Question

The protein in foods is commonly determined by Kjeldahl titration. This involves three steps. First, the sample is digested with boiling sulfuric acid, which hydrolyzes the protein and converts the nitrogen from the protein into ammonium ions.

protein + H₂SO₄ → NH₄⁺ + other

In the second step, the solution is made basic, which converts the ammonium ions to ammonia molecules. Ammonia is volatile so it can be removed from a solution by steam distillation. The mixture from the first step is distilled into a container of hydrochloric acid. The ammonia from the first step mixes with the HCl, which neutralizes some of the HCl.

NH₃ + HCl → NH₄Cl

In the third step, the unreacted HCl is back titrated with NaOH. The amount of NH₄⁺ that was produced in the first step can then be calculated and used to determine the amount of protein in the sample. Use the index of your textbook to find a more complete discussion of Kjeldahl titrations.

The protein in a 1.1180-g sample of an oat cereal is determined by a Kjeldahl analysis. The sample is digested with H₂SO₄, the resulting solution made basic with NaOH, and the NH₃ distilled into 50.00 mL of 0.09435 M HCl. The excess HCl is back titrated using 33.72 mL of 0.06286 M NaOH. Given that the proteins in grains average 17.54% w/w N, report the %w/w protein in the sample.

% protein

Answer 1