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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?

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  • $\begingroup$ Probably transient formation of $\ce{NO2}$. $\endgroup$
    – Zhe
    Commented Oct 26, 2017 at 13:21
  • $\begingroup$ See this question and links therein. $\endgroup$
    – Nilay Ghosh
    Commented Nov 3, 2017 at 11:19

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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.

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  • $\begingroup$ So you mean that excess of concentrated H2SO4 makes brown ring disappear. $\endgroup$
    – Pole_Star
    Commented Oct 26, 2017 at 16:28
  • $\begingroup$ But if we add excess then it turns black... $\endgroup$
    – Pole_Star
    Commented Oct 26, 2017 at 16:32
  • $\begingroup$ I tried it out too $\endgroup$
    – Pole_Star
    Commented Oct 26, 2017 at 16:33

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