Kjeldahl Method for Determining Nitrogen

Electrothermal MQ Series Macro-Kjeldahl Extraction Heaters

Kjeldahl Method for Determining Nitrogen

Digestion    Titration

The Kjeldahl method was developed over 100 years ago for determining the nitrogen contents in organic and inorganic substances. Although the technique and apparatus have been modified over the years, the basic principles introduced by Johan Kjeldahl still endure today.

Kjeldahl nitrogen determinations are performed on a variety of substances such as meat, feed, grain, waste water, soil, and many other samples. Various scientific associations approve and have refined the Kjeldahl method, including the AOAC International (formerly the Association of Official Analytical Chemists), Association of American Cereal Chemists, American Oil Chemists Society, Environmental Protection Agency, International Standards Organization, and United States Department of Agriculture.

The Kjeldahl method may be broken down into three main steps: digestion, distillation, and titration.


Digestion

Digestion is accomplished by boiling a homogeneous sample in concentrated sulfuric acid. The end result is an ammonium sulfate solution. The general equation for the digestion of an organic sample is shown below:

Organic N H2SO4
(NH4)SO4 H2O CO4 other sample matrix byproducts

Distillation: Excess base is added to the digestion product to convert NH4 to NH3 as indicated in the following equation. The NH3 is recovered by distilling the reaction product.

ammonium
sulfate
heat
ammonia
gas
(NH4)2SO4 2NaOH
2NH3 Na2SO4 2H2O


Titration

Titration quantifies the amount of ammonia in the receiving solution. The amount of nitrogen in a sample can be calculated from the quantified amount of ammonia ion in the receiving solution.

There are two types of titration—back titration and direct titration. Both methods indicate the ammonia present in the distillate with a color change.

In back titration (commonly used in macro Kjeldahl), 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 until the solution is "back titrated" with base.

ammonia standard
sulfuric acid
acid
excess
ammonium
sulfate
sulfuric
acid
2NH3 2H2SO4
(NH4)2SO4 H2SO4
(no color change)

ammonia
sulfate
measured
excess
acid
measured
sodium
hydroxide
ammonium
sulfate
(NH4)2SO4 H2SO4 2NaOH
Na2SO4 (NH4)2SO4 2H2O
(color change occurs)

In direct titration, if boric acid is used as the receiving solution instead of a standardized mineral acid, the chemical reaction is:

ammonia
gas
boric
acid
ammonium-
borate complex
excess
boric acid
NH3 H3BO3
NH4 H2BO-3 H3BO3
(color change occurs)

The boric acid captures the ammonia gas, forming an ammonium-borate complex. As the ammonia collects, the color of the receiving solutions changes.

ammonium-
borate
complex
sulfuric
acid
ammonium
sulfate
boric
acid
2NH4 H2BO-3 H2SO4 (NH4)2SO4 2H3BO3
(color change occurs in reverse)

The boric acid method has the advantages that only one standard solution is necessary for the determination and that the solution has a long shelf life.


Kjeldahl System for Protein Analysis

Distilling Apparatus for Kjeldahl Determinations