The Kjeldahl method involves digesting a sample with sulfuric acid to convert organic nitrogen into ammonium sulfate. The ammonium sulfate is then distilled to produce ammonia gas, which is trapped in an acid solution. The amount of ammonia trapped is determined by back-titration with a base. This allows the nitrogen content of the original sample to be calculated. The method involves digestion, distillation, and titration steps and is a standard way to determine protein content in materials like food, fertilizer, and waste.

1. BY: Anant Kushwaha
Archita Aryal
Alija Khanal

2. KJELDAHL METHOD OF NITROGEN
ESTIMATION
Introduction:
 Nitrogen is one of the five major elements found in various organic compounds such as
protein.
 This fact was recognized by Danish chemist , Johan kjeldahl (1883)
 And use it as a method of determining the amount of nitrogen in mixture of substances
containing ammonia salts, nitrate or organic nitrogen compound .
 The central idea used in procedure is the oxidation of the organic compound using strong
sulfuric acid .
 As the organic material is oxidized the carbon contains in it converts into carbon dioxide and
hydrogen into water.
 The nitrogen from amine groups is converted into ammonium ion , which dissolves in the
oxidizing solution and later converted into ammonia gas .

3.  This method of nitrogen analysis is the worldwide standard
for calculating the protein content in a wide variety of
materials ranging from human and animal food , fertilizer
,waste water and fossil fuel.
 Generally , this method is discussed under three sub
heading briefly . they are :
a) Digestion of sample
b) Distillation
c) Titration

5. a) Digestion of sample:
 This step is most time consuming .
 The purpose of this step is to break down bonds that hold
polypeptides together and convert them to simpler
chemical such as water, carbon dioxide and ammonia.
 This reaction can be speeded up by catalyst and neutral
substance, such as potassium sulfate(K2SO4) , this raise the
boiling point of the digesting acid and thus temperature of
the reaction .
 Catalyst generally uses are selenium(Se) , copper(Cu) ,
mercury(Hg).

6.  METHOD TO PERFORM DIGESTION :
 Weight accurately about 1 g sample containing nitrogen and
place it into kjeldahl flask , along with 12-15 ml of
concentrated sulfuric acid (H2SO4) .
• And add 7 g potassium sulfate and a catalyst usually
copper

7.  Now , put the flask over burner and heat at the temperature about
370oc to 400oc .
 Continue heating until fume can be seen or sample is completely
dissolved .
 It takes about 60-90 min and then after allow flask to cool and add
250ml water carefully.
Sample containing nitrogen +H2SO4 catalyst (NH4)2SO4.

8. a) Distillastion :
 The purpose of this step is to separate ammonia from
digestion mixture.
 Methodology to perform distillation:
 Raise the PH of mixture by using sodium hydroxide(45%
NaOH solution) .This effects in changing ammonium(NH4
+)
ions to ammonia gas .
 Thus formed ammonia gas is trapped/absorbed by special
trapping /absorbing solution such as 15 ml Hcl in 70 ml of
water .
 And trapping solution is used in excess amount so to
completely dissolve the complete amount of nitrogen
containing ammonia gas .
 And this excess solution of trapping solution is determined
by titration in next step.

9. • Remove the trapping flask and rinse with water so as to make sure that all
the ammonia has been dissolved .
(NH4)2SO4 + NaOH 2NH3 +Na2SO4+2H2O

10. a) Titration:
 Actually this step help to calculate the amount of nitrogen
in the sample.
 Here dissolved ammonia in trapping solution of HCL
neutralizes it, which is determined by back titration using
standard known solution of base usually NAOH .
 In this way the amount of ammonia distilled off from the
digested solution is calculated and hence the amount of
nitrogen in the sample.
 Methodology of titration:
 Add an indicator to the acid trapping ammonia the
indicator should turn a strong color ,indicates that
significant amount of trapping acid is still present
 Put a standard solution of NaOH into a buret and slowly
add small amounts of NaOH solution to the acid solution .
 The point at which the indicator turns its color into orange
is endpoint indicating the reaction is completed .

11. •Record the volume and perform a calculation to find out the amount of ammonia ,
hence nitrogen in the sample
NH3 + HCl NH4Cl + HCl
excess

12.  Calculation:
 One mole of ammonia coming from the digestion mixture will neutralize exactly one mole of the acid in the
trapping flask.
 Therefore , the first step is to find out the no. of moles of ammonia that have been produced and trapped .
Moles of acid =molarity of acid × vol.
used in flask
mole of base= molarity of base ×vol.
added from buret
 Substract the moles of base from moles of acid to get moles of ammonia coming from sample .
 The number of moles of ammonia is the same as the moles of nitrogen.
 To calculate the no. of grams of nitrogen in the sample ,multiply the moles of nitrogen by the atomic mass of
nitrogen.
gms nitrogen= moles of nitrogen × atomic mass
gms nitrogen = moles of nitrogen × 14.0067.