有關化學的問題

"Brown ring test" is used to test for aqueous nitrate ion.

Procedure:
Equal volumes of the unknown solution and dilute FeSO4 solution are mixed in a test tube. Then, the test tube its tilted, concentrated sulphuric acid is slowly added along the inner wall until a layer of concentrated sulphuric acid is formed. The test tube is then allowed to stand for a few minuites.

Results:
If the unknown contains nitrate ion, a brown ring is formed at the boundary of the two layers.

Explanation:
Consider the boundary. In the presence of acidic solution, nitrate ion acts as an oxidizing agent to oxidized some of the Fe2+ ions, and some of the nitrate ions are reduced to NO gas.
8H+(aq) + 2NO3-(aq) + 3Fe2+(aq) → 4H2O(l) + 2NO(g) + 3Fe3+(aq)
NO(g) reacts with FeSO4 solution to form a brown complex.
FeSO4 + NO → FeSO4•NO
=

Brown Ring Test can be used to test nitrate. Add FeSO4 to the sample and then run in concentrated H2SO4 slowly to form a bottom layer. A brown ring forms in present of nitrate.
Equations:


NO3- + H2SO4 HNO3 + HSO4-
HNO3 + 3Fe2+ + 3H+ 2H2O + NO + 3Fe3+
FeSO4 + NO FeSO4 ．NO (brown complex)


2008-12-27 15:48:00 補充：
NO3- + H2SO4 ----> HNO3 + HSO4-


HNO3 + 3(Fe2+) + 3H+ ----> 2H2O + NO + 3Fe3+


FeSO4 + NO -------> FeSO4 ．NO (brown complex)