A brown ring is formed in the ring test for NO3- ion. It is due to the formation of:

The nitrate ion brown ring test is a qualitative test that verifies the presence of nitrate ion. A freshly produced ferrous sulphate solution is added to a solution containing nitrate ion in the ring test for NO3 ion. A ring is produced, containing a brown-colored compound. The existence of nitrate ion is confirmed by the creation of this brown-colored complex. The complex is brown in colour [Fe(H2O)5NO]SO4 .

NO3 + 3Fe2+ + 4H+ → NO + 3Fe3+ +2 H2O

[Fe(H2O)6]2+ + NO → [Fe(H2O)5NO]SO4 + H2O Ferrous ions react with nitrate ions in the presence of protons to create ferric ions and nitric oxide in the first stage. Nitric oxide combines with hydrated ferrous ions in the second stage to generate a brown ring complex. The oxidation state of nitrogen drops by 3 in the first stage. The oxidation status of ferrous ions does not change in the second phase of the process.

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