An aqueous solution of a salt containing a cation Z^ n+ reacts with...

In the qualitative analysis of Group III cations, the $Fe^{3+}$ ion reacts with potassium hexacyanoferrate(II), $K_{4}[Fe(CN)_{6}]$, to form a characteristic Prussian blue precipitate.

The chemical reaction is given by:
$4Fe^{3+} + 3K_{4}[Fe(CN)_{6}] \rightarrow Fe_{4}[Fe(CN)_{6}]_{3} \downarrow + 12K^{+}$

The Prussian blue precipitate is Iron(III) hexacyanoferrate(II), which has the formula $Fe_{4}[Fe(CN)_{6}]_{3}$.

Answer: $Fe^{3+}$ and $Fe_{4}[Fe(CN)_{6}]_{3}$