Kjeldahl Nitrogen Analysis

This method was developed by Johan Kjeldahl in 1883 for determining nitrogen in both organic and inorganic compounds. Ammonia and ammonium are determined for their nitrogen value. This is an invaluable tool for analyzing proteins, mostly in food.

A sample is heated with sulfuric acid for oxidation with the help of a catalyst such as selenium or copper. Potassium or sodium sulfates accelerate the reaction.  Further steps include distillation and condensation. Boric acid standard solutions react with the condensate, allowing for calculations. Sodium hydroxide then frees up the ammonia, which can be determined by titration with hydrochloric or sulfuric acids.

Later in the method, a weak boric acid solution is added in excess to trap the ammonia, with both direct and back titrations involved. One useful indicator for this is bromcresol green (or bromocresol green) – methyl red. It can be added separately, or conveniently is already provided in commercially available boric acid solutions.

The Kjeldahl method is quite a long procedure, and most testing labs today employ automated equipment. 

Exaxol offers a boric acid solution with an indicator for your convenience (catalog # B04261 and B0426), for both 2% and 4% boric acid. Also, Exaxol offers sodium hydroxide suitable for Kjeldahl analysis in different percentage strengths. Other reagents include Sodium Hydroxide 40% – Sodium Thiosulfate 2.5%  (such as S-65941), Digestion Reagent for Organic Nitrogen APHA (D-1801) and Standardized Hydrochloric and Sulfuric Acids solutions, all prepared and tested in Exaxol’s ISO 17025 accredited laboratory.