Brown ring from the anion detection of NO3-

Question

Today in our practicals I observed that when I did the wet test for nitrate (Making a 1 ml solution of salt solution+$\ce{FeSO4}$ solution+hot concentrated $\ce{H2SO4}$); The brown ring appeared for a very short time and then disappeared.

But later on I came to know that it was nitrate only. What may be the possible reason for this temporary appearance of a tinge of a brown colour and then its sudden disappearance?

Probably transient formation of $\ce{NO2}$. – Zhe Oct 26 '17 at 13:21 — Zhe, Oct 26 '17 at 13:21
See this question and links therein. – Nilay Ghosh Nov 03 '17 at 11:19 — Nilay Ghosh, Nov 03 '17 at 11:19

score 4 · Answer 1 · edited Oct 26 '17 at 16:45

4

The brown ring formed in the confirmatory test for the nitrate ion is due to the complex called nitroso ferrous sulphate, or pentaaqua nitrosyl iron (II) sulfate with the formula $\ce{[Fe(H2O)5NO]SO4}$. The concentrated sulfuric acid decomposes this complex and brown ring disappers in a few seconds when $\ce{H2SO4}$ moves to the upper layer. This is why slow addition of the acid (through the sides of the test tube) is advised.

edited Oct 26 '17 at 16:45

looneysnoop

90
9

answered Oct 26 '17 at 16:22

Pauling0304

448
2
11

So you mean that excess of concentrated H2SO4 makes brown ring disappear. – Pole_Star Oct 26 '17 at 16:28
But if we add excess then it turns black... – Pole_Star Oct 26 '17 at 16:32
I tried it out too – Pole_Star Oct 26 '17 at 16:33

Brown ring from the anion detection of NO3-

1 Answers1