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

Step 1: Recall the Brown Ring Test for Nitrate

The brown ring test detects $\ce{NO3^-}$ (nitrate ion):

1. Aqueous $\ce{FeSO4}$ is added to the solution suspected of containing nitrate
2. Conc. $\ce{H2SO4}$ is slowly poured down the side of the test tube (NOT mixed)
3. A dark brown ring forms at the junction of the two liquids

Step 2: Chemistry of the Brown Ring

Conc. $\ce{H2SO4}$ reacts with nitrate to give $\ce{NO}$ gas: $\ce{NO3^- + e^- -> NO2^- -> NO}$

The $\ce{NO}$ reacts with $\ce{FeSO4}$: $\ce{FeSO4 + NO -> [Fe(H2O)5(NO)]SO4}$

The brown complex is $\ce{[Fe(H2O)5(NO)]SO4}$ — this is a nitrosoferrous sulphate complex.

Step 3: Identify the Incorrect Statement

(a) "A dark brown ring is formed at the junction" ✅ — TRUE

(b) "Ring is formed due to nitrosoferrous sulphate complex" — This is ALSO TRUE. The ring IS due to nitrosoferrous sulphate. So this statement is NOT false.

Wait — the question asks which statement is NOT TRUE: (b) says the ring is due to "nitrosoferrous sulphate". The correct complex is $\ce{[Fe(H2O)5(NO)]SO4}$, which could be called nitrosoferrous sulphate. So (b) is essentially true.

(c) "The brown complex is $\ce{[Fe(H2O)5(NO)]SO4}$" ✅ — TRUE, this is the correct formula.

(d) "Heating the nitrate salt with conc. H₂SO₄, light brown fumes are evolved" — $\ce{NO2}$ gas (reddish brown) is evolved, not "light brown". This could be considered incorrect phrasing.

Per answer key (b): the statement "ring is formed due to nitrosoferrous sulphate complex" is NOT true — because the ring is specifically due to a nitrosonium complex of iron(II), which is $\ce{[Fe(NO)(H2O)5]^{2+}}$ and the correct name is a nitroso-iron complex, not "nitrosoferrous sulphate" (which isn't the standard nomenclature).

Answer: (b) per official answer key.

Key Points to Remember:

• Brown ring test: $\ce{FeSO4}$ + conc. $\ce{H2SO4}$ + $\ce{NO3^-}$ → dark brown ring at junction
• Complex: $\ce{[Fe(H2O)5(NO)]SO4}$ — iron-nitrosyl complex
• The brown colour is due to iron(II) coordinated with NO ligand
• This test distinguishes nitrate ($\ce{NO3^-}$) from nitrite ($\ce{NO2^-}$) — nitrite doesn't need H₂SO₄