THE DETERMINATION OF KJELDAHL NITROGEN (TKN) IN WATER
ASN 3503
Range
Detection limit: 0.5 mg N/l
Sample Preparation
Samples should be collected in Polyethylene or glass bottles. They should be analysed as soon as possible, or else
stored between 2-5°C until analysed. Acidification with Sulphuric Acid to pH <2 may also be used as an aid to
preservation, provided that possible contamination of the acidified sample by adsorption of any atmospheric
ammonia is avoided.
If the approximate Nitrogen concentration of the sample is known, the test portion volume can be selected
according to:
Nitrogen concentration / mg/l Volume used / ml
up to 20 100
20 - 50 50
50 - 100 25
Use 250 ml digestion tube with one boiling rod, except for Kjeltec 1035/1038 where boiling stones >6 mm should
be used.
NOTE! Interference may occur if the Nitrate concentration in the sample is 10 x or more higher than the TKN
level. Use an anion exchange resin (chloride form) to remove the Nitrate prior to the TKN analysis.
Digestion
Add 2 Kjeltabs S/3.5 (or 7 g K2SO4 + 7 mg Se). Add 12 ml H2SO4. Put boiling rod with open end downwards or
boiling stones in place, position the exhaust and turn on the aspirator or scrubber. Digest for 70 minutes in a
preheated block digester at 420°C. Depending on sample content, foaming can occur. If this is the case, the
samples have to be heated slowly.
Using large sample volumes it is advisable to start digestion at ~ 250°C and slowly raise temperature to 420°C.
Remove rack with exhaust and leave to cool for at least 15 minutes.
Distillation
On some systems part or all of this is performed automatically. Dilute cooled digest with 75 ml H2O. Add 25 ml
of receiver solution(1) to the receiver flask. Add 50 ml 40 % NaOH to the tube. Allow reaction to settle (delay).
Distil for the prescribed time (see below) and titrate distillate with standardised titrant (2).
(1) For levels below 10 mg/l the Boric acid should be omitted from the receiver solution.
(2) The normality of the titrant is required to 4 decimal places. Perform a reagent blank before each batch of
samples.
Calculation
(T-B) x 14.007 x N x 1000
mg TKN / l = ---------------------
Volume of sample (ml)
