Question

Discuss the chemistry of Lassaigne’s test for detection of nitrogen in the organic compound. Why is the extract boiled with $HNO_{3}$ before testing for halogens and sulphur?

Answer 1

Lassaigne test:  

This test is used in detection of the exsistence of an organic compound of nitrogen, sulphur, halogens, and phosphorus. These elements are present in an “organic compound” in a covalent form. They are converted to the ion form by “fusing” the sodium metal compound.

$Na\quad +\quad C\quad +\quad N\quad \rightarrow \quad NaCN$

$2Na\quad +\quad S\quad \rightarrow \quad { Na }_{ 2 }S$

$Na\quad +\quad X\quad \rightarrow \quad NaX\quad (\quad X\quad =\quad Cl,Br,I)$

Cyanide, sulphide, and halide formed by sodium are extracted by boiling in distilled water from the fused mass. The resulting extract is called Lassaigne extract. This extract of Lassaigne is then tested for “nitrogen”, “sulphur”, “halogens”, and “phosphorus”.

$Test\quad for\quad nitrogen:\\ Lassaignes's\quad extract\quad +\quad Ferrous\quad sulphate\quad +\quad Conc.\quad Sulphuric\quad acid$

The fusion extract of sodium is boiled with “iron (II) sulphate” in the Lassaigne nitrogen test in an “organic compound” and then acidified with sulphuric acid.  Then the sodium cyanide first reacts with “sulphate of iron (II)” and forms “sodium hexacyanoferrate (II)”.

Then, when sulphuric acid is heated, some iron (II) becomes oxidized to form “iron (III) hexacyanoferrate (II)”, which is color Prussian blue.  

The reactions,

$6C{ N }^{ - }\quad +\quad { Fe }^{ 2+ }\quad \rightarrow \quad [Fe(CN{ ) }_{ 6 }{ ] }^{ 4- }$

$\begin{matrix} 3\left[ Fe(CN{ ) }_{ 6 } \right] \\ \quad \end{matrix}\quad +\quad \begin{matrix} 4{ Fe }^{ 3+ } \\ \quad \end{matrix}\quad \rightarrow \quad \begin{matrix} { Fe }_{ 4 }[Fe(CN{ ) }_{ 6 }{ ] }_{ 3 }x{ H }_{ 2 }O \\ Prussian\quad blue\quad colour \end{matrix}$