## Werkwijze

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.

- Cut the sample in to small cubes (maximum dimension: 2 x 2 x 2 cm).
- Using an analytical balance,weigh (in duplo) 0.5 g of the sample on a nitrogen free paper. (filtreerpapiertje)
- Scramble the paper with the sample and put it at the bottom of the destruction tube.
- Also put a scrambled paper in a destruction tube. This is your blanco.
- Add 25ml (69%) sulfuric acid and 8 ml (35%) hydroxen peroxide tot the destruction tubes.
- Now you can start the destruction: preheat the system for 15 min at heating position 10.
- Put the destruction tubes in the system and set the system to heating position 9.
- Wait 15 min.
- After 15 min add another 8 ml (35%) hydrogen peroxide to the destruction tubes.
- Wait another 15 min.
- Cool the destruction tubes down to room temperature before you proceed to the destillation.

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.

- Put your destruction tube in the destillation unit.
- Take a erlenmeyer flask and add 30ml of boric acid (4%).
- Place the erlenmeyer flask in the destillation unit
- Now start the destillation with the programmed parameters as seen in the schedule above.

Titration

- After the destillation remove the destruction tube (carefully because the tube will be hot!!)
- Remove the erlenmeyer flask.
- Add 3 drops of tashiro indicator.
- Titrate the mixture with HCl (0,1M) till eq-point.