1746

Sulfuric Acid

John Roebuck (1718–1794), Peregrine Phillips (1800–1888)

Sulfuric acid is a workhorse indeed. As a key component in many industrial processes, it has been a valuable commodity for centuries, but it’s not something you’ll find springing up out of the ground (fortunately); it has to be made on an industrial scale. The techniques required to create sulfuric acid—known to medieval alchemists as oil of vitriol—have always had the same last step: dissolving sulfur trioxide in water. But reaching the sulfur trioxide stage has been the tricky part . . . even though it involves nothing more than burning sulfur.

In earlier days, just producing equipment that could handle being soaked in sulfuric acid was a challenge. Five hundred years ago, sulfur was burned while suspended over water in glass jars, but that set limits on how much acid could be made per batch. Large glass vessels were not easy to come by and were unpredictably fragile—not a desirable property for vessels full of corrosive acid—but in 1746, English industrialist John Roebuck invented a better manufacturing process. He realized that lead was resistant to sulfuric acid, so he introduced large lead vessels, which produced ten times more acid than the glass ones. On any scale, though, this method needed several repeated doses of burning sulfur, and the resulting sulfuric acid had to be concentrated via boiling. Both of these processes are as hard to be downwind of as they sound, but the demand for the acid was so strong that Roebuck-style plants were opened throughout the industrialized world.

The lead-chamber technique was used until 1831, when English vinegar merchant Peregrine Phillips discovered a way to turn the more easily available sulfur dioxide gas into the trioxide by flowing it over a heated metal catalyst. This route, known as the contact process, is still used today, as sulfuric acid is more in demand than it ever was. Vast amounts are used for making fertilizers, and it shows up as a reagent in almost every part of the chemical industry.

Sulfates (sulfuric acid salts) show up in many fertilizer mixtures. Sulfuric acid itself is not recommended for the garden, however!