Determination of crude protein in a sample

Principle  

       Conversion of nitrogenous compounds of the sample into ammonium sulfate by boiling with sulphuric acid and subsequent decomposition of ammonium sulfate with fixed alkali and the collection of titrated ammonia in an acid solution is titrated against an acid of known strength and the N content of the sample is computed.

Apparatus

  • Kjeldahl digestion apparatus
  • Volumetric flask (250 ml)
  • Kjeldahl micro distillation apparatus pipette 20 ml
  • Titration apparatus
  • Beakers
  • Digital weighing balance
  • Funnel
  • Heating arrangement
  • Coldwater circulation

Reagents

  • N/10 H2SO4
  • Boric acid (H3BO3)
  • 40% sodium hydroxide
  • Methyl red indicator
  • Distilled water
  • Digestion mixture (K2SO4, FeSO4, and CuSO4)
  • Feed sample
  • Commercial H2SO4

Preparation of digestion mixture

  1. K2SO4                    85 parts                     To rice the boiling point of the acid
  2. FeSO4                     5 parts                       To check the bumping
  3. CuSO4                    10 parts                      To act as a catalyst

Procedure

Digestion 

        Take 1 gram of the dried feed sample, add 5 gram digestion mixture in Kjeldahl digestion flask (500 ml), add 25-30  ml commercial sulphuric acid, and provide heat till transparent or colorless content is appeared (3-5 hours).

Dilution

       Transfer the digested content in a 250 ml volumetric flask and add distilled water to make 250 ml.

Distillation

      Take 10 ml of the above feed sample solution in the micro Kjeldahl distillation flask and also add 10 ml of 40% NaOH solution, take another flask or beaker containing 10 ml 4% boric acid solution, add 1-2 drops of methyl red as an indicator, provide steam, ammonia gas fumes produce. Ammonia gas is trapped in 4% boric acid solution. The endpoint will be golden color. after this wait for two minutes and remove the flask.

Titration

         Titrate the boric acid solution with N/10 H2SO4 till pink color is obtained, record the volume of the acid used.

Calculation

          Vol. of N/10 H2SO4 used X vol. of sample solution X 0.0014
N% = ---------------------------------------------------------------------------------------------   X  100
        weight of the sample X volume of the sample solution used (10ml)

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