Question

Why potassium ferricyanide cannot be used as internal indicator?

Answer 1

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.

Answer 2

When we use Potassium Ferricyanide as an internal indicator, it reacts with $Fe^{2+}$ ions and gives us a blue color which will lead to rusting of metal.

• Potassium Ferricyanide consists of ferroxyl indicator and phenolphthalein which turns blue when reacted with $Fe^{2+}$ and it leads to oxidation of metal resulting in rusting of the metal.
• An internal indicator must not react with ferrous iron during titration instead there should only be a color change at the exact point when the ferrous ion is converted to ferric, but Potassium Ferricyanide does react therefore it cannot be used as an internal indicator.