Exp. 10: TKN Measurement
Theory:
Total kjeldal nitrogen (TKN) is the expression that describes the whole amount of nitrogen in
water sample which exists in two forms:
a) NH3-N : which is the inorganic nitrogen. This type of nitrogen is determined using Nessler
experiment which we used in previous experiment.
b) Organic nitrogen : which must be converted to other forms before we can make the
experiment on it.
In this experiment, we are going to find the total kjeldal nitrogen in the water sample in the
procedure we are going to discuss later.
The preparation of the sample for titration process is consisted of two steps:
a) Digestion: in which we add a complex substance to the sample and heat them on a specific
temperature to convert them to (NH4)SO4.
b) Distillation process: in which we evaporate the sample and then use an acid to condensate it
and then use the solution in the titration process.
Reagents:
1) Digestion reagent:

Dissolve 134 gram K2SO4 and 7.3 g CuSO4 in 800 ml dH2O

Add gently 134 ml conc. H2SO4

Dilute to 1000 ml by (dH2O)
2) Sodium hydroxide- sodium thiosulfate reagent:

Dissolve 500 g NaOH

25g Na2S2O3.5H2O

Dilute to 1000 ml (1L) by dH2O

Dilute the volume up to 1 litter
3) Mixed indicator solution (one month)
1

Dissolve 200mg Methyl Red indicator in 100ml 95% ethyl or (isopropyl)Alcohol,
dissolve 100mg methylene blue in 50ml 95% ethyl or (isopropyl) alcohol, combine
solution
Environmental Measurements Laboratory | Islamic University of Gaza
Exp. 10: TKN Measurement
4) Indicating boric acid solution

20g H3BO3 in Ammonia free distilled, Add 10 ml mixed indicator, dilute to 1L dH2O.
5) Standard sulfuric acid titrant 0.02 N
Apparatus:
1) Distillation apparatus.
2) Hot plate.
3) Other apparatus as (burette, pipette, graduated cylinder,…)
Procedures:
Digestion step:
1) In a kjeldal flask (500 ml), place 50 ml water sample (or 25 ml) of sewage water sample
2) Place glass beads (to prevent our bulbs formation).
3) Add 50 ml digestion solution.
4) Add glass beeds.
5) Heat under hood to remove acid fumes.
6) Heat to continue the digestion until the colored or turbid samples become transparent and
pale green.
Note :
*Acid fumes are evaporated white fumes are observed dark fumes are observed during boiling.
* Sample (if drinking water) will be pale straw to clear color.
Distillation step:
7) Cool, dilute to 300 with dH2O
8) mix well
2
9) Tilt the flask and carefully add :

50 ml NaOH/Na2S2O3 to form alkaline layer at flask bottom
Environmental Measurements Laboratory | Islamic University of Gaza
Exp. 10: TKN Measurement

Connect the flask to steamed out distillation apparatus and shake flask, mix well, a
block precipitate, HgS will form, pH >11.
10) Distill; collect 200 ml distillate below surface of 50 ml absorbent solution (indicating boric
acid solution)

Color will be lavender (violet)

As distillation proceeds, color change to pale green.
Titration step
11) Titrate with 0.02N H2SO4; the end point is obtained when, >> Color green → violet
(lavender)

Note : H2SO4 must standardized by Na2CO3
Calculations:
NH3-N (mg /L) =
(𝐴−𝐵)∗280
𝑚𝑙 𝑠𝑎𝑚𝑝𝑙𝑒
A = volume H2SO4 titrated for sample ml
B = volume H2SO4 titrated for blank ml
mls Sample = 50 or 25
3
Environmental Measurements Laboratory | Islamic University of Gaza