Protein analysis by the Kjeldahl method

An example for practicing


You have to blindly control two yogurts. You know that one of them is a classic yogurt, while the other is a protein-rich yogurt. To this end, you carry out a determination of total nitrogen for each of the products (A and B) by the Kjeldahl method (see experimental procedure).
  • the initial test sample mass (mproduct) is 1.035 g for each yogurt.
  • the titration volumes of NaOH 0.1 mol/L (Vsample) are 20.4 mL for yogurt A and 15.9 mL for yogurt B.
  • the titration volume of NaOH for the control (Vcontrol) is 25.1 mL.

The formula for determining the protein content in a product, in g for 100 g of product, is:


(CNaOH x (Vcontrol – Vsample) x 10-3 x MN x 6,25 x 100) / mproduct
(CNaOH x (Vcontrol - Vsample) x 10-3 x MN x 6,25 x 100) / mproduct
(CNaOH x (Vcontrol – Vsample) x 10-3 x MNH3 x 6,25 x 100) / mproduct

Yogurt A is the protein-rich yogurt.


True
False

The protein content in yogurt A, in g for 100 g of product, is:


3.97
5.25
8.03

The protein content in yogurt B, in g for 100 g of product, is:


3.97
5.25
8.03

If you are presented with a third yogurt (yogurt C), which is said to be half as high in protein as the classic yogurt, what volume of sodium hydroxide (0.1 mol/L) would you expect to pour to verify this information if the mass of the initial sample is 1.000 g?


7.8mL
16.0 mL
18.6 mL
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