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Destruction

A common procedure for the complete destruction of organic material according to Kjeldahl, is to oxidize the sample with concentrated sulfuric acid at high tempera­ture.

Do not use concentrated sulfuric acid, (98%) in the destruction mixture. In this case the hydrogen peroxide solution is violently decomposed. Best results are obtained using sulfuric acid, (69%) in the destruction mixture.

 

Step                        Heating position  Time      

Preheating               10                         15 min                      

1                             9                           15 min

2                             Additional portion ( 8 ml) of hydrogen peroxide is added

3                             9                           10 min

Cooling                    -                            30 min

If the samples are not analyzed on the same day, dilute them with 50 ml of water in order to prevent crystallization. Add water only to cooled samples, otherwise the reaction with the concentrated acid is violent and the sample may be lost. Gently swirl the tube to mix the digested sample with the water.

 

Destillation

The Kjeldahl Unit B-323 was set according to the parameters listed  below:


Distillation-parameters

 

Water

50 ml

 

 

NaOH

90 ml (stellen

 

 

Reaction time

  5 s

 

 

Distillation time

    240 s

 

 

 

 

 

 

If the samples have already been diluted with 50 ml water, add only 20 ml water for the distillation and adjust volume NaOH to 65 ml.


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