Question

The confirmatory reactions that identify nitrate and nitrite both produce brown NO 2 gas. What criterion will you use to de termine which anion produced the gas?

Answer 1

NITRITE

The nitrite ion, which has the chemical formula NO₂⁻, is a symmetric anion with equal N–O bond lengths and an O–N–O bond angle of approximately 120°.^{[citation needed]} Upon protonation, the unstable weak acid nitrous acid is produced. Nitrite can be oxidized or reduced, with the product somewhat dependent on the oxidizing/reducing agent and its strength. The nitrite ion is an ambidentate ligand, and is known to bond to metal centers in at least five different ways.^[1] Nitrite is also important in biochemistry as a source of the potent vasodilator nitric oxide. In organic chemistry the NO₂ group is present in nitrous acid esters and nitro compounds.

conformation test

A common nitrate test, known as the brown ring test^[1] can be performed by adding iron(II) sulfate to a solution of a nitrate, then slowly adding concentrated sulfuric acid such that the acid forms a layer below the aqueous solution. A brown ring will form at the junction of the two layers, indicating the presence of the nitrate ion.^[2] Note that the presence of nitrite ions will interfere with this test.^[3]

The overall reaction is the reduction of the nitrate ion by iron(II) which is oxidised to iron(III) and formation of a nitrosonium complex where nitric oxide is oxidised to NO⁺.

NO₃⁻ + 3Fe²⁺ + 4H⁺ → 3Fe³⁺ + NO + 2H₂O

[Fe(H₂O)₆]²⁺ + NO → [Fe(H₂O)₅(NO)]²⁺ + H₂O

NITRATE

The anion is the conjugate base of nitric acid, consisting of one central nitrogen atom surrounded by three identically bonded oxygen atoms in a trigonal planar arrangement. The nitrate ion carries a formal charge of -1. This results from a combination formal charge in which each of the three oxygens carries a −²⁄₃ charge, whereas the nitrogen carries a 1+ charge, all these adding up to formal charge of the polyatomic nitrate ion. This arrangement is commonly used as an example of resonance. Like theisoelectronic carbonate ion, the nitrate ion can be represented by resonance structures:

conformation test

I₃-

When it reacts with the I₃- anion, the nitrite ion reduces itself to nitric oxide. The student should check the oxidation states of nitrogen in both the cases.

NO₂^- + 2I₃^- + 2H⁺ --> NO(g) + 3I₂(aq) + H₂O(l)

The yellow color of the I₃- ions changes to a greenish tinge which later turns violet. This observation is made in the presence of an oxidizing agent which oxidizes iodine from -⅓ to 0. The O.A. in our case is of course, the nitrite ion!

both nitrite and nitrate produces nitrous oxide because of this reason brown ring is preduced.