Ferric chloride 29 %

One application of 29% ferric chloride solution in histology is the Perl's Prussian blue reaction, a staining method for the detection of iron deposition, particularly hemosiderin, in tissue specimens. Hemosiderin is a breakdown product of hemoglobin and occurs in certain diseases such as hemochromatosis, hemolysis, or bleeding within tissues.
In the Perl's Prussian-Blue reaction, a solution of potassium ferricyanide(III) (also known as potassium ferricyanide) and an acid, usually hydrochloric acid, is applied to the tissue. The 29% ferric chloride solution is then added. The ferric chloride reacts with the potassium ferricyanide in the presence of the stored iron in the hemosiderin granules, producing an insoluble, blue-colored product known as Turnbull blue or Berlin blue. These blue deposits can then be identified and localized under the microscope to assess the degree and distribution of iron deposits in the tissue under examination.

Another application is in metallography the use as a non-copper etchant for the detection of phosphorus segregation (for steels up to 0.5%C), grain area etchant, grain figure and hatch etch for mild steels, for pretreatment for the detection of force action figures, change detection.