Labconco Open Combination (Macro) Kjeldahl Systems 2123Series - Manual (Page 23)

Appendix C: Total Kjeldahl Nitrogen Method
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the flask neck on the fume removal system. If necessary
lower the burner heat setting to prevent sample from
foaming up into neck of flask. Once, the sample comes to
a smooth rolling boil return heat setting to maximum.
When digestion sample clears (no black specs with pale
green tint color), continue to boil for an equal length of
time as required to reach the clear point, (total time
approx. 1 hr). If black specs occur in the flask neck, rotate
the flask 180 degrees to allow refluxing acid to rinse the
internal flask surface. Allow digestion mixture to cool and
carefully add 50 ml of distilled water to prevent K
2
S0
4
salt
solids from forming. All salt crystals must be dissolved
before proceeding to the next step. If necessary partially
reheat sample and agitate to dissolve crystals.
6. Distillation
6.1. The distillation burners should be preheated and condenser cooling
water turned on. The receiving flask should be in-place with enough
boric acid H
3
BO
3
plus the mixed indicator solution to submerge the
tip of condenser delivery tube well below the level of the boric acid
receiving solution in the receiving flask. (Erlenmeyer 250 ml flasks
recommended)
6.2. In a fume hood, carefully adding of 100 ml of 45% sodium
hydroxide solution without mixing to make the digested sample
alkaline. Tilt the flask in to the hood while adding the sodium
hydroxide to the sulfuric acid digestion solution. Slowly add the
heavy sodium hydroxide solution down the neck of the flask. The
heavier sodium hydroxide solution will slip under the aqueous
sulfuric acid solution without mixing and with out loss of free-
ammonia. Do not mix until the flask has been connected to the
distillation apparatus.
6.3. Immediately connect the flask to a condenser using the rubber
stopper on the distillation connecting bulb attached to a condenser
column.
6.4. Vigorously swirl the Kjeldahl flask to mix contents thoroughly; heat
until all NH
3
has been distilled (>8-10 minutes). During the ammonia
transfer the receiving solution with change color from purple to
green. Lower receiving flask and let liquid drain from condenser tip.
Turn off distillation burners.
6.5. For nitrogen concentrations above 1 mg/1, the ammonia can be
determined titrimetrically. For concentrations below this level,
colorimetric or potentiometric determination methods are
recommended.