Velp Scientifica
– UDK139 Operating Manual
10004762/B5
96
26 - Kjeldahl method to determine total nitrogen on water
The value obtained by this method corresponds to the sum of organic nitrogen
(proteins, nucleic acids, urea, synthetic organic chemicals) and ammonia nitrogen.
The measurement of this parameter is required by Italian laws 319/76 and 650/79 for
the discharge of wastewaters in lakes, direct or indirect, included in a band of 10 km
from the coast line (o a n nitric = 10 mg N/l maximum).
Kjeldahl’s method doesn’t measure, or measures partly, some nitrogen compounds:
azides, azocompounds, hydrazones, hydrazine and hydroxylamine, nitrite and
nitrate.
Procedure
1) Sample:
50-200 ml of water (0,1 - 0,5 mg of nitrogen in most cases) in a standard
test tube.
With DKL12 it is possible to use the Jumbo test tube 400 ml (code A00000185) with
the dedicated sample rack (code A00000181).
2) Reagents for digestion:
for each sample add in the test tube:
2 catalyst tablets CM (code CT0006650)
10 ml concentrated sulphuric acid (96-98%)
3) Digestion:
mix well the test tube content.
Heat for 30 min at 150°C, 30 min. at 250 °C and then for 1 hour at 420 °C.
The initial heating is used to evaporate water.
4) Cooling:
let cool the tubes to 50-60 °C.
5) Distillation-titration:
place in position the tube with digested sample in the unit
UDK 139.
factor: 0.00
Addition of reagents:
dilution water = 50 ml
H
3
BO
3
= 30 ml
NaOH = 50 ml
Titrant: HCl 0.01N
Distillation time= 4 min
Steam power= 100%
Press the button starting distillation cycle.
6) References:
Environmental Protection Agency, EPA PAI-DK01, AOAC 973.48,
ISO 5663-1984, ISO11732
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