What does the Danish chemist Johan Kjeldahl have to do with boats? Initially nothing, and yet a boat plays a decisive role in the analytical method he developed for the determination of nitrogen.
At a meeting of the Danish Chemical Society in 1883 Kjeldahl presented his method for the determination of nitrogen. Today it is used throughout the world as a routine method in numerous laboratories, for example for the determination of the total protein content of foodstuffs or for the detection of nitrogen in soil samples.
The method has three working steps: 1. Sample digestion, in which the sparingly soluble or insoluble nitrogen compounds are converted into ammonium sulphate by heating in a solution of concentrated acid; 2. Distillation, in which the nitrogen compound, which has first been converted into ammonia by the addition of excess alkali, is separated from the digestion solution; and finally 3. Titration, in which the amount of ammonia that reacts with a standard solution of an acid is quantified; the nitrogen content of the sample can then be calculated from this amount.
Even today the safe and complete transfer of the sample through the narrow neck of a Kjeldahl flask and into the digestion solution often presents a challenge. This is where the boat mentioned above comes into play, as Schleicher & Schuell MicroScience offers pre-folded weighing boats specially designed for this manoeuvre. These are made from nitrogen-free paper and contain no adhesives or additives. This means they can be added to the acid solution with the sample without influencing the analytical results.
The weighing boat is placed on the balance and the required sample is weighed out. Then the weighing boat containing the sample is placed in the Kjeldahl flask or digestion tube containing the acid. Because of its shape it has no problem in passing through the narrow neck and enters the flask or digestion tube without any loss of sample.
Schleicher & Schuell MicroScience GmbH is based in Dassel, Germany, www.schleicher-schuell.de
