Kjeldahl Analysis Step By Step; Sample Preparation - Selecta DQO 6 Manual De Instrucciones

MANUAL DE INSTRUCCIONES 80158 REV D OCT/2019 (Sujetas a modificaciones sin previo aviso) Pag.: 30
9.

Kjeldahl analysis step by step

9.1 Sample Preparation

• Grind, mix thoroughly and mix the sample.
• Weigh between 1 and 2 grams of sample.
• In samples with very small nitrogen content (sewage, etc) take enough samples
containing at least 5 mg of nitrogen.
9.2 Digestion
• Add between 10 and 15 ml (tube macro) 96-98% H2SO4 and 15-20 g of catalyst
(for the micro tube is maximum 5ml H2SO4).
• Set up a system for the extraction of fumes or scrubber with NaOH.
• Perform digestion in three steps:
1. Depending on the water content of the sample digestion start evaporating
water at 150 for 20 to 60 minutes.
2. Perform a second step at 280°C for 30 minutes to reduce the production of
white fumes.
3. Continue digestion at 400 for 60-90 minutes.
Visual Control: The result is a clear liquid with clear, green or blue coloration
depending on the catalyst used. There should be no black debris attached to
the tube wall.
Note: During digestion foam production in the samples must be controlled. If
this is excessive, should be extended step # 1.
9.3 Dilution
• Remove the tubes shows the block digester and allow to cool to room
temperature (It may be forced immersing the tubes cautiously in a little water).
• Add about 25ml of water to each tube (10 Tube MICRO).
• Add the water slowly and moving the tube if allowed to solidify the sample.
If necessary, gently heat the tube (for example introducing it into the heating
block still hot).
• Allow to cool back to room temperature.
• Avoid loss of nitrogen and violent reactions to insert the distillers still hot tube.
9.4 Distillation
• Check the level of NaOH and Boric Acid in the tank. Check that the metering
pumps are primed and meter the correct volumes.
• Place a 250ml Erlenmeyer flask to the condenser with 50 ml of boric acid and
a few drops of indicator.
• Schedule a dosage of 50-75 ml of NaOH (25ml for MICRO).
• Insert the sample tube into the distiller.
• Distill to collect in 250ml Erlenmeyer (50ml distillate Boric 200ml).
Visual Control: Once the NaOH was added, the sample must have a bluish color, if
not, add more NaOH.
9.5 Tritation and calculation
• Titrate the distillate with HCl or H2SO4 0.1 or 0.25 N to change colour (Endpoint:
pH 4.65).
• Perform the calculation: Nitrogen mg = 14 * Volumen HCl * Normality.
• To move to protein corrected by the appropriate factor depending on the nature of
the sample (6.25 by default).
• Periodically perform a blank test and subtract the result.
J
.P. SELECTA s.a.u. Ctra. NII Km 585.1 Abrera 08630 (Barcelona) España Tel 34 93 770 08 77 Fax 34 93 770 23 62
e-mail: selecta@jpselecta.es - website: http://www.jpselecta.es