Turnbull's Blue stain detects which iron species?

• A. Ferrous iron
• B. Ferric iron
• C. Ferric oxide
• D. Ferritin-bound iron

Answer: (A)

Turnbull's blue staining is specific for ferrous iron (Fe2+). In this reaction, ferrous iron reduces potassium ferricyanide to ferrocyanide, producing a blue precipitate known as Turnbull's blue. Ferric iron (Fe3+), which is the common form in ferritin and hemosiderin, does not produce this blue under the Turnbull method, so it isn’t detected by this stain. For ferric iron, the Prussian blue (Perls) stain is used, which forms ferric ferrocyanide and reflects Fe3+. In short, a blue product from Turnbull's blue indicates Fe2+ rather than Fe3+.