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Cation and anion are called basic and acidic radicals, respectively, because during salt formation cation comes from base and anion comes from acid.
$\text{ }{{\text{K}}_{\text{4}}}\left[ \text{Fe}{{\left( \text{CN} \right)}_{\text{6}}} \right]\text{ }$ Can be used to detect one or more out of $\text{ F}{{\text{e}}^{\text{2+}}}\text{ }$, $\text{ F}{{\text{e}}^{\text{3+}}}\text{ }$, $\text{ Z}{{\text{n}}^{\text{2+}}}\text{ }$,$\text{ C}{{\text{u}}^{\text{2+}}}\text{ }$and $\text{ C}{{\text{d}}^{\text{2+}}}\text{ }$
A)$\text{ F}{{\text{e}}^{\text{2+}}}\text{ }$, $\text{ F}{{\text{e}}^{\text{3+}}}\text{ }$only
B)$\text{ F}{{\text{e}}^{\text{3+}}}\text{ }$, $\text{ Z}{{\text{n}}^{\text{2+}}}\text{ }$,$\text{ C}{{\text{u}}^{\text{2+}}}\text{ }$ only
C) All but not $\text{ C}{{\text{d}}^{\text{2+}}}\text{ }$
D) All of these

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Last updated date: 13th Jun 2024
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Answer
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Hint: Potassium Ferrocyanide is an inorganic compound. The Ferrocyanide ion $\text{ }{{\left[ \text{Fe}{{\left( \text{CN} \right)}_{\text{6}}} \right]}^{\text{4}-}}\text{ }$ forms complex with transition metal ion. Metal ions or cations are basic radical reactions with acidic radicals. This results in the formation of salt. Most of the metal- Ferrocyanide complexes are coloured. However in some cases, metal forms a potassium metal Ferrocyanide complex . This is a colorless complex.

Complete step by step answer:
Potassium Ferrocyanide $\text{ }{{\text{K}}_{\text{4}}}\left[ \text{Fe}{{\left( \text{CN} \right)}_{\text{6}}} \right]\text{ }$ is an inorganic compound.it is potassium salt of coordinated to $\text{ }{{\left[ \text{Fe}{{\left( \text{CN} \right)}_{\text{6}}} \right]}^{\text{4}-}}\text{ }$complex.
Copper ion $\text{ C}{{\text{u}}^{\text{2+}}}\text{ }$ forms a chocolate brown colour precipitate with potassium ferrocyanide .the reaction between the $\text{ C}{{\text{u}}^{\text{2+}}}\text{ }$and potassium Ferrocyanide is given as follows,
$\text{ }\begin{matrix}
\text{C}{{\text{u}}^{2+}} & + & \text{ }{{\text{K}}_{\text{4}}}\left[ \text{Fe}{{\left( \text{CN} \right)}_{\text{6}}} \right]\text{ } & \to & \text{ C}{{\text{u}}_{2}}\left[ \text{Fe}{{\left( \text{CN} \right)}_{\text{6}}} \right]\text{ }\downarrow & + & \text{4}{{\text{K}}^{\text{+}}} \\
{} & {} & {} & {} & (\text{Chocolate-brown ppt)} & {} & {} \\
\end{matrix}$
Ferric chloride when treated with potassium ferrocyanide forms a coloured precipitate. The $\text{ F}{{\text{e}}^{\text{3+}}}\text{ }$ ion from ferric chloride displaced the primary covalent potassium ion from the potassium ferrocyanide complex. This reaction results in the ferric ferrocyanide complex.it is a Prussian blue colour complex. A reaction of formation of ferric ferrocyanide is as shown below,
$\text{ }\begin{matrix}
\text{4FeC}{{\text{l}}_{\text{3}}} & \text{+} & \text{ 3}{{\text{K}}_{\text{4}}}\left[ \text{Fe}{{\left( \text{CN} \right)}_{\text{6}}} \right]\text{ } & \to & \text{ F}{{\text{e}}_{\text{4}}}\left[ \text{Fe}{{\left( \text{CN} \right)}_{\text{6}}} \right]\text{ }\!\!\bar{\ }\!\!\text{ } & \text{+} & \text{12KCl} \\
\text{Ferric Chloride} & {} & {} & {} & \begin{align}
& \text{(Ferric ferrocyanide)} \\
& \text{ (Purssian blue)} \\
\end{align} & {} & {} \\
\end{matrix}$
Zinc ion $\text{ Z}{{\text{n}}^{\text{2+}}}\text{ }$ when treated with potassium Ferrocyanide form a colourless or white precipitate. The $\text{ Z}{{\text{n}}^{\text{2+}}}\text{ }$ ion from the zinc solution displaced the potassium ion from the potassium Ferrocyanide complex. This reaction results in the ferric zinc hexacyanoferrate complex .It is a white complex. A reaction of formation of the white complex is as shown below,
$\text{ }\begin{matrix}
\text{ 3Z}{{\text{n}}^{\text{2+}}}\text{ } & \text{+ 2}{{\text{K}}^{\text{+}}} & \text{ + 2}{{\left[ \text{Fe}{{\left( \text{CN} \right)}_{\text{6}}} \right]}^{\text{4}-}}\text{ } & \to & \text{ }{{\text{K}}_{\text{2}}}\text{Z}{{\text{n}}_{\text{3}}}\left[ \text{Fe}{{\left( \text{CN} \right)}_{\text{6}}} \right]\downarrow & {} & {} \\
{} & {} & {} & {} & \text{(white ppt)} & {} & {} \\
\end{matrix}\text{ }$
The white precipitate obtained has various compositions. the reaction takes place in excess of the reagent.
Ferrous chloride when treated with potassium Ferrocyanide forms a colourless precipitate. The $\text{ F}{{\text{e}}^{\text{2+}}}\text{ }$ ion from ferrous chloride displaced the potassium ion from the potassium Ferrocyanide complex. This reaction results in the mixed complex of $\text{ F}{{\text{e}}^{\text{2+}}}\text{ }$ion and potassium ion.it is a white complex. A reaction of formation of a white precipitate is as shown below,
$\text{ }\begin{matrix}
\text{ F}{{\text{e}}^{\text{2+}}}\text{ } & \text{+ 2}{{\text{K}}^{\text{+}}} & \text{ + }{{\left[ \text{Fe}{{\left( \text{CN} \right)}_{\text{6}}} \right]}^{\text{4}-}}\text{ } & \to & \text{ }{{\text{K}}_{\text{2}}}\text{Fe}\left[ \text{Fe}{{\left( \text{CN} \right)}_{\text{6}}} \right]\downarrow & {} & {} \\
{} & {} & {} & {} & \text{(white ppt)} & {} & {} \\
\end{matrix}\text{ }$
This precipitate is formed in the absence of air.
Similarly, cadmium ion $\text{ C}{{\text{d}}^{\text{2+}}}\text{ }$ reacts with potassium Ferrocyanide to form a cadmium ion coordinated to the Ferrocyanide complex. The reaction of the $\text{ C}{{\text{d}}^{\text{2+}}}\text{ }$ salt to the potassium Ferrocyanide is as shown below,
$\text{ C}{{\text{d}}^{\text{2+}}}\text{ + }{{\text{K}}_{\text{4}}}\left[ \text{Fe(CN}{{\text{)}}_{\text{6}}} \right]\text{ }\to \text{ Cd}{{\text{K}}_{\text{2}}}\left[ \text{Fe(CN}{{\text{)}}_{\text{6}}} \right]\text{+ 2}{{\text{K}}^{\text{+}}}$
It is a bluish-white colour precipitate.

Thus $\text{ F}{{\text{e}}^{\text{2+}}}\text{ }$, $\text{ F}{{\text{e}}^{\text{3+}}}\text{ }$, $\text{ Z}{{\text{n}}^{\text{2+}}}\text{ }$,$\text{ C}{{\text{u}}^{\text{2+}}}\text{ }$ and $\text{ C}{{\text{d}}^{\text{2+}}}\text{ } $are detected by potassium Ferrocyanide reagent.

Hence, (D) is the correct option.

Note: Iron from two types of the complex: Ferrocyanide and ferricyanide.many metals react with Ferrocyanide complex and form precipitate. Thus potassium Ferrocyanide is not a good separating reagent for the metal ion. Thus it is used commonly as the confirmatory test not as a separating reagent.