INSTRUCTION MANUAL
CODE 80187
REV D
01/2021
Page: 46
J
.P. SELECTA s.a.u.
Autovía A-2 Km 585.1 Abrera 08630 (Barcelona) España
Tel 34 937 700 877 Fax 34 937 702 362
e-mail: [email protected] - website: http://www.grupo-selecta.com
12 Kjeldahl analysis step by step
12.2 Digestion
• Add between 10-15ml (macrotube) of H
2
SO
4
(for microtube maximum H
2
SO
4
is 5ml).
• Rig up a system for smoke extraction or scrubber with NaOH.
• Make this digestion in 3 steps:
1.
In function of the level of water in the sample. Begin digestion at 150ºC for 20
or 60 minutes.
2.
Make a second step by 280ºC for 30 minutes to reduce white smoke produc
-
tion.
3.
Continue the digestion with 400ºC for 60 - 90 minutes.
Visual checking:
the result is a neat transparent liquid with a clear blue colora-
tion or green or yellow depending on the catalyser used. It must not still black
rests stuck on the tube’s wall.
Note:
During digestion you must control foam production in the samples. If there
is too much foam, length step 1.
12.1 Sample preparation
• Triture, homogenize and mix the sample.
• Weight between 1 and 2 grams of the sample.
• In low nitrogen content sample, (waste water, etc...) take enough samples to
get 5mg nitrogen.
12.3 Dilution
• Take out the sample tube off the digester block and let it cool to ambient tem
-
perature.
• Add about 25ml of water in each tube (10ml for microtube).
• Slow add water, moving the tube to not let solidify the sample. Heat the tube if
necessary.(e.g. introducing the tube into the digester block)
• Let it cool again.
• To avoid nitrogen loss and violent reactions do not introduce the hot tube
inside the distiller.
12.4 Distillation
• Check the tank level of NaOH and Boric acid. Check the dispensing bombs are
primed and give the correct volume.
• Put a 250ml Erlenmeyer in the outlet cooler with 50ml Boric acid and some
indicator drops.
• Program a 50 to 75 ml NaOH dosage (25ml for microtube).
• Insert the tube with the sample in the distiller.
• Distillate till recollecting 250ml in the Erlenmeyer (50ml Boric acid+200ml of
distillate).
Visual checking
: Once the NaOH has been added, the sample must get a blue
coloration, if not add more NaOH.
12.5 Titration and calculations
• Titrate the distilled with HCl or H
2
SO
4
0.1 or 0.25 N till colour changes. (end
point: pH 4.65)
• Calculate: mg of Nitrogen = 14 * Volume
HCl
* Normality
• Calculate protein contents correct with the right factor based on the sample
nature. (6.25 default value)
• Periodically make a blank test and subtract it from the result.
