Question

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

• A. To dissolve ${\mathrm{Ag}}_2\mathrm{S}$
• B. To dissolve $\mathrm{AgCN}$
• C. To remove silver halides
• D. To decompose ${\mathrm{Na}}_2\mathrm{S}$ and $\mathrm{NaCN}$

Answer 1

The correct option is D

To decompose ${\mathrm{Na}}_2\mathrm{S}$ and $\mathrm{NaCN}$

Explanation for Correct Answer:

Option (D): To decompose ${\mathrm{Na}}_2\mathrm{S}$ and $\mathrm{NaCN}$

Step 1: Lassigne Test

A small amount of dilute Nitric acid $\left({\mathrm{HNO}}_3\right)$ is added in the test of halogens from sodium extract. This is done in order to expel all the evolved gases.

The blood red product Ferric thiocyanate $\left(\mathrm{Fe}{\left(\mathrm{SCN}\right)}_3\right)$ is formed when Ferric chloride $\left({\mathrm{FeCl}}_3\right)$ reacts with Sodium thiocyanate $\left(\mathrm{NaSCN}\right)$.

This test is considered as as the confirmatory test for organic compounds that contain both Nitrogren and Sulfur.

$\underset{\mathrm{Sodium}\mathrm{cyanide}}{8\mathrm{NaCN}}+\underset{\mathrm{Octasulfur}}{{\mathrm{S}}_8}\stackrel{∆}{\to }\underset{\mathrm{Sodium}\mathrm{thiocyanate}}{8\mathrm{NaSCN}}$

$\underset{\mathrm{Iron}\mathrm{ion}}{{\mathrm{Fe}}^{3+}}+\underset{\mathrm{Sodium}\mathrm{thiocyanate}}{3\mathrm{NaSCN}}\to \underset{\left(\mathrm{Blood}\mathrm{red}\mathrm{colouration}\right)}{\underset{\mathrm{Ferric}\mathrm{thiocyanate}}{\mathrm{Fe}{\left(\mathrm{SCN}\right)}_3}}$

Step 2: Violet Colouration

Sodium reacts with sulphur from the organic compound during the preparation of Lassaigne's extract to form Sodium sulphide $\left({\mathrm{Na}}_2\mathrm{S}\right)$.

When this reacts with Sodium nitroprusside $\left({\mathrm{Na}}_2\left[\mathrm{Fe}{\left(\mathrm{CN}\right)}_5\mathrm{NO}\right]\right)$ it gives the product Sodium thionitroprusside $\left({\mathrm{Na}}_4\left[\mathrm{Fe}{\left(\mathrm{CN}\right)}_5\mathrm{NOS}\right]\right)$ which has a violet colouration.

$2\mathrm{Na}+\underset{\left(\mathrm{From}\mathrm{organic}\mathrm{compound}\right)}{\mathrm{S}}\to {\mathrm{Na}}_2\mathrm{S}$

$\underset{\mathrm{Sodium}\mathrm{sulphide}}{{\mathrm{Na}}_2\mathrm{S}}+\underset{\mathrm{Sodium}\mathrm{nitroprusside}}{{\mathrm{Na}}_2\left[\mathrm{Fe}{\left(\mathrm{CN}\right)}_5\mathrm{NO}\right]}\to \underset{(\mathrm{Violet}\mathrm{colouration})}{\underset{\mathrm{Sodium}\mathrm{thionitroprusside}}{{\mathrm{Na}}_4\left[\mathrm{Fe}{\left(\mathrm{CN}\right)}_5\mathrm{NOS}\right]}}$

Hence, the correct answer is an option (D).