Prep. By dissolving a quarter of an ounce of ferrocyanide of potassium (crystals) in five fluid ounces of distilled water.

Use. Ferrocyanide of potassium forms insoluble precipitates with many of the metals, and is used as a test for their presence; the colour of the precipitate is sometimes sufficient to indicate the nature of the metal present in solution. Thus it gives a blue precipitate with the persalts of iron, a reddish-brown one with those of copper, and a white or nearly white precipitate with protosalts of iron, manganese, zinc, tin, cadmium, lead, bismuth, antimony, mercury, and silver.