Example of Kjeldahl Method for Nitrogen Determination

1. Nitrogen Determination by
Kjeldahl Method
Marian Danielle Fidelis S. Borongan
BS PHARM III
PHAR 32 TTh 3:00-4:30

2. ¤ Nitrogen Determination in Beef Extract
The Kjeldahl Apparatus A
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3. ¤ Nitrogen Determination in Beef Extract STEP 1: DIGESTION
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1 g organic sample in
Kjeldahl digestion
flask
10 g powdered
K2SO4
500 mg powdered
Cu2SO4
20 mL H2SO4
Heat the mixture
below boiling point
until frothing has
ceased
Increase heat until
acid boils for 30
minutes
Heat this way!
Clear green or
colorless solution
150 mL H2O,
cool
Raise the boiling point of
the digesting acid and the
temperature of the
reaction
Used as a catalyst
to hasten digestion
process
Oxidize organic matter
to liberate nitrogen as
ammonium sulfate
Organic N + H2SO4  CO2 + H2O + (NH4)2SO4

4. ¤ Nitrogen Determination in Beef Extract STEP 2: DISTILLATION
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100 mL NaOH Granulated zinc
Raise the pH of the mixture
to change the ammonium
ions to ammonia
Connect the Kjeldahl flask
to a Kjeldahl connecting
bulb (trap), which is
connected to a condenser.
The delivery tube from the
condenser is dipped
beneath the 50 mL H2SO4
contained in a conical flask.
Mix the contents in
the flask by gentle
rotation and distill
about two-thirds of
the contents of the
flask.
Let the NaOH solution
flow into the inner side of
the flask to form a layer
under the acid solution
Assemble this way!
Reddish pink or
bluish green Liberation of NH3
by the sample
(NH4)2SO4 + 2NaOH  2NH3 + Na2SO4 + 2H2O
3NH3 + H2SO4  (NH4)2SO4
Direct titration

5. ¤ Nitrogen Determination in Beef Extract STEP 3: TITRATION
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Methyl red-methylene blue
indicator
Clear green or
colorless solution
0.5 N Sodium Hydroxide
Endpoint:
faint pink color
2NaOH + H2SO4  Na2SO4 + 2H2O
Ammonia from the digestion flask was distilled
and was collected in the conical flask with
sulfuric acid.
It was then titrated with 0.5N sodium hydroxide ,
which neutralized the excess sulfuric acid in the
solution which did not react with ammonia.
The number of moles equal to the nitrogen
content is the difference between the number of
moles of acid in the trap and the number of moles
of the acid neutralized by the standard solution.

6. ¤ Nitrogen Determination in Beef Extract
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Step 1: Digestion of Sample
The purpose of this step is to break down the bonds
that hold the polypeptides together, and convert them to
simpler chemicals such as water, carbon dioxide, and
ammonia.
Step 2: Distillation
The purpose of this next step is to separate the
ammonia (the nitrogen) from the digestion mixture.
Step 3: Titration
To determine the total amount of nitrogen in the
sample by using a standard solution.

7. ¤ Nitrogen Determination in Beef Extract
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Sample Problem:
A 0.200g of a urea (FW = 60) sample is analyzed by the Kjeldahl method. The
ammonia collected in a 50 mL of 0.05 M H2SO4 . The excess acid required 3.4 mL
of 0.05 M NaOH. Find the percentage of the compound in the sample.
Solution:
2NH3 + H2SO4  (NH4)2SO4
mmol H2SO4 reacted = mmol H2SO4 taken ˗mmol H2SO4 back-titrated
mmol H2SO4 back-titrated = ½ mmol NaOH
½ mmol NH3 = mmol H2SO4 reacted

8. ¤ Nitrogen Determination in Beef Extract
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Sample Problem:
A 0.200g of a urea (FW= 60) sample is analyzed by the Kjeldahl method. The
ammonia collected in a 50 mL of 0.05 M H2SO4 . The excess acid required 3.4 mL
of 0.05 M NaOH. Find the percentage of the compound in the sample.
Solution:
½ mmol NH3 = (0.05 M × 50mL) ˗ ½(0.05 M 3.4mL)
mmol NH3 = 2[(0.05 M × 50mL) ˗ ½(0.05 M 3.4mL)]
mmol urea = ½ mmol NH3
mmol urea = (0.05 M × 50mL) ˗ ½(0.05 M 3.4mL) = 2.415 mmol

9. ¤ Nitrogen Determination in Beef Extract
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Sample Problem:
A 0.200g of a urea (FW= 60) sample is analyzed by the Kjeldahl method. The
ammonia collected in a 50 mL of 0.05 M H2SO4 . The excess acid required 3.4 mL
of 0.05 M NaOH. Find the percentage of the compound in the sample.

10. ¤ Nitrogen Determination in Beef Extract
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Metho
d
Type Main
titrant
Amo
unt
MW No. or
rep
ions/
electro
ns
mEw Back
titrant
Amo
unt
Alkalim
etry
Resid
ual
Sulfuric
acid
50
mL
98.0
8
2 0.049
04
Sodium
hydroxi
ide
qs