Question

Explain the brown ring test with the help of the chemical equation.

Answer 1

Hint: The molecular formula of nitric acid is \[\text{N}{{\text{O}}_{3}}\]. The compound is highly corrosive and can be detected through the organic tests in the laboratory. Aqua fortis is another name for the nitric acid.

Complete answer:
To detect the presence of the nitric oxide, the brown ring test is conducted.
Firstly, we have to take a test tube in which a solution of nitric acid is present. Now, we will add a solution of iron (III) sulphate in it. After that add a concentrated sulphuric acid so that it will form a layer below the aqueous solution. After adding the concentrated sulphuric acid, we will see the formation of a brown ring separating the two layers.
In this test, the reduction of nitric acid and oxidation of iron (III) takes place.
The balanced chemical reaction is:

\[\text{2HN}{{\text{O}}_{3}}\text{ + 3}{{\text{H}}_{2}}\text{S}{{\text{O}}_{4}}\text{ + 6FeS}{{\text{O}}_{4}}\to \text{ 3F}{{\text{e}}_{2}}{{\left( \text{S}{{\text{O}}_{4}} \right)}_{3}}\text{ + 2NO + 4}{{\text{H}}_{2}}\text{O}\]

Here, the oxidation state of nitrogen changes from +5 to +2 which indicates that the nitrogen is reduced. Also, in this reaction nitrate ion acts as an oxidising agent. Whereas the oxidation state of iron changes from +2 to +3 which indicates that the iron is oxidised.

Note: The brown ring which is formed is made up of the nitrosonium complex. The layer is made between the two layers i.e. one layer is of iron sulphate solution and the sample whereas another one is of concentrated sulphuric acid. The test helps determine the quality of food because if the amount of nitric acid is more than 10 milligrams then it is toxic for us.