Velp Scientifica
– UDK139 Operating Manual
10004762/B5
97
27 - Kjeldahl method to determine total nitrogen on soil
A soil of good fertility contains 2-3% of organic matter mainly humus which is
composed by 4 - 6% of nitrogen.
There is also some ammonia nitrogen (some ppm) which is to be determined
separately and substracted from the
value of Kjeldahl’s nitrogen obtained,if organic
nitrogen is needed.
Procedure
1) Sample:
1 g about of air dried soil, sieved to 2 mm (3÷12 mg of organic nitrogen),
dried at 105 °C up to constant weight, accurately weighed and quantitatively
transferred in a test tube.
2) Reagents for digestion:
2 catalyst tablets ST (code CT0006609)
12 ml concentrated sulphuric acid (96-98%).
3) Digestion:
heat for 60 min at 420 °C.
4) Cooling:
let cool the test tubes to 50-60°C.
5) Distillation-titration:
place in position the test 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 0.2 N
Distillation time= 4 min
Steam power= 100%
Press the button starting distillation cycle.
To avoid damage to the distillator it’s strongly recommended to NOT use the
“removal of distillation residues” function.
References:
“Methods of soil analysis” part 2 – Chemical and microbiological
properties, 2
a
ed.
284