Protein Determination of Food by the Kjeldahl Procedure                Answer Key

 

Materials

 

Buret containing 0.1N HCl

Kjeldahl sample*

 

*A 5.12g food sample was digested with sulfuric acid and the appropriate catalysts. After complete digestion, the sample was neutralized with excess sodium hydroxide. The ammonia released from the sample was steam-distilled into a boric acid solution. The resulting solution is the sample labeled Kjeldahl.

 

 

Procedure

 

1.  Obtain a sample of protein digest (labeled Kjeldahl).

 

2.  Titrate the sample using 0.1N hydrochloric acid until you see a color change.

 

3.  Record the volume of acid used of the bottom of this sheet.

4.  Answer the questions on the bottom and back side of this sheet.

 

 

 

Record of Observation

 

Volume of acid required for titration (8 points): __________________________________________

 

 

 

 

Questions & Answers

 

 

1.  Calculate % nitrogen in the sample using the follow equation. (4 points)

 

         (mL acid x 0.1 N x 0.014 g nitrogen/milliequivalent) x 100 = _____________________

                           sample weight, g

    

 

2.  The % nitrogen for this sample can be converted to % protein by using a factor of 6.25. That is 6.25 x % nitrogen = % protein. What is the % protein? (4 points)

 

_______________________________________________________________________________

 

 

3.  A second sample weighed 6.3 grams and required 17.22 mL of acid for titration. What is the % nitrogen in this sample? (4 points)

 

 

     _______________________________________________________________________________

 

 

 

4.  For this sample, the conversion factor is 6.38. How much protein did it contain? (4 points)

 

 

_______________________________________________________________________________

 

 

5.  A third sample weighed 7.32g and required 13.42 mL of acid for titration. What is the % nitrogen in this sample? (4 points)

 

 

     _______________________________________________________________________________

 

 

 

6.  For this sample, the conversion factor is 5.7. How much protein did it contain? (4 points)

 

 

     _______________________________________________________________________________

 

 

7.  Using the molecular weights (MWs) provided, derive the factor of approximately 6.25 used to convert the % nitrogen to % protein. (8 points)

 

     MW of Nitrogen  = 14

     MW of Carbon    = 12

     MW of Oxygen   = 16

     MW of Hydrogen = 1