Hydrogen cyanide, originally called Prussic acid, is a notorious poisonous gas. It was used by Nazi Germany in extermination camps, and it is used to carry out the death penalty in some US states. Its 26 °C boiling point makes it easy to liquefy. In aqueous solution, HCN is called hydrocyanic acid, but its 9.2 pKamakes it a very weak acid. HCN is a valuable feedstock in industrial processes, and the cyanide anion is a widely used ligand in metal complexes.
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