Question

The ${{\text{K}}_{4}}\left[ \text{Fe}{{\left( \text{CN} \right)}_{6}} \right]$ can be used to detect.
A.$\text{F}{{\text{e}}^{2+}},\text{ F}{{\text{e}}^{3+}}$
B.$\text{F}{{\text{e}}^{3+}},\text{ Z}{{\text{n}}^{2+}},\text{ C}{{\text{u}}^{2+}}$
C.$\text{F}{{\text{e}}^{2+}},\text{F}{{\text{e}}^{3+}},\text{Z}{{\text{n}}^{2+}},\text{ C}{{\text{u}}^{2+}}$
D.All of the above.

Answer 1

Hint: ${{\text{K}}_{4}}\left[ \text{Fe}{{\left( \text{CN} \right)}_{6}} \right]$ = full name of compound is potassium Ferrocyanide or potassium hexacyanoferrate (11). On a large scale it is prepared by coal gas which contains $0\cdot 2$ percent of hydrogen cyanide as an impurity. Further this gas passed through a washer containing iron (11) sulphate and potassium hydroxide. The reaction takes place as follows.
$\begin{align}
  & \text{HCN}+\text{KOH}\to \text{ KCN}+{{\text{H}}_{2}}\text{O} \\
 & \text{6 KCN}+\text{FeS}{{\text{O}}_{4}}\to \text{ }{{\text{K}}_{4}}\left[ \text{Fe}{{\left( \text{CN} \right)}_{6}} \right]+{{\text{K}}_{2}}\text{S}{{\text{O}}_{4}} \\
\end{align}$

Complete step by step answer:
Potassium Ferrocyanide used in analytical chemistry to detect the ions of several metal in unknown compound these ions are mainly $\text{F}{{\text{e}}^{2+}}$ ,$\text{F}{{\text{e}}^{3+}}$,$\text{Z}{{\text{n}}^{2+}}$ ,$\text{C}{{\text{u}}^{2+}}$
For the detection of $\text{F}{{\text{e}}^{2+}}$ : When unknown compound which contains $\text{F}{{\text{e}}^{2+}}$ ion react with Potassium Ferro cyanide to form white ppt.
$\text{F}{{\text{e}}^{2+}}+\text{2}{{\text{K}}^{+}}{{\left[ \text{Fe}{{\left( \text{CN} \right)}_{6}} \right]}^{4}}\to \text{ }{{\text{K}}_{2}}\text{Fe}\left[ \text{Fe}{{\left( \text{CN} \right)}_{6}} \right]\downarrow $ White ppt.
For the detection of $\text{F}{{\text{e}}^{3+}}$ : When unknown compound containing $\text{F}{{\text{e}}^{3+}}$ions react with Potassium Ferro cyanide to form Prussian blue ppt.
$4\text{F}{{\text{e}}^{3+}}+3{{\text{K}}_{4}}\left[ \text{Fe}{{\left( \text{CN} \right)}_{6}} \right]\to \text{ F}{{\text{e}}_{4}}{{\left[ \text{Fe}{{\left( \text{CN} \right)}_{6}} \right]}_{3}}+12{{\text{K}}^{+}}$
For the detection of $\text{C}{{\text{u}}^{2+}}$ : For the detection of $\text{C}{{\text{u}}^{2+}}$ ion in unknown compound, it combines with cupric salts to form reddish brown ppt of cupric Ferrocyanide.
$2\text{C}{{\text{u}}^{2+}}+{{\text{K}}_{4}}\left[ \text{Fe}{{\left( \text{CN} \right)}_{6}} \right]\to 4{{\text{K}}^{+}}+\text{C}{{\text{u}}_{2}}\left[ \text{Fe}{{\left( \text{CN} \right)}_{6}} \right]$ Reddish Brown ppt
For the detection of $\text{Z}{{\text{n}}^{2+}}$ Ions: In order to detect $\text{Z}{{\text{n}}^{2+}}$ ions in unknown salt potassium Ferro cyanide react with Zn compound to form white ppt
\[\text{Z}{{\text{n}}^{2+}}+2{{\text{K}}^{+}}{{\left[ \text{Fe}{{\left( \text{CN} \right)}_{6}} \right]}^{4-}}\to \text{ }{{\text{K}}_{2}}\text{Zn}\left[ \text{Fe}{{\left( \text{CN} \right)}_{6}} \right]\downarrow \] White ppt

Hence Option-D is correct.

Note:
Noted that in two cases in the detection of $\text{Z}{{\text{n}}^{2+}}$ and $\text{F}{{\text{e}}^{2+}}$ some color ppt formed. In order to find out which of salt whether it is $\text{F}{{\text{e}}^{2+}}$or$\text{Z}{{\text{n}}^{2+}}$ we will further done confirmatory test iron and zinc.
Respect to that potassium Ferrocyanide also used to detect the calcium ion.
$\text{C}{{\text{a}}^{2}}+{{\text{K}}_{4}}\left[ \text{Fe}{{\left( \text{CN} \right)}_{6}} \right]\to \text{ }{{\text{K}}_{4}}\text{Ca}\left[ \text{Fe}{{\left( \text{CN} \right)}_{6}} \right]$