Sulphuric acid, H2SO4, known also as oil of vitriol, is an odourless, dense, oily liquid having a specific gravity of 1.842. Pure sulphuric acid is colourless, but the commercial acid is of a straw to brown colour. It is a typical acid. It occurs but rarely in a free natural state, but combined with certain elements it is common in the animal, vegetable, and mineral kingdoms. A simple method of preparing sulphuric acid on a small scale is to boil sulphur in aqua regia or in nitric acid; the oxidation of the sulphur will produce the sulphuric acid. The two principal commercial methods of preparing the acid are based on discoveries made in the fifteenth century by Valentine. By one process, sulphate of iron (green vitriol, hence the term " oil of vitriol") is distilled in earthenware retorts, the vapour passing into a receiver containing a little ordinary sulphuric acid and forming a brown, fuming, oily liquid having a specific gravity of 1.9; this is the process employed at Nordhausen, Germany, the product being known commercially as Nordhausen acid. The English process may have two forms (1) in which sulphur is used, and (2) in which sulphide of iron (iron pyrites) is used; both of the processes depend on the production of sulphurous acid.

Sulphur is ignited and burnt in a conical brickwork oven; just above the sulphur is supported a pot, known as the nitre pot, which is filled with a mixture of sulphuric acid and either soda nitrate or potash nitrate, from 81b. to 101b. of nitre with from 511). to 61b. of acid being allowed for every hundredweight of sulphur. If iron pyrites is used, it is roasted in arched chambers. Under the action of the heated sulphuric acid the nitre decomposes, the nitric acid fumes passing into another chamber along with the sulphurous acid obtained by burning the sulphur. The sulphurous acid abstracts from the nitric acid sufficient oxygen for its conversion into sulphuric acid, the nitric acid becoming nitric oxide, which quickly becomes nitric peroxide by taking oxygen from the air supplied for the combustion of the sulphur. Steam is introduced, and the sulphurous acid constantly being produced takes oxygen from the nitric peroxide and continues the supply of sulphuric acid; thus the cycle of actions and reactions continues until the whole of the sulphur is consumed.

The sulphuric acid falls into water, which is drawn off for concentration when it reaches the specific gravity of 1.4. The solution is concentrated first by evaporation in lead pans until the specific gravity is 1.6, and then by boiling in vessels of platinum or flint glass.