Sodium fusion test

The sodium fusion test, or Lassaigne's test, is used in elemental analysis for the qualitative determination of the presence of foreign elements, namely halogens, nitrogen, and sulfur, in an organic compound. It was developed by J. L. Lassaigne.[1][2]

The test involves heating the sample strongly with clean sodium metal, "fusing" it with the sample. A variety of techniques has been described. The "fused" sample is plunged into water, and the usual qualitative tests are performed on the resultant solution for the respective possible constituents.[2]

Theory

The halogens, nitrogen and sulfur are covalently bonded to the organic compounds. In order to detect them, the elements need to be converted into their ionic forms. This is done by fusing the organic compound with sodium metal. The ionic compounds formed during the fusion are extracted in aqueous solution and can be detected by simple chemical tests. The extract is called sodium fusion extract or Lassaigne's extract.


When an organic compound is heated strongly with sodium, any halogens, nitrogen, and sulfur will be converted into inorganic sodium salts such as sodium halide (for halides), sodium cyanide (for nitrogen), sodium sulfide (for sulfur), and sodium thiocyanate (for sulfur and nitrogen).The nitrogen is confirmed with ferrous sulfate.

FOR NITROGEN ELEMENT :
                             
          Na + (C+N)  --------------> NaCN
          2NaCN + FeSO4--------------->Na2SO4 + Fe(CN)2
          Fe(CN)2+ 4NaCN -------------> Na4[Fe(CN)6]

References

  1. Lassaigne (1843) "Mémoire sur un procédé simple pour constater la présence de l'azote dans des quantités minimes de matière organique" [Memoir on a simple procedure for confirming the presence of nitrogen in minimal quantities of organic matter], Comptes rendus,16 : 387-391.
  2. Gower, R. P.; Rhodes, I. P. (1969). "A review of techniques in the Lassaigne sodium-fusion". Journal of Chemical Education. 46 (9): 606. Bibcode:1969JChEd..46..606G. doi:10.1021/ed046p606.


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