Note:  Do not rely on this information. It is very old.

Prussian Blue

Prussian Blue. When solutions of ferric salt and yellow prussiate of potash are mixed together, a heavy dark-blue precipitate is formed known as Prussian blue, and employed largely as a pigment. Before being used for this purpose, the precipitate is washed well with water and a dilute acid. It has the composition represented by the formula Fe7(CN)18, but its exact nature has yet to be determined. It dissolves readily in oxalic acid, yielding a liquid employed as a blue ink. A compound obtained from red prussiate and a ferrous salt is very similar if not identical, and is known as Turnbull's blue.