1752

Hydrogen Cyanide

Pierre Macquer (1718–1784), Carl Wilhelm Scheele (1742–1786), Claude-Louis Berthollet (1748–1822), Joseph-Louis Gay-Lussac (1778–1850)

Prussian blue was still at the cutting edge of chemistry throughout the eighteenth century, as its precise composition was a mystery. Then, in 1752, French chemist Pierre Macquer found that it could be broken down into iron salts and some sort of volatile gas, and that the process could be run in reverse to generate the pigment again. But what was that volatile material? Swedish chemist Carl Wilhelm Scheele had the good (or bad) fortune of working that one out: He reacted Prussian blue with sulfuric acid, noting that it produced a “strong, peculiar, and not unpleasant odor.” A modern chemist would have told him to run for the door (or dive out the window), but he went on to actually taste the stuff (it was slightly sweet and gave a sensation of heat on the tongue). Scheele was describing famously poisonous hydrogen cyanide, and he was lucky to live to tell anyone about it.

The cyanides in Prussian blue are complexed (arranged in space and pointing inward) very tightly to iron atoms, and given the chance, they will do the same to the iron atoms found in the hemoglobin molecules of red blood cells—a reaction that makes them useless for carrying oxygen. Despite its dangers, hydrogen cyanide helped advance chemistry quite a bit. It is a weak acid, since in water it partly dissociates into H+ and CN−, and the presence of those H+ ions makes it an acid. At the time, though, it was believed that all acids had to contain oxygen in their formulae, the way sulfuric and nitric acids do. However, in 1787, prussic acid (named for its source of Prussian blue) was shown by French chemist Claude-Louis Berthollet to be oxygen-free, and in 1815 his fellow Frenchman Joseph-Louis Gay-Lussac worked out its precise chemical formula: HCN. Once cyanide ions were recognized as a separate species, the name for them suggested itself immediately, as cyan is the Greek word for “blue.”

A statue of Scheele in Stockholm, erected in 1892. The tradition of decorating parks with statues of chemists should perhaps be revived.