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In the brown ring test for nitrite and nitrate ions, the characteristic brown ring is produced because a complex ion is formed with which formula?

1. [Fe(H2O)6]2+
2. [Fe(NO)(CN)5]2+
3. [Fe(H2O)5NO]2+
4. [Fe(H2O)6]2+

Correct answer: [Fe(H2O)5NO]2+

Solution

In the brown ring test the NO produced replaces one water ligand of [Fe(H2O)6]2+ to give the brown nitrosyl complex [Fe(H2O)5NO]2+ (iron in +1, NO+). Hence the formula is [Fe(H2O)5NO]2+.

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