Potassium ferricyanide is also one of two compounds present in ferroxyl indicator solution (along with phenolphthalein) which turns blue (Prussian blue) in the presence of Fe2+ ions, and which can therefore be used to detect metal oxidation that will lead to rust. It is possible to calculate the number of moles of Fe2+ ions by using a colorimeter, because of the very intense color of Prussian blue Fe4[Fe(CN)6]3.

SAFETY :

Potassium ferricyanide has very low toxicity, its main hazard being that it is a mild irritant to the eyes and skin. However, under very strongly acidic conditions, highly toxic hydrogen cyanide gas is evolved, according to the equation:

6 H+ + [Fe(CN)6]3− → 6 HCN + Fe3+[9]

The reaction with hydrochloric acid is as follows:

6 HCl + K3[Fe(CN)6] → 6 HCN + FeCl3 + 3 KCl