Procedure

Apparatus:

Kjeldahl Digestion Assembly, Ammonia Distillation Assembly.

Determination of Available Nitrogen Content in Soil

1. Weigh 50 g of processed soil sample in a 500 mL Kjeldahl flask.
2. Add 1 g CuSO₄, 10 g K₂SO₄, and 30 mL concentrated H₂SO₄.
3. Shake the contents thoroughly and allow to stand for 30 minutes with intermittent shaking until complete solution forms.
4. Digest until a greenish color appears. K₂SO₄ raises the boiling point of H₂SO₄ to prevent loss. CuSO₄•5H₂O catalyzes digestion.
5. Run a blank with reagents only and subtract its value from the sample result.
6. Digest on Kjeldahl rack: low flame (10–30 mins) until frothing stops, then raise heat until sample is charred and acid reaches 1/3 of the flask height. Avoid overheating above the liquid level.
7. Cool and dilute to ~100 mL with distilled water. Swirl for 2 mins, transfer fluid to a 1000 mL distillation flask.
8. Wash residue 4–5 times with 50–60 mL distilled water. Decant into the distillation flask.
9. Add glass beads to prevent bumping.
10. Set up distillation with a dropping funnel (for 40% NaOH), a Kjeldahl trap, and a condenser tube into 50 mL 0.1N HCl with methyl red indicator.
11. Add 125 mL (or 100 mL if bumping occurs) of 40% NaOH until contents are alkaline (approx. 5× H₂SO₄ used). Heat the flask.
12. Allow NH₃ to be absorbed in the standard HCl. Wash the end of the delivery tube. Add 150 mL distilled water to the conical flask. Stop distillation once no NH₃ is detected (red litmus stays red).
13. Titrate excess HCl with 0.1 N NaOH until methyl red changes from pink to yellow.
14. Calculate the amount of nitrogen from titration results.

a) Calculation:

• Blank:
  Volume of HCl = a mL
  Volume of NaOH = b mL
  HCl consumed = z = a – b mL
• Sample:
  Volume of HCl = v mL
  Volume of NaOH = u mL
  HCl consumed = w = v – u mL
  HCl for sample = y = w – z mL

Conversions:
1000 mL 1N HCl = 17 g NH₃ = 14 g N
1 mL 1N HCl = 0.014 g N
1 mL 0.1N HCl = 0.0014 g N
Nitrogen in 5 g sample = q = y × 0.0014 g N

% Nitrogen in Sample:
(q × 100) / 5

Points to Remember While Performing the Experiment:

• Always wear a lab coat and gloves.
• Switch on the exhaust fan and check availability of chemicals.
• Adjust the Bunsen burner flame to a small blue cone.
• Clean apparatus with chromic acid and distilled water. Ensure they are dry.
• Calibrate the electronic balance before use.
• Ensure desiccator contains silica gel.
• Clean the crucible with chromic acid, heat, and cool before placing in the desiccator. Use tongs or tissue only—never hands.
• Set oven temperature to 130°C. Use cotton gloves when handling hot items.
• Ensure Kipp’s apparatus is clean and contains iron sulfide and H₂SO₄.
• Recap reagent bottles after use. Turn off the fan, light, gas, and oven before leaving.
• Discard used gloves in the waste bin.