Which statement is not true with respect to nitrate ion test?

We need to identify which statement is NOT true with respect to the brown ring test for nitrate ions.

Recall the Brown Ring Test procedure

In the brown ring test:

1. The suspected nitrate salt is dissolved in water.

2. Freshly prepared ferrous sulfate ($$FeSO_4$$) solution is added.

3. Concentrated $$H_2SO_4$$ is carefully added along the side of the test tube so that it forms a separate layer below the aqueous solution.

Analyze each statement

Option A: "A dark brown ring is formed at the junction of two solutions."

This is TRUE. The brown ring appears at the interface of the $$H_2SO_4$$ layer and the aqueous solution.

Option B: "Ring is formed due to nitroferrous sulphate complex."

The brown ring is due to the formation of $$[Fe(H_2O)_5(NO)]^{2+}$$ complex, which is called nitrosonium ferrous sulphate (or nitroso ferrous sulphate), NOT "nitroferrous sulphate." The correct name involves "nitroso" prefix. However, the key issue is whether the name used is acceptable.

Option C: "The brown complex is $$[Fe(H_2O)_5(NO)]SO_4$$"

This is TRUE. The brown ring is formed by the complex $$[Fe(H_2O)_5(NO)]SO_4$$, where NO is a neutral ligand coordinated to $$Fe^{2+}$$ (though the actual oxidation involves $$Fe^+ $$ with $$NO^+$$).

Option D: "Heating the nitrate salt with conc. $$H_2SO_4$$, light brown fumes are evolved."

When a nitrate salt is heated with concentrated $$H_2SO_4$$, brown fumes of $$NO_2$$ are evolved. This is TRUE and is a confirmatory test for nitrate ions.

Identify the incorrect statement

Option B states the ring is due to "nitroferrous sulphate complex." The correct name is nitrosoferrous sulphate (or nitrosonium pentaaquairon(II) sulphate), not "nitro" ferrous sulphate. "Nitro" would imply $$NO_2$$ as the ligand, but the actual ligand is $$NO$$ (nitrosyl/nitroso). This makes Option B the incorrect statement.

The correct answer is Option B: Ring is formed due to nitroferrous sulphate complex.