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Sulphuric Acid

Although sulfuric acid is now one of the most commonly used chemicals, it was probably little known before the 16th cent. It was prepared by Johann Van Helmont (c.1600) by destructive distillation of green vitriol (ferrous sulfate) and by burning sulfur. The first major industrial demand for sulfuric acid was the Leblanc process for making sodium carbonate (developed c.1790). Sulfuric acid was produced at Nordhausen from green vitriol but was expensive.

A process for its synthesis by burning sulfur with saltpeter (potassium nitrate) was first used by Johann Glauber in the 17th cent. and developed commercially by Joshua Ward in England c.1740. It was soon superseded by the lead chamber process, invented by John Roebuck in 1746 and since improved by many others. The contact process was originally developed c.1830 by Peregrine Phillips in England; it was little used until a need for concentrated acid arose, particularly for the manufacture of synthetic organic dyes.


In the pure state, sulphuric acid is a clear, colorless, oily liquid. One-hundred percent H2SO4 has its melting point at 10°C; when heated, it gives off SO3 until the concentration of the acid has fallen to 98.5%, and it then boils at a constant temperature of 338°C. Considerable evolution of heat occurs when concentrated sulphuric acid is diluted with water. Substantial amounts of SO3 can dissolve in the acid. The resulting solution is known commercially as fuming sulphuric acid.

Sulphuric acid does not occur as a free acid in nature. It is found only in the form of its salts (sulphates): gypsum (CaSO4.2H2O), Epsom salts (MgSO4.7H2O), barite (BaSO4) and Glauber’s salt (Na2SO4.1OH2O). Up to about the eighteenth century, sulphuric acid was made by heating alum (aluminum potassium sulphate) or iron vitriol (hydrous ferrous sulphate).

This method was superseded by the burning of natural sulphur with saltpeter, which eventually evolved into the so-called lead-chamber process, which has the advantage that the acid can be obtained in any desired concentration, whereas the highest attainable concentration with the lead chamber process is 78%.

The contact process is as follows, Sulphur dioxide (SO2) is obtained by roasting iron pyrites (FeS2) in a rotary kiln, shelved roasting kiln or fluidized bed kiln. Which of these kiln types is employed depends on the particle size and nature of the pyrites to be processed. When the gases from the roasting process have cooled in gas ducts, by radiation of heat from 1000 oC to about 400°C -500°C, the dust they contain is removed in electrostatic precipitators (electric fitters).

Next, the SO2 gas is passed through a washing tower, where constituents that are present in vapor form mainly compounds of arsenic, selenium and chlorine are removed with sulphuric acid serving as the washing liquid. Remaining traces of impurities present as very fine suspended droplets (fog) are removed in an irrigated electrostatic precipitator (wet Precipitator). Then the gas is dried by being brought into contact with concentrated (98%) sulphuric acid.

A blower draws in the cold dried SO2 gas and delivers it into the converter, which is a tank or tower in which a suitable catalyst e.g., vanadium pentoxide (V2O5) is placed in layers on shelves or arranged in some other appropriate manner to ensure through contact with the gas. The reaction whereby SO2 is converted to SO3 by oxidation (2SO2+O2-> 2SO3) takes place at 430°C to 550°C. A heat exchanger installed before the converter serves to cool the gas discharged from the converter and at the same time preheats the incoming gas flowing to the converter.

Sulfuric acid is one of the most important industrial chemicals. More of it is made each year than is made of any other manufactured chemical; more than 40 million tons of it were produced in the United States in 1990. It has widely combined uses and plays some part in the production of nearly all manufactured goods.

The major use of sulfuric acid is in the production of fertilizers, e.g., superphosphate of lime and ammonium sulfate. It is widely used in the manufacture of chemicals, e.g., in making hydrochloric acid, nitric acid, sulfate salts, synthetic detergents, dyes and pigments, explosives, and drugs.

It is used in petroleum refining to wash impurities out of gasoline and other refinery products. Sulfuric acid is used in processing metals, e.g., in pickling (cleaning) iron and steel before plating them with tin or zinc. Rayon is made with sulfuric acid. It serves as the electrolyte in the lead-acid storage battery commonly used in motor vehicles acid for this use, containing about 33% H2SO4 and with specific gravity about 1.25, is often called battery acid.