Get Kjeldahl Method

The Kjeldahl method permits the available nitrogen to be precisely determined in the plant and in the soil. The method of determination involves three successive phases which are, Digestion of the organic material to convert nitrogen into HNO3. Distillation of the released Ammonia into an absorbing surface or medium.

The Kjeldahl method or Kjeldahl digestion (Danish pronunciation: [ k el t l]) in analytical chemistry is a method for the quantitative determination of nitrogen contained in organic substances plus the nitrogen contained in the inorganic compounds ammonia and ammonium (NH3/NH4+).

The Kjeldahl method was developed by a brewer called Johann Kjeldahl in 1883. The protocol is built on the principle that strong acid helps in the digestion of food so that it releases nitrogen which can be determined by a suitable titration technique.

Kjeldahl. The method consists essentially of transforming all nitrogen in a weighed sample into ammonium sulfate by digestion with sulfuric acid, alkalizing the solution, and determining the resulting ammonia by distilling it into a measured volume of standard acid, the excess of which is determined by titration.

In the classical Kjeldahl method, the proteins are 'digested' in sulfuric acid with a catalyst (selenium, mercury, or copper salts). An acid deposit of ammonium sulfate is formed, which is then dissolved in water. ... The excess acid is back-titrated with standard NaOH to determine the amount of ammonia and hence nitrogen.

The Kjeldahl digestion method has some disadvantages. This method only measures nitrogen bound to organic components (proteins, amino acids, nucleic acids) and ammonium in the sample. Other nitrogen forms, such as nitrate and nitrite, cannot be measured through this procedure.

Step One: Digestion of the Sample This is the most time-consuming step in the analysis. 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, of course, ammonia.

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.