The Kjeldahl Method for Protein

A titrimetric method that has been used for many years for the determination of nitrogen or protein in a sample is the Kjeldahl method. Examples of samples include grain, protein supplements for animal feed, fertilizers, and food products. It is a method that often makes use of the back titration concept mentioned above. We will now describe this technique in detail. 

The acid in the receiving vessel can be either a dilute (perhaps 0.10 N) standardized solution of a strong acid, such as sulfuric acid, or a solution of boric acid. If it is the former, it is an example of a back titration. If it is the latter, it is an example of an indirect titration. 

In the indirect method using boric acid (H3B03), the ammonia reacts with the boric acid, producing a partially neutralized salt of boric acid (H,BOj)  

The amount of standardized acid needed is proportional to the amount of ammonia that bubbled through. It is an indirect method because the ammonia is determined but not titrated. It is determined indirectly by titration of H2B03. In a direct titration, the analyte would be reacted directly with the titrant, as per the discussion in Section 4.6. The concentration of the boric acid in the receiving vessel does not enter into the calculation and need not be known. Equation (4.40) is used for the calculation. 

FIGURE 5.14 A flow chart of the Kjeldahl titration procedures. 

---

O—H, and N—H. These bands enable the quantitative characterization of polymers, chemicals, foods, and agricultural products for analytes such as water, fatty acids, proteins, and the like. In many cases, the use of NIR reflectance spectroscopy has been able to replace time consuming, classical wet chemical analyses, such as the Kjeldahl method for protein nitrogen and the Karl Fischer titration for water content. The NIR region has been used for qualitative studies of hydrogen bonding, complexation in organometallic compounds, and solute-solvent interactions because the NIR absorptions are sensitive to intermolecular forces. 

---