From Dean:

Kjeldahl determination of nitrogen In the Kjeldahl method the sample is digested with sulfuric acid and a catalyst.

Organic material is destroyed and the nitrogen is converted to ammonium hydrogen sulfate. The heating is continued until the solution becomes colorless or light yellow. Selenium, copper, and mercury, and salts of each, have been used as the catalyst. Potassium sulfate added to the catalyst raises the temperature and thereby speeds the decomposition. A black should be carried though all the steps of the analysis.

After the digestion is complete, allow the flask to cool. Cautiously dilute with distilled water and cool to room temperature. Arrange a distillation apparatus with a water-jacketed condenser the adapter tip of which extends just below the surface of the solution in the receiver. Carefully pour a concentrated solution of NaOH down the side of the digestion flask so that little mixing occurs with the solution in the flask. Add several pieces of granulated zinc and piece of pH test paper. Immediately connect the flask to a spray trap and the condenser. Swirl the solution until mixed; the test paper should indicate an alkaline value. Bring the solution to a boil and distill at a steady rate until only one-third of the original solution remains.

When the reaction mixture is made alkaline, ammonia is liberated and removed by steam distillation. (1) In the classical method, the distillate is collected in a known excess of standard HCl solution. The unused HCl is titrated with a standard solution of NaOH, using as indicator methyl red or bromocresol green These indicator change color at the pH that corresponds to a solution of ammonium ions. (2) In the boric acid method, the distillate is collected in an excess of boric acid (crystals). The borate ion formed is titrated with a standard solution of HCl using methyl red or bromocresol green as the indicator.

Developed in 1883, the Kjeldahl mitrogen analysis remains one of the most accurate and widely used methods for determining the nitrogen in substances such as protein, milk, cereal, and flour.

Problem A typical protein contains 16.2 wt % nitrogen. A 0.500-mL aliquot of protein

solution was digested, and the librated NH3 was distilled into 10.00 mL of 0.021 40 mol/L HCl. The unreacted HCl required 3.26 mL of 0.0198 mol/L NaOH for complete titration. Find the concentration of protein (mg protein/mL) in the original sample.

(25.8 mg protein/mL)