Question

Write the reactions involved and procedure in brief to detect Nitric acid by brown ring test.

Answer 1

Brown ring test

Take aqueous solution of a Nitrate or Nitric acid in a test tube.

(a) Add freshly prepared saturated solution of Iron sulphate.

(b) Now add concentrated Sulphuric acid carefully from the sides of the test tube, such that t should not fall dropwise in the test tube.

(c) Cool the test tube in water.

(d) A brown ring appears at the junction of two liquids.

Reaction:

$6{\mathrm{FeSO}}_4\left(\mathrm{s}\right)+3{\mathrm{H}}_2{\mathrm{SO}}_4\left(\mathrm{aq}\right)+2{\mathrm{HNO}}_3\left(\mathrm{aq}\right)\to 3{\mathrm{Fe}}_2{\left({\mathrm{SO}}_4\right)}_3\left(\mathrm{s}\right)+4{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)+2\mathrm{NO}\left(\mathrm{g}\right)$

(Ferrous sulphate) (Sulphuric acid) (Nitric acid) (Iron (lll)sulpahte) (Water) (Nitric oxide)

${\mathrm{FeSO}}_4\left(\mathrm{aq}\right)+\mathrm{NO}\left(\mathrm{g}\right)\to {\mathrm{FeSO}}_4.\mathrm{NO}\left(\mathrm{s}\right)$

(Ferrous sulphate) (Nitric oxide) (Nitroso ferrous sulphate)

Hence we can say that brown ring test is performed to detect Nitric acid and Nitrates. And it form brown ring on addition of Sulphuric acid and Ferrous sulphate to Nitric acid. This brown ring formed is Nitroso ferrous sulphate.