some easy steps to be better understand the kjeldahl method. Hope this ppt will help students and interested person to learn about it.

1. PARTHAPRATIMKANDAR&SWAGATACHAKRABORTY

2. HISTORY OF KJELDAHL METHOD
IN THE YEAR OF 1883, MARCH 7TH JOHAN GUSTAV
CHRISTOFFER THORSAGER KJELDAHL , A DANISH CHEMIST
DEVELOPED THIS METHOD.
OBJECTIVE
“ QUALITATIVE DETERMINATION OF ORGANIC AS WELL
AS INORGANIC NITROGEN (N2) IN THE SUBSTANCES .”
PG 2

3. PROCEDURE
PG 3

4. DIAGRAM OF KJELDAHL METHOD PG 4

5. THE PROCEDURE INVOLVES THREE MAJOR STEPS
 DIGESTION: For decomposition of nitrogen sample utilizing a concentrated acid
solution(H2SO4) a catalyst(CuSO4) and K2SO4 for boiling point increasing agent. As
a result Ammonium Sulphate produce.
Nitrogenous Compound + H2SO4 +CuSO4 + K2SO4 =
(NH4)2SO4 + H2O + CO2 + Other by-products
DISTILLATION: Adding excess base to the acid digestion mixture to convert NH4+
to NH3, followed by boiling and condensation of the NH3 gas in a receiving solution.
(NH4)2SO4 + 2NaOH = 2NH3 ↑ + Na2SO4 + 2H2O
TITRATION: To quantify the amount of ammonia in the receiving solution.
pg 5

6. TITRATION
 Reaction with Boric Acid (H3BO3): [Direct titration]
If Boric Acid is receiving solution then the Ammonia produce in
distillation is react with H3BO3 and produce AMMONIUM BORATE
(NH4+H2BO3-), it can directly titrated against standard acid by using
METHYL RED as an indicator.
NH3 + H3BO3 = NH4+H2BO3-
ADVANTAGES:
1) Prevent loss of ammonia by volatilization,
2) No need to back titration, too weak acid to interfere further titration. Pg 6

7. TITRATION
 Reaction with Hydrochloric Acid (HCl): [Back titration]
The ammonia is captured by a, carefully measured excess of a
standardized acid solution in the receiving flask. The excess of acid
in the receiving solution keeps the pH low, and the indicator does not
change color.
NH3 + HCl (in excess) = NH4+Cl- + HCl (left back)
The excess acid solution is exactly neutralized by a carefully
measured standardized alkaline base solution such as sodium hydroxide. A
color change is produced at the end point of the titration.
HCl + NaOH = NaCl + H2O
V ml of X (N) NaOH = V ml of X (N) HCl = V ml of X (N) NH3
pg 7

8. AdvantagesANDDisadvantages
ADVANTAGES
• It is a standard method of determining nitrogen content on protein in food sample.
• It can measured the total nitrogen in soil sample.
DISADVANTAGES
• It does not give a measure of the true protein, since all nitrogen in food is not in the form of protein.
• Different proteins need different correction factors because they have different amino acid sequence .
• The use of concentrated sulphuric acid at high temperatures make some kind of hazards, as does the use of
some of the possible catalyst.
• Kjeldahl method is not applicable to compounds containing nitrogen in nitro and azo
groups and nitrogen present in rings (e.g. pyridine , quinoline, isoquinoline) as nitrogen of these compounds
does not convert to ammonium sulfate under the conditions of this method.
• This is a time consuming method.