Sulphuric Acid

Sulphuric Acid. Of all the acids which find employment in the various industries of the country this compound is by far the most important, being, directly or indirectly, useful in almost every industrial process. It appears to have been known in the alchemistic period, and was then prepared by beating copperas. Its manufacture by heating sulphur and nitre was carried on in the 17th century. The first sulphuric acid works were established at Battersea in 1772, and the erection of other similar works was soon followed by various improvements in the methods of its production. The present method of carrying out the process is as follows:- By heating pyrites in suitable kilns or furnaces sulphur dioxide is formed, which passes with the excess of air into a large lead chamber, into which pass also jets of steam and nitric acid fumes produced by the action of sulphuric acid upon Chili saltpetre. Here by the interaction of these compounds sulphuric acid is formed, and collects at the bottom of the chamber. The nitric fumes get reduced to lower oxides, but are again oxidised by the air present also. The remaining gases pass into a second chamber and again meet steam jets, more acid being deposited, and are then drawn into a third chamber, where a similar effect is produced. What then remains of the gases, now almost solely nitric fumes, and air are drawn up a chimney - the "Gay Lussac tower" - in which they meet a descending spray of strong sulphuric acid, which absorbs the nitric fumes, this nitrated acid afterwards giving up the fumes to the sulphur dioxide in the "Glover's tower" before the gas enters the lead chambers. The sulphuric acid is drawn off from the chambers, and then contains about 64 per cent. pure acid. It is concentrated first in leaden pans, and finally in glass or platinum vessels. The manufacturing process depends on the power of nitric oxides to oxidise sulphur dioxide and of the nitrous oxides to absorb oxygen, and so make the process continuous. The following equations represent these reactions:-

(i) SO₂ + OH₂ + NO₂ = H₂SO₄ + NO;

(ii) NO + O = NO₂. "

The pure acid has the composition H₂SO₄, and is a thick, colourless, oily liquid, known also as oil of vitriol. If mixed with water, much heat is evolved, the mixture being attended by a contraction in volume. Owing to the readiness with which it absorbs water, it forms an admirable desiccator for gases, etc. It is a powerful acid forming a series of salts known as sulphates. If both hydrogen atoms are replaced by metals, normal sulphates result, acid sulphates being salts in which only one hydrogen is so replaced. The uses of the acid, both in pure chemistry and in technical and industrial processes, are too manifold for enumeration, whilst many of its salts also are important commercially. Thus ferrous sulphate, or copperas, is largely employed in chemistry, in dyeing, and in the manufacture of inks; copper sulphate or blue vitriol is used in electroplating, as an insecticide for pbylloxera, in dyeing, etc.; calcium sulphate is well known under the name of gypsum, zinc sulphate as vitriol, while the sulphates of sodium, potassium, and ammonium are important compounds. The numerous compounds known as alums are also all double sulphates - i.e. the sulphuric acid radical united with two different metals.