LimsPulse
Food & proximate analysis

Kjeldahl digestion & distillation unit

Digests a food sample in hot sulphuric acid to turn its organic nitrogen into ammonium, then distils the ammonia and titrates it. The nitrogen found, multiplied by a conversion factor, gives crude protein.

  • Food & beverage
  • Dairy
  • Animal feed
  • Agriculture
  • Environmental
  • Fertiliser
Photo of a Kjeldahl digestion & distillation unit
Photo: Giancarlo Dessì, CC BY-SA 3.0 (cropped), via Wikimedia Commons
Follow your site SOP
This is a typical operating sequence for learning. Your instrument model and approved SOP take precedence.
STAGE 1

Before you start

  1. 1Check the digestion block and distillation unit are within their calibration due date.
  2. 2Check scrubber solution, alkali (typically 32–40% NaOH), boric acid receiver solution and titrant levels.
  3. 3Standardise the titrant (for example 0.1 M HCl) and record the factor.
  4. 4Confirm the fume hood or scrubber is working before heating any acid.
  • Concentrated sulphuric acid and 40% sodium hydroxide cause severe burns. Wear a face shield, acid-resistant gloves and a lab coat.
STAGE 2

Digestion

  1. 1Weigh the sample into a clean, dry tube. Use nitrogen-free paper for sticky samples and include it in the blank.
  2. 2Add catalyst tablets and the volume of sulphuric acid stated in the method (for example 12–15 mL).
  3. 3Include a reagent blank and a nitrogen recovery standard in every batch.
  4. 4Place the rack in the preheated block, fit the exhaust manifold and switch on the scrubber.
  5. 5Digest until the solution is clear and then continue for the extra time set in the method.
  6. 6Lift the rack and let the tubes cool to room temperature with the exhaust still running.
  • Foaming samples (high fat or sugar) can froth over. Start at a lower temperature or add an antifoam.
  • Never add water to a hot digest. Cool first, then dilute slowly.
STAGE 3

Distillation and titration

  1. 1Run a steam-only blank distillation to warm up the unit.
  2. 2Fit the tube, then let the unit add water and excess alkali (the digest should turn dark blue/brown with copper).
  3. 3Distil into boric acid with mixed indicator for the set time or distillate volume.
  4. 4Titrate with standard acid to the end point (green to pink/grey for bromocresol green–methyl red), or let the automatic titrator do it.
  5. 5Record titre, blank and sample mass, and calculate nitrogen and protein.
  • Make sure the tube is seated firmly before alkali is added. Hot alkaline steam escaping from a loose tube is a serious hazard.
STAGE 4

Shutdown

  1. 1Run a water distillation to rinse the steam path.
  2. 2Empty and rinse the tubes once cool; neutralise acid waste as per site procedure.
  3. 3Switch off the block and leave the scrubber running until fumes have cleared.
  4. 4Complete the instrument log.
In LimsPulse · Calibration Management

Keep every Kjeldahl in control: usage logs, calibration and maintenance schedules, and automatic blocking of overdue instruments.

See how