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 sulphur, 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 sulphur 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.

Method

A small amount of the organic compound is added to a melted globule of sodium in a test tube and is heated strongly till it becomes red hot. Then it is boiled in water for some time, cooled, and filtered. The filtrate (sodium extract) contains ionic compounds of sodium as:

Na + C + N -> NaCN
Na + C + N + S -> NaSCN
2Na + S -> Na2S
Na + X -> NaX

Test for Nitrogen

The sodium extract is made alkaline by adding NaOH. To this mixture freshly prepared FeSO4 solution is added and boiled for sometimes and then cooled. A few drops of FeCl3 are added and Prussian blue (bluish green) color forms due to formation of ferric ferrocyanide along with NaCl. This shows the presence of nitrogen in the organic compound.

Test for Halogens

The sodium extract is boiled with concentrated HNO3 followed by the addition of AgNO3 solution which yields a white (Cl) or yellow (Br or I) precipitate if halogen is present.

NaX + AgNO3 -> AgX + NaNO3

Test for Phosphorus

Sodium peroxide is added to sodium extract to oxidise phosphorus to sodium phosphate. It is boiled with concentrated HNO3 and then ammonium molybdate is added. A yellow precipitate indicates the presence of phosphorus.[3]

Test for Sulphur

1)Lead acetate test

Na-extract is acidified by acetic acid and lead acetate is added to it.Black precipitate of lead sulphide is formed along with Sodium acetate.

2)Sodium nitroprusside test

Freshly prepared sodium nitroprusside solution is added to Na-extract, deep violet colouration appears due to formation of sodium thionitroprusside.

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.
  3. "Lassaigne's Test - Test for Nitrogen, Sulphur, Halogens with Examples". BYJU'S.


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