Potassium ferrocyanide
Potassium ferrocyanide (K4Fe(CN)6·3H2O), also known as yellow prussiate of potash, is a coordination compound forming lemon-yellow monoclinic crystals at room temperature and decomposing at its boiling point. It is insoluble in alcohol but a litre of water can dissolve just under three hundred grams of the crystals, and the solution can be reduced with acid to release cyanide gas. The resulting hydrogen cyanide (HCN) boils at 26°C and, being slightly lighter than air, quickly evaporates clear of the release point.
On February 20, 2002 four Moroccans were arrested while in possession of detailed maps of the United States embassy in Rome, the Rome water supply network, and four kilograms of potassium ferrocyanide.