Animated Solution for Chemistry - Salt Analysis: Comprehension Passage

Let's dive into the fascinating world of coordination chemistry and salt analysis. The passage begins by describing the reaction of potassium ferrocyanide, ${\text{K}}_4[\text{Fe}(\text{CN}{)}_6]$, with a freshly prepared ferrous sulfate (${\text{FeSO}}_4$) solution.

When this reaction occurs in the complete absence of air, a double displacement-like process takes place. The ${\text{Fe}}^{2+}$ ions from the ferrous sulfate replace two of the ${\text{K}}^{+}$ ions in the coordination complex. This results in the formation of a white precipitate, which the problem labels as $\mathbf{X}$. The chemical equation for this transformation is:

Thus, we can confidently identify precipitate $\mathbf{X}$ as potassium ferrous ferrocyanide, ${\text{K}}_2\text{Fe}[\text{Fe}(\text{CN}{)}_6]$.

But the story doesn't end there. The passage notes that this white precipitate turns blue when exposed to air. Why does this happen?

The oxygen present in the air acts as an oxidizing agent. It specifically targets the iron(II) ions located outside the coordination sphere, oxidizing them to iron(III) ions. This oxidation transforms the white ${\text{K}}_2\text{Fe}[\text{Fe}(\text{CN}{)}_6]$ into the famous Prussian Blue, ${\text{Fe}}_4[\text{Fe}(\text{CN}{)}_6{]}_3$. The deep blue color arises from an intervalence charge transfer between the ${\text{Fe}}^{2+}$ and ${\text{Fe}}^{3+}$ centers, confirming our initial identification of $\mathbf{X}$.

Moving to the second half of the passage, we encounter one of the most visually striking experiments in chemistry: the Brown Ring Test for nitrate ions (${\text{NO}}_3^{-}$).

The setup involves mixing a nitrate-containing solution with freshly prepared ferrous sulfate. Then comes the critical step: concentrated sulfuric acid (${\text{H}}_2{\text{SO}}_4$) is poured very slowly down the inner side of the test tube. Because concentrated sulfuric acid is highly dense, it sinks to the bottom without mixing, creating two distinct liquid layers.

At the exact junction where the dense acidic layer meets the lighter aqueous layer, a powerful redox reaction is triggered. The strongly acidic medium allows the ${\text{Fe}}^{2+}$ ions to reduce the ${\text{NO}}_3^{-}$ ions into nitric oxide ($\text{NO}$) gas, while the ferrous ions themselves are oxidized to ferric (${\text{Fe}}^{3+}$) ions:

The newly formed nitric oxide doesn't just bubble away. It immediately reacts with the excess hydrated ferrous ions, $[\text{Fe}({\text{H}}_2\text{O}{)}_6{]}^{2+}$, present in the upper layer. A ligand substitution occurs where one water molecule is replaced by the $\text{NO}$ ligand:

This resulting complex, pentaaquanitrosyliron(I), is responsible for the iconic brown ring that forms precisely at the liquid junction. A crucial detail to remember for JEE is that the oxidation state of iron in this specific complex is +1, as the $\text{NO}$ ligand exists as the nitrosonium ion, ${\text{NO}}^{+}$.

By piecing together these chemical clues, we find that precipitate $\mathbf{X}$ is ${\text{K}}_2\text{Fe}[\text{Fe}(\text{CN}{)}_6]$ and the brown ring complex is $[\text{Fe}({\text{H}}_2\text{O}{)}_5(\text{NO}){]}^{2+}$.