The potassium ferrocyanide solution gives a prussian blue colour, when added to
(a) CoCl3
(b) FeCl2

Q: Why doesn't $\text{FeCl}_2$ give the Prussian blue color?

$\text{FeCl}_2$ contains $\text{Fe}^{2+}$ (ferrous) ions. When reacted with potassium ferrocyanide, it initially forms a white precipitate of $\text{K}_2\text{Fe}[\text{Fe}(\text{CN})_6]$, which slowly turns blue upon exposure to air due to oxidation. It doesn't give the immediate deep Prussian blue color that $\text{Fe}^{3+}$ does.

Q: What is the oxidation state of iron in the Prussian blue complex?

In the complex $\text{Fe}_4[\text{Fe}(\text{CN})_6]_3$, the iron atoms outside the coordination sphere are in the +3 oxidation state (Ferric), while the iron atoms inside the cyanide coordination sphere are in the +2 oxidation state (Ferrous).

Q: Is Turnbull's blue different from Prussian blue?

Historically, they were thought to be different. Turnbull's blue was made by reacting $\text{Fe}^{2+}$ with potassium ferricyanide, while Prussian blue was made from $\text{Fe}^{3+}$ and potassium ferrocyanide. However, modern techniques like X-ray diffraction have proven that they are chemically identical!

Q: Why is the Prussian blue color so intense?

The intense blue color is due to a phenomenon called 'intervalence charge transfer' (IVCT). Light energy causes an electron to jump from the $\text{Fe}^{2+}$ ion to the adjacent $\text{Fe}^{3+}$ ion. This absorption of red/orange light makes the compound appear deep blue.

Q: Can this test be used for other metals like Cobalt?

Potassium ferrocyanide does react with other metal ions, but it forms different colored precipitates. For example, with $\text{Cu}^{2+}$ it forms a chocolate brown precipitate, and with $\text{Zn}^{2+}$ it forms a white precipitate. The deep blue color is highly specific to Iron(III).

Question

The potassium ferrocyanide solution gives a prussian blue colour, when added to

Options:

(A) $	ext{CoCl}_3$

(B) $	ext{FeCl}_2$

(C) $	ext{CoCl}_2$

(D) $	ext{FeCl}_3$

Sigma Percentile · Accepted Answer

### The Detective Work of Chemistry

Imagine you are a chemical detective, and your job is to identify an unknown metal ion hiding in a clear solution. You have a toolkit of special reagents, and one of your most powerful tools is **Potassium ferrocyanide**, chemically known as $	ext{K}_4[	ext{Fe}(	ext{CN})_6]$. This compound is famous in qualitative analysis because it acts like a homing missile for certain transition metals, producing vivid, unmistakable colors when it finds its target.

### The Magic of Prussian Blue

When you add a few drops of potassium ferrocyanide to a solution containing **Ferric ions** ($	ext{Fe}^{3+}$), something spectacular happens. The solution instantly transforms, yielding a brilliant, deep blue precipitate. This iconic color is known as **Prussian Blue**.

Historically, Prussian blue was one of the first modern synthetic pigments, widely used in paints and even giving us the term "blueprint" from its use in architectural drawings. But chemically, what exactly is this beautiful substance?

### The Chemical Equation

Let's look at the exact chemical reaction that takes place. When Ferric chloride ($	ext{FeCl}_3$) reacts with Potassium ferrocyanide, they undergo a double displacement reaction:

$$4	ext{FeCl}_3 + 3	ext{K}_4[	ext{Fe}(	ext{CN})_6] \longrightarrow 	ext{Fe}_4[	ext{Fe}(	ext{CN})_6]_3 + 12	ext{KCl}$$

The resulting complex, $	ext{Fe}_4[	ext{Fe}(	ext{CN})_6]_3$, is formally called **Iron(III) hexacyanoferrate(II)**. Notice the oxidation states: the iron atoms outside the coordination sphere are in the +3 state, while the iron atom trapped inside the cyanide ligands is in the +2 state. The intense blue color arises from an electron jumping back and forth between these two different iron atoms—a phenomenon known as intervalence charge transfer.

### Finding the Culprit

Now, let's evaluate our options to see which one can produce this magical reaction. We need a source of $	ext{Fe}^{3+}$ ions.

- **$	ext{CoCl}_3$ and $	ext{CoCl}_2$**: Cobalt salts react with ferrocyanide to form greenish or grayish precipitates, not Prussian blue.
- **$	ext{FeCl}_2$**: This contains Ferrous ($	ext{Fe}^{2+}$) ions. Reacting it with ferrocyanide initially forms a white precipitate, which only turns blue slowly as it oxidizes in the air.
- **$	ext{FeCl}_3$**: This is Ferric chloride. It provides the exact $	ext{Fe}^{3+}$ ions required to instantly form the deep Prussian blue precipitate.

Therefore, the correct answer is undoubtedly **$	ext{FeCl}_3$**.

Animated Solution for Chemistry - Coordination Compounds: The potassium ferrocyanide solution gives a prussian blue colour, when added to

Select Answer:

Visualized Solution

$Identifying the Reagent$

$The Prussian Blue Test$

$The Chemical Reaction$

$Analyzing the Options$

The Sigma Insight: Nomenclature, Isomerism, Importance and Werner's Theory

The Detective Work of Chemistry

The Magic of Prussian Blue

The Chemical Equation

Finding the Culprit

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