Potassium ferrocyanide
Potassium ferrocyanide (K4Fe(CN)6·3H2O), also known as yellow prussiate of potash or potassium hexacyanoferrate(II), 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 react with acid to release hydrogen cyanide gas. The resulting hydrogen cyanide (HCN) boils at 26 °C and, being slightly lighter than air, quickly evaporates clear of the release point.
Potassium ferrocyanide is also one of two compounds present in ferroxyl indicator solution (along with phenolphthalein) which turns blue (Prussian blue) in the presence of Fe3+ ions, and which can therefore be used to detect rust. It is possible to calculate the number of moles of Fe3+ ions by using a colorimeter, because the very intense color of Prussian blue Fe4[Fe(CN)6]3.
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.
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