Kjeldahl method

The Kjeldahl method is a method for determining the amount of total nitrogen in a compound or mixture. That is, in the presence of a catalyst, digest the sample with concentrated sulfuric acid to convert organic nitrogen into inorganic ammonium salt, and then convert the ammonium salt into ammonia under alkaline conditions, distill out with water vapor and absorb for excess acid, and then Titration with standard alkali can calculate the amount of nitrogen in the sample. Because the nitrogen content of protein is relatively constant, the protein content can be calculated from its nitrogen content, so this method is a classic protein quantification method.

The Kjeldahl method is based on the test of natural nitrogen-containing organic compounds and concentrated sulfuric acid, which will be oxidized into carbon dioxide and water,

Then alkalization distillation frees ammonia, absorbs it with boric acid and then titrates with sulfuric acid or hydrochloric acid standard solution, and multiplies the conversion factor by the acid consumption, which is the protein content.

1. The amine radicals in organic matter are nitrified to form (NH4) 2SO4 under the action of strong heat, CuSO4 and concentrated H2SO4

The reaction formula is: 2NH2 + H2S04 + 2H = (NH4) 2S04 (with CuSO4 as catalyst)

2. In the Kjeldahl nitrogen generator, it reacts with alkali and releases NH3 by distillation, collected in H3BO3 solution

The reaction formula is: (NH4) 2SO4 + 2NaOH = 2NH3 + 2H2O + Na2SO4

2NH3 + 4H3BO3 = (NH4) 2B4O7 + 5H2O

3. Titrate with a known solution of H2SO4 (or HCI) standard solution, calculate the nitrogen content according to the amount of HCI consumption, and then multiply by the corresponding conversion factor to obtain the protein content

The reaction formula is: (NH4) 2B4O7 + H2SO4 + 5H2O = (NH4) 2SO4 + 4H3BO3

(NH4) 2B4O7 + 2HCl + 5H2O = 2NH4Cl + 4H3BO3