Detection of Nitrogen and Sulfur in Lassaigne's Test

In Lassaigne's test for the detection of nitrogen and/or sulfur in organic compounds, we prepare Lassaigne's extract. In this extract, NaCN is formed if nitrogen is present, and $\ce{Na2S}$ is formed when sulfur is present. NaCN on treatment with $\ce{FeSO4}$ and $\ce{FeCl3}$ gives a Prussian blue precipitate of $\ce{Fe4[Fe(CN)6]3}$, and $\ce{Na2S}$ on treatment with $\ce{Na2[Fe(CN)5NO]}$ (sodium nitroprusside) will give a violet color due to the formation of $\ce{Na4[Fe(CN)5NOS]}$. However, when both are present, NaCNS is formed. NaCNS can be detected by an $\ce{FeCl3}$ test, which gives a blood red color of $\ce{Fe(CNS)3}$ (ferric sulfocyanide). My question is, when both nitrogen AND sulfur are present, will the individual tests for nitrogen and sulfur (Prussian blue and sodium nitroprusside test) be shown? Doesn't this depend on whether the sodium extract contains NaCN, NaCNS, or $\ce{Na2S}$? Is it possible to have a mixture of all 3 (NaCN, NaCNS, and $\ce{Na2S}$), in which case all the above mentioned tests will be answered?




Please note that nobody uses Lassaigne's test of the 18th century to test $\ce{S}$, $\ce{N}$ and halogens in organic compounds. It is unsafe and really obsolete method of qualitative analysis. I know that it is still taught in high schools in certain parts of the world. Just looking at the chemistry's point of view, your hypothesis is partially correct. One may see the formation of thiocyanate. However, all those products which you mentioned are dependent on the relative ratio of sodium metal and the organic compounds. If an excess of sodium metal is used, thiocyanate ion is destroyed during fusion and you get the regular products. In short it is possible to simultaneously detect $\ce{N}$ and $\ce{S}$ with the proper amount of compound to sodium ratio during fusion.


**Reference:**
Tucker, S. Horwood. "A lost centenary: Lassaigne's test for nitrogen. The identification of nitrogen, sulfur, and halogens in organic compounds." *Journal of Chemical Education* **1945**, *22(5)*, 212-215 (DOI: <https://doi.org/10.1021/ed022p212>).


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