SULPHURIC ACID, oil of vitriol, H2SO4, is commercially of the greatest importance. The urgency of the market demand for this acid is recognized as one of the most accurate and sensitive indications of the general state of the trade.
Pure sulphuric acid is an oily, colourless, heavy liquid (sp. gr. 1.85), freezing at io.8° C and boiling with partial decomposition at 338° C. It is highly corrosive, charring many organic sub stances, largely owing to its great avidity for water. So much heat is evolved that on mixing with water it is unsafe to add water to the strong acid, the acid being always poured slowly into water with thorough stirring. When the vapour is heated to C it is completely dissociated into water and sulphur trioxide, the latter decomposing into sulphur dioxide and oxygen on further heating. The hot concentrated acid is a weak oxidizing agent, converting carbon into carbon dioxide, sulphur into dioxide, and many metals, such as copper, into their sulphates with evolution of sulphur dioxide. The cold dilute acid dissolves certain of the more electropositive metals such as zinc and iron with evolution of hydrogen. Sulphuric acid is dibasic, giving rise to two series of salts, acid and normal. Most sulphates are readily soluble in water, the commonest exceptions being those of calcium, stron tium, barium and lead. Its efficacy in many common chemical processes depends on its power of liberating other acids from their compounds, not because it is necessarily stronger (see CHEMICAL ACTION), but because it is less volatile. Its value in organic chemistry depends partly on its great affinity foe water, whence its use in nitration processes (see DYES, SYNTHETIC; and EXPLO SIVES), and partly on its power of forming sulphonic acids with many aromatic compounds (see CARBOLIC ACID, INDIGO, and NAPHTHALENE).
This acid fumed strongly in the air, whence its old name of "fuming" or Nordhausen sulphuric acid. A great extension arose from Liebig's discovery in 1840 of a method of rendering the phosphorus content of mineral phosphates available in soluble form for plant growth by treatment with sulphuric acid, this process affording the greatest outlet for this acid. Subsequently, a more highly concentrated form of acid became essential to the manufacture of dyes and the refining of oil, and the "contact" process was developed to meet this requirement.
The original raw material was brimstone from Sicily, but by the middle of the I9th century this source had been largely replaced by iron pyrites which occurs abundantly in many Countries, for example, in North Wales and County Wicklow, Ireland; but after the introduction of the wet process for copper extraction the pyrites most generally used in Great Britain and the eastern States of America was the copper-containing variety found in immense quantities in South-Western Spain where it is mined by the Rio Tinto, Tharsis and other companies. In recent years, however, the enormous production of brimstone (see SULPHUR) in the United States by the "Frasch" process coupled with changed economic conditions in copper extraction has led to renewed use of brimstone. Spent oxide from the purification of coal gas and zinc blende or "concentrates" from the Australian lead-zinc ores are sulphur-centaining materials which furnish sulphur dioxide in appropriate furnaces. Large quantities of acid are produced in America from the waste gases from copper smelters, and during the World War a promising process was developed in Germany whereby sulphur dioxide is produced from naturally-occurring calcium sulphate, gypsum or anhydrite by calcining with clay in a rotary kiln, yielding cement as a by-product. Brimstone and spent oxide contain free sulphur, whereas in pyrites and blende the sulphur is present as sulphides of iron and zinc respectively; nevertheless, the latter materials will burn in air without extra neous heat, although with blende it is necessary to use fuel in the finishing stages in order to assist in its complete desulphurization.