# Brown ring from the anion detection of NO3-

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
• See this question and links therein. – Nilay Ghosh Nov 3 '17 at 11:19

## 2 Answers

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.

• 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 formed due to a complex formed by reaction between Fe+2 and NO3- .complex is of formula [Fe(H2O)5NO]SO4 .The ring is unstable because Fe is present in +1 oxidation state thats why H2SO4 is added slowly because it can oxides Fe and can break the ring

• This is wrong. See here. – Jan Nov 3 '17 at 13:49