 g AL2(SO4)3.18H20, 12.8 g H3BO3, and 0.7222 g KNO3 in 8 liter of deionized water. Dilute to 5 L and check pH. Adjust to pH 3.0 with NaOH.  Dilute final volume to 10 liter.

Boric Acid Preservation Solution

Dissolve 6.2 g boric acid in 100 mL hot distilled water.
Ammonium Sulfate Ionic Strength Adjustor (ISA)-  Prepare a 2 M ammonium sulfate solution by placing 26.42 g reagent-grade (NH4)2SO4 in a 100 mL volumetric flask. Dissolve and dilute to the mark with distilled water.

Standard Stock Solution (1000 mg/l NO3-N) -

  • Dissolve 0.7218 g KNO3 (dry) in deionized water in a 100 mL volumetric flask. Add 1 mL preservation solution then dilute to 100 mL.
  • Silver sulfate - Reagent grade Ag2SO4.
  • Outer filling solution - Dilute 2 mL ISA to 100 mL with distilled water
  • Inner filling solution - Orion cat. 900002

Procedure (Soils):

  1. Measure 20 g soil (16 mL scoop) into a paper cup.
  2. Add 40 mL NO3 extracting solution, mix thoroughly, and allow to equilibrate for 1 (one) hour.
  3. Filter through # 4 filter paper and collect filtrate in 50 mL beakers.
  4. Working Standards (2, 10, 100 mg/l NO3-N)
    1. To prepare 2, 10, and 100 ppm working standards from commercial 1000 ppm NO3-N stock solution, pipette 0.2, 1, and 10 mL respectively of the 1000 ppm stock into 100 mL volumetric flasks. Bring to volume with NO3 extracting solution.
      (Prepare fresh standards every two weeks.)
  5. Determine ppm NO3-N by specific ion electrode.
  6. Instrument calibration and operation for soils:
    1. Remove electrodes from storage solution and rinse thoroughly.
    2. Place electrodes in 2 ppm standard. Press calibration then enter 3-yes for number of standards. Wait for beep then enter 4.0-yes.
    3. Rinse electrodes then place in 10 ppm standard. Wait for beep then enter 20.0-yes.
    4. Rinse electrodes then place in 100 ppm standard. Wait for beep then enter 200-yes. Slope should read 54-60.
    5. To read samples press measure. Red check sample should read 15-1 Brown check sample should read 7-9.
  7. Record as ppm NO3-N (no decimals) on lab result sheets for special analyses.
    Recheck standards periodically and recalibrate as necessary.
  8. After analyses are completed, rinse electrodes, return to storage solution, turn off stirrer, and return instrument to Standby.

Procedure (Saturated Extract):

  1. Working Standards
    1. To prepare 2, 10, and 100 ppm working standards, add 0.2, 1, and 10 mL respectively standard stock solution (1000 mg/l NO3-N) to 100 mL volumetric flasks and bring to volume with distilled water.
    2. Prepare fresh standards every two weeks.
  2. Add 1 mL preservation solution to each sample and standards.
  3. Add 2 mL ISA to the standards and the samples.
  4. Check levels of inner and outer filling solutions in reference electrode (see reference electrode manual for specifications).
  5. Instrument calibration for SE: (see NO3-N instrument calibration and operation for soils - page 7 except enter exact value for standards, i.e.., 2, 10 and 100).
  6. Rinse electrodes and place into sample. Record reading directly as mg/l NO3-N (no decimal) on Report on Special Soils or Artificial Mixes form.
  7. Recheck standards frequently.
  8. When finished, place meter in Standby mode and submerge electrodes in Blank.