A Short History of Hydrogen Sulfide
From the sewers of Paris to physiological messenger
Chemical Similarities Between Cyanide and Sulfide
In addition to the similarities in their physiological effects, cyanide and sulfide have chemical similarities. The undissociated forms of both hydrogen cyanide and hydrogen sulfide are flammable, volatile gases. Hydrogen sulfide (whose vapor density, or d, is 1.19) is heavier than air (d = 1.0) whereas hydrogen cyanide (d = 0.941) is lighter. Both are weak acids with acid dissociation constants (pKa) that are of some physiological significance: hydrogen cyanide 9.2–9.3 and hydrogen sulfide 7.04. Both form salts with sodium and potassium as well as with some alkaline earths. Both anions bind to methemoglobin as noted above, and each of those complexes has its distinct visible absorption spectrum. And both are inhibitors of cytochrome c oxidase, the terminal enzyme in the electron transport chain that reacts with molecular oxygen in aerobic organisms. Blockage of that key enzyme is believed to be the mechanism of action in the rapidly lethal form of cyanide and sulfide poisonings.