Another important class of carbon compounds consists of the cyanides, which contain the anion group C N: Cyanide ions are extremely toxic because they bind almost irreversibly to the Fe(Ill) ion in cytochrome oxidase, a key enzyme in metabolic processes. Hydrogen cyanide, which has the aroma of bitter almonds, is even more dangerous because of its volatility (b.p. 26 C). A few tenths of l percent by volume of HCN in air can cause death within minutes. Hydrogen cyanide can be prepared by treating sodium cyanide or potassium cyanide with acid. Because HCN (in solution, called hydrocyanic acid) is a very weak acid (Ka 4.9 x 10 most of the HCN produced in this reaction is in the nonionized form and leaves the solution as hydrogen cyanide gas. For this reason acids should never be mixed with metal cyanides in the laboratory without proper ventilation