Animated Solution for Chemistry - Salt Analysis: The correct option(s) to distinguish nitrate salts of ${\text{Mn}}^{2+}$ and ${\text{Cu}}^{2+}$ taken separately is (are) :-

Welcome to the fascinating world of qualitative salt analysis! Imagine you are a chemical detective, and you are handed two test tubes. One contains a solution of Copper(II) nitrate, and the other contains Manganese(II) nitrate. Both are clear, aqueous solutions, and your mission is to definitively identify which is which.

This is not just a theoretical exercise; it is a classic laboratory puzzle that tests your understanding of solubility products, the common ion effect, and electrochemistry. Let's break down the given options and see how we can solve this mystery.

A Deceptive Glow

Our first instinct might be to perform a simple flame test. When we introduce a metal salt into a hot flame, the electrons get excited to higher energy levels. As they fall back down, they emit light of characteristic wavelengths.

For Copper (${\text{Cu}}^{2+}$), the flame typically burns with a beautiful blue-green hue. However, Manganese (${\text{Mn}}^{2+}$) can also impart a yellowish-green color to the flame. Because both ions produce a greenish tint, relying solely on the flame test to distinguish them is risky and inconclusive. Therefore, the flame test is not our best tool here.

Now, let's dive into the core of qualitative analysis. The separation of cations into different groups is based on the solubility of their salts, particularly their sulfides.

Copper (${\text{Cu}}^{2+}$) is a Group II cation, while Manganese (${\text{Mn}}^{2+}$) belongs to Group IV. The reagent used to separate these groups is Hydrogen Sulfide (${\text{H}}_2\text{S}$) gas. But there is a catch: the pH of the medium changes everything!

The Common Ion Effect

When we pass ${\text{H}}_2\text{S}$ gas through the solutions in an acidic medium (usually acidified with dilute $\text{HCl}$), we invoke the powerful Common Ion Effect.

Hydrogen sulfide is a weak diprotic acid:

In an acidic medium, the high concentration of ${\text{H}}^{+}$ ions from the $\text{HCl}$ pushes the equilibrium to the left, drastically reducing the concentration of sulfide (${\text{S}}^{2-}$) ions.

This low concentration of sulfide ions is sufficient to exceed the extremely low solubility product ($K_{sp}$) of Copper Sulfide ($\text{CuS}$), causing it to precipitate as a dense black solid. However, it is not enough to exceed the higher $K_{sp}$ of Manganese Sulfide ($\text{MnS}$). Thus, Manganese remains happily dissolved in the solution.

This is a perfect way to distinguish them! Copper precipitates, Manganese does not.

Opening the Floodgates

What happens if we change the rules and use a faintly basic medium instead?

In a basic medium, the hydroxide (${\text{OH}}^{-}$) ions react with the ${\text{H}}^{+}$ ions produced by the dissociation of ${\text{H}}_2\text{S}$, forming water. This removes ${\text{H}}^{+}$ from the system, pulling the equilibrium to the right and generating a massive amount of sulfide (${\text{S}}^{2-}$) ions.

With such an abundance of sulfide ions, the solubility products of both $\text{CuS}$ and $\text{MnS}$ are easily exceeded. Both ions will precipitate out of the solution. Since both form precipitates, we are back to square one—we cannot distinguish them this way.

The Ultimate Decider

Finally, let's step out of the test tube and look at the fundamental electrochemical properties of these metals. The Standard Reduction Potential (SRP) tells us how eagerly an ion wants to gain electrons and reduce to its metallic state.

For Copper:

For Manganese:

Copper has a positive reduction potential, meaning it is easily reduced (it is a noble metal compared to hydrogen). Manganese, on the other hand, has a highly negative reduction potential, meaning it prefers to stay oxidized as an ion.

This massive difference in their reduction potentials is a definitive, measurable property that perfectly distinguishes the two metals.

By carefully analyzing the chemical and physical properties of Copper and Manganese, we have successfully solved the puzzle. The flame test is too ambiguous, and a basic medium precipitates both. However, passing ${\text{H}}_2\text{S}$ in an acidic medium selectively precipitates Copper, and their standard reduction potentials are vastly different.

This is the beauty of chemistry—every element has its own unique fingerprint if you know exactly how to look for it!