Sodium fusion test

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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]

The test involves heating the sample with sodium metal, "fusing" it with the sample. A variety of techniques has been described. The "fused" sample is plunged into water, and the qualitative tests are performed on the resultant solution for the respective possible constituents.[2][3]

Theory[]

The halogens, nitrogen, and sulphur are covalently bonded to the organic compounds are converted to various sodium salts ionic compounds formed during the fusion. Typically proposed reactions are:

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

The fate of the hydrocarbon portion of the sample is disregarded.

The aaqueous extract is called sodium fusion extract or Lassaigne's extract.

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.[4]

Test for Sulphur[]

1)

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 .

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. ^ 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.
  4. ^ "Lassaigne's Test - Test for Nitrogen, Sulphur, Halogens with Examples". BYJU'S.{{cite web}}: CS1 maint: url-status (link)

External links[]


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