Kjeldahl Digestion

Kjeldahl digestion is a method of heating substances in concentrated acids in order to dissolve or breakdown the substance to give a solution. The method was originally developed to quantify the nitrogen content in proteins by producing a solution of ammonium ions, but the method is also used for other difficult to dissolve substances, such as metals.

The Kjeldahl flask consists of a flask bulb with a long neck and is suitable for bringing concentrated acids to their boiling point. Sulfuric acid is most commonly used as the dissolving acid with a boiling point around 340 °C. Additives are sometimes used to increase the boiling point further, or catalysts may be added to speed up the reaction.

Procedure

1. The Kjeldahl flask should be securely clamped above the heat source and should be angled towards the back of the fume hood. The heat source is usually a Bunsen burner with tripod and gauze, as a strong heat source is required to boil the acid.

2. The substance of interest is added to the flask and acid is carefully added down the neck of the tilted flask.

3. The mixture is brought carefully to the boil. The reaction may be vigorous and the heat source should be removed to allow the reaction to subside.

4. Once a solution has been obtained, the heat should be removed and the mixture allowed to cool.

5. The solution is usually diluted with water, by carefully adding water along the neck of the flask. Note that in this instance water is being added to acid and the reaction may be very vigourous and evolve heat.

The procedure will require adaptation depending on the desired reaction. For example more than one acid-digestion step may be involved.