Kjeldahl Digestion for Total Nitrogen

Reagents

  1. DIGESTION MIXTURE - 3 kg K2S04 and 600 g CuS04.5H20 (or 384 g CuS04 anhydrous) (use CuS04.5H20 if available). (This mixture can be purchased as Kjeltab catalyst tablets.)
  2. CONC H2S04.
  3. NITROGEN STANDARD - Accurately weigh 4.7166 g (NH4)2S04 (dried and stored in the desiccator), dissolve in deionized water, then dilute to 100 mL. This will give you a 1% N solution. To check distillation procedure, pipet 1 mL into digestion tube, dilute with about 80 mL deionized water and follow the distillation procedure. For calculations, 1 mL = 1 g.
  4. Glycine - Weigh 0.1000 mg (stored in desiccator) and digest along with samples. Value should be 18.66 + 0.9% N.

Apparatus

  1. DIGESTION BLOCK
  2. FUME MANIFOLD
  3. 250 mL DIGESTION TUBES

Procedure

  1. Weigh 0.500 g plant tissue or 0.500 g feed sample into a 250 mL digestion tube.
  2. Add 2 Kjeltab catalyst tablets.
  3. Add 12 mL of conc. H2S04.
  4. Place on digestion block with front and back shields.
  5. Heat at 400oC for 45 min. (should be 30 min. after samples have cleared).
  6. Remove from block and allow to cool 15 - 20 min. with shields off.
  7. Add approximately 75 mL dH20.
  8. Sample is ready for distillation.

Quality Control

  1. Ash 0.1 g glycine along with samples on first set. Ensure value of N is 18.66 + 0.9%.
  2. Prior to distilling samples, pipet 1 mL of (NH4)2SO4 solution into a digestion tube for each distillation unit, dilute each with 80 mL deionized water, and distill to ensure each sample is 1.00 + 0.05% N.

References

  1. Brenner, J. M. 1965. Total Nitrogen. In C. A. Black (ed) Methods of Soil Analysis, Part II, SSSA., Madison, Wis. p.1149-1178.
  2. Horwitz, W. (ed). 1970. Official Methods of Analysis of the AOAC, 11th Edition, Washington, D. C. p.16-17.
  3. Tecator Manual. Kjeltec System 2300 Distilling Unit. Tecator, Hoganas, Sweden.