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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?

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    $\begingroup$ Why do you want individual tests? It will give either NaCN or Na2S if either is present and NaSCN if both are present and you can distinguish NaCN, Na2S and NaSCN. Na2S is yellow solid and while NaCN and NaSCN are white, they can be distinguished by their solubility (NaCN is 63.7 g/100ml and NaSCN is 139 g/100ml at rt) and once you identified the sodium salt, you proceed with corresponding reaction. Tbh, Lassaigne test has become obsolete and nobody uses obviously due to its hazard factors. $\endgroup$ Apr 8, 2023 at 5:47

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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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  • $\begingroup$ So it will show a positive test for all 3 tests (ferric sulfocyanide, sodium nitroprusside, and ferric ferrocyanide) if the correct ratio of sodium metal to organic compound is used? $\endgroup$
    – R H
    Apr 8, 2023 at 1:27
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    $\begingroup$ I do not know the exact answer because a general answer cannot be stated, what if a molecule has two S for each N or any other ratio. The above reference suggests to vary the amount of sodium and see the effect for each compound. $\endgroup$
    – AChem
    Apr 8, 2023 at 1:31

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