Potassium ferricyanide(II) 5% (blood leach salt yellow) is a chemical compound used in histology and pathology to identify and visualize iron deposits in tissues. The solution is often used in the Perls' Prussian Blue reaction, where it is mixed with a hydrochloric acid solution.

First, the tissue is treated with a hydrochloric acid solution to oxidize the iron stored in the tissue to its trivalent form (Fe3+). Then, the tissue is treated with the potassium hexacyanoferrate(II) solution. The reaction between Fe3+ and potassium ferricyanide(II) leads to the formation of insoluble Berlin blue, which adheres to the iron deposits in the tissues. The blue deposits then allow microscopic assessment of iron storage in tissues.

The use of potassium ferricyanide(II) 5% in combination with hydrochloric acid provides a specific and sensitive method for visualizing iron deposits, which may be important in the diagnosis of diseases such as hemochromatosis, hemolysis, and sideroblastic anemia.