Potassium Ferricyanide

Potassium Ferricyanide K3[Fe(CN)6]

These micrographs are a bit different. Many metallographic etching solutions are a combination of solvent(s), an acid or base, and a salt. Murakami's reagent calls for these lovely crystals of potassium ferricyanide, potassium hydroxide, and DI water.

The translucent nature of these red-orange crystals allowed for utilizing transmitted illumination, which isn't something I commonly use in day-to-day metallography.

Potassium ferricyanide is readily dissolved into water and will react with concentrated strong acids, producing hydrogen cyanide gas - not something you want to breath in! Safely working with Murakami's reagent requires careful control of waste streams, among other things.

When potassium ferricyanide is oxidized it takes the form of Fe4[Fe(CN)6]3. This compound is commonly referred to as Prussian Blue, the first modern synthetic pigment. It is what makes blueprints, blue. When dissolved in water, potassium ferricyanide forms a fluorescent yellow-green solution. Quite the colorful compound!