Question

In Lassaigne's test for the detection of halogens, the sodium fusion extract is first boiled with concentrated nitric acid. This is :

• A. to remove silver haldies
• B. to decompose $N{a}_{2}S$ and $NaCN$, if present
• C. to dissolve $A{g}_{2}S$
• D. to dissolve $AgCN$, if formed
• E. because $A{g}_{2}S$ and $AgCN$ are insoluable in nitric acid

Answer 1

Answer: (B)

to decompose $N{a}_{2}S$ and $NaCN$, if present

In the test of halogens from sodium extract, a small amount of dilute HNO3 is added. This is because in testing the halogen from the Lassigne's extract, it contains halogens. Therefore, dilute nitric acid is added before testing halogens to expel all the gases if evolved.

Ferric Chloride $\left(FeC{l}_3\right)$ that reacts with Sodium Thiocyanate $\left(NaCNS\right)$ to form Ferric Thiocyanate $\left[Fe\right(CNS{)}_3]$ which is blood red in color. This test can be used as a confirmatory test for organic compounds containing both Nitrogen and Sulfur (e.g. Methionine amino acid).

$NaCN+S_8\stackrel{\Delta }{\to }\underset{SodiumThiocynate}{NaSCN}$

$F{e}^{+3}+3NaSCN\to \underset{Ferricthiocynate\left(Bloodredcolouration\right)}{Fe(SCN{)}_3}$

During the preparation of Lassaigne's extract, sulphur from the organic compound reacts with sodium to form sodium sulphide. It gives a purple colour with sodium nitroprusside due to the formation of sodium thionitroprusside.

$2Na+\underset{\left(Fromorganiccompound\right)}{S}\to N{a}_{2}S$

$\underset{fromsodiumexteracted}{N{a}_{2}S}+N{a}_2\left[Fe\right(CN{)}_{5}NO]\to \underset{violetcolouration}{N{a}_4\left[Fe\right(CN{)}_{5}NOS]}$

$Hencethecorrectanswerisoption\left(B\right).$