Prussian blue is a complex compound that has the formula $ \text{Fe}_{4}[\text{Fe(CN)}_{6}]_{3} $. It is formed by the reaction of ferric ions with ferrocyanide ions.

The reaction can be explained as follows:

1. Potassium ferrocyanide, $ \text{K}_{4}[\text{Fe(CN)}_{6}] $, dissociates in water to release ferrocyanide ions, $ [\text{Fe(CN)}_{6}]^{4-} $.




2. The ferric ions, $ \text{Fe}^{3+} $, present in the acidic ferric chloride solution will react with the ferrocyanide ions.




3. In an acidic medium, the ferric ions hydrolyze, forming a complex with ferrocyanide ions, leading to the formation of Prussian blue.

The overall reaction can be represented as:

$ \text{Fe}^{3+} + 4[\text{Fe(CN)}_{6}]^{4-} \rightarrow \text{Fe}_{4}[\text{Fe(CN)}_{6}]_{3}$

The Prussian blue complex is a colloidal species and exhibits an intense blue color. It has been historically used as a pigment in paints and also has applications in medical treatments as an antidote for certain types of heavy metal poisoning.