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.
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.
Theory
The halogens, nitrogen, and sulfur are covalently bonded to the organic compounds are converted to various sodium salts formed during the fusion. Typically proposed reactions are:
:Na + C + N → NaCN
:Na + C + N + S → NaSCN
:2Na + S → Na<sub>2</sub>S
:Na + X → NaX
The fate of the hydrocarbon portion of the sample is disregarded.
The aqueous extract is called sodium fusion extract or Lassaigne's extract.
Test for nitrogen
The sodium fusion extract is made alkaline by adding NaOH. To this mixture, freshly prepared FeSO<sub>4</sub> solution is added and boiled for some time and then cooled. A few drops of FeCl<sub>3</sub> 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.
<chem display="block">6CN- +Fe^2+ -> [Fe(CN)6]^4-</chem>
<chem display="block">{3[Fe(CN)6]}^{4-} +4Fe^3+ ->[\ce{xH2O}]Fe4[Fe(CN)6]3\cdot xH2O</chem>
Test for sulfur
Lead acetate test
The sodium fusion extract is acidified with acetic acid and lead acetate is added to it. A black precipitate of lead sulfide indicates the presence of sulfur.
Sodium nitroprusside test
Freshly prepared sodium nitroprusside solution is added to the sodium fusion extract, turning the solution deep violet due to formation of sodium thionitroprusside.