Will-Varrentrapp nitrogen determination
The determination of nitrogen according to Will-Varrentrapp is a no longer common method in analytical chemistry for determining the nitrogen content of organic samples . It was developed in the first half of the 19th century by the two German chemists Heinrich Will and Franz Varrentrapp and was based on a method that went back to the French chemist Anselme Payen .
The method is based on heating the sample with potassium or sodium hydroxide and calcium oxide and measuring the amount of ammonia produced by titration or precipitation reactions . From its publication in 1841 to the presentation of Kjeldahl's nitrogen determination in 1883, Will and Varrentrapp's method was the dominant method for quantitative nitrogen analysis in almost all areas of chemical analysis.
execution
The determination of nitrogen according to Will-Varrentrapp is based on heating the sample to be examined with a mixture of potassium or sodium hydroxide and calcium oxide in a combustion tube. A sample amount of around 200 milligrams for nitrogen-rich compounds and around 400 milligrams for nitrogen-poor compounds is considered sufficient. For analysis of liquid samples, these are placed in an open, spherical vessel of suitable size in the filled combustion tube. The nitrogen from the sample by heating in red heat to gaseous ammonia converted, in hydrochloric acid is collected. In addition, depending on the chemical composition of the sample, carbon dioxide , hydrogen and various gaseous and liquid hydrocarbons can be released.
The amount of ammonia is proportional to the content of nitrogen in the sample and can thus be calculated after a quantitative determination of the ammonium chloride formed in the hydrochloric acid through various titration or precipitation reactions . In this regard, Franz Varrentrapp and Heinrich Will described the precipitation of ammonium platinum chloride with a platinum (IV) chloride solution added in excess and subsequent weighing of the washed and dried ammonium platinum chloride or of the metallic platinum remaining after the ammonium platinum chloride has been annealed .
Advantages and disadvantages
The Will-Varrentrapp method, a further development of a method by Anselme Payen , was considered to be easy to carry out in terms of equipment and the necessary chemicals and comparable to the analytical methods for carbon and other chemical elements that were common at the time . In particular with regard to its accuracy and its lower susceptibility to interference, it was superior to the previously used methods for nitrogen determination such as the combustion method according to Jean-Baptiste Dumas . It generally provides reliable results for proteins and amines . In the case of nitro compounds , on the other hand, the nitrogen from the sample is only partially converted into ammonia. The release of nitrogen from many oxygen-free organic bases such as aniline is also incomplete. Another major disadvantage of the method was the time required to carry it out.
Historical meaning
Due to its advantages, the method for nitrogen determination developed by Franz Varrentrapp and Heinrich Will found widespread use in almost all areas of application of analytical chemistry in the decades after its introduction. Justus von Liebig , Will and Varrentrapp's academic teacher, described it as early as 1841 when it was published as “one of the most important improvements in organic analysis”. With the spread of Kjeldahl's nitrogen determination , which was introduced by the Danish chemist Johan Kjeldahl in 1883, Will-Varrentrapp's method was increasingly out of use, as the method developed by Kjeldahl was less time-consuming and thus enabled a higher sample throughput . Kjeldahl himself used Will-Varrentrapp's nitrogen determination to validate his newly developed method on the basis of various samples of animal and plant origin.
literature
- Franz Varrentrapp, Heinrich Will: New method for the determination of nitrogen in organic compounds. In: Annals of Chemistry and Pharmacy. Volume 39 / 1841. Issue 3, pp. 257-296
- Method of nitrogen determination by Varrentrapp and Will. In: Justus von Liebig : Instructions for the analysis of organic bodies. Second edition. Published by Friedrich Vieweg, and son, Braunschweig 1853, pp. 88–97
- Organic Analysis. In: Aaron John Ihde: The Development of Modern Chemistry. Courier Dover Publications, New York 1984, ISBN 0-48-664235-6 , pp. 296-298