Chapter XVII: Sulphur Trioxide

Sulphur trioxide does not exist by itself in nature. Moreover it is but little known even as an artificial product. It is not an article of ordinary sale, though it is occasionally made by the chemist. Yet it is a constituent part of one of the most important compounds known to modern industry. That compound is sulphuric acid.

Sulphur trioxide is a white solid, but it cannot easily be kept so. This is because it has very strong affinity for moisture. It fact it readily absorbs that water-vapor which is distributed through the atmosphere, even in dry weather and when the ordinary observer would suppose that the air contained no moisture at all. When it absorbs moisture it chemically combines with it, forming sulphuric acid.

The chemical change is represented by the following equation:

=SO₃= + =H₂O= = =H₂SO₄=

One molecule of One molecule of One molecule of
Sulphur trioxide, Water, Sulphuric acid,
80 18 98
parts by weight. parts by weight. parts by weight.
\____________________________________/ \_______________/
| |
98 98

On account of this reaction, sulphur trioxide is often spoken of as sulphuric anhydride, the term anhydride being intended to suggest that the substance so named is derived from an acid by the removal of water from the latter. Thus sulphuric acid _minus_ water produces sulphuric anhydride. And this harmonizes with what has before been declared, namely, that sulphuric anhydride—or sulphur trioxide—_plus_ water produces sulphuric acid.

Sulphuric Acid.

This substance is known to commerce chiefly under the name of oil of vitriol. It is an oily liquid nearly twice as heavy as water. It has very powerful chemical action upon most substances with which it comes in contact. Moreover, its market price is very low, that is, between one and two cents a pound at wholesale. To these two facts last mentioned—that is, the marked chemical power and the low price is referable the enormous demand for the substance. To be sure, increase of demand and fall in price have a reciprocal action; for even a slight cheapening of a substance widens considerably the range of its possible uses and increases the amount consumed. Again, increase of demand and consumption, lead manufacturers to increase their production, a circumstance which is generally followed by lower price. The manufacture of sulphuric acid exemplifies these well-known principles of political economy. The manufacture of this substance has risen within the last hundred years from almost nothing to a present annual production of about nine hundred thousand tons in Great Britain alone. The price meanwhile has fallen to about one-thirtieth of what it was in the middle of the last century. At the present time the price of oil of vitriol seems to be steadily decreasing, while the amount produced is steadily increasing in England, France, Germany and the United States—indeed in all countries pervaded by active industrial enterprises. It will be generally admitted, as M. Dumas has said, that the amount of sulphuric acid consumed affords a very precise measure of the advancement in industrial arts of a given country or of a historical epoch.

Uses of Oil of Vitriol.

It would be difficult to enumerate the many industries that demand the use of sulphuric acid. It must likewise be admitted that there are but few manufacturing operations which do not directly or indirectly involve its employment. The industries that stand in the front rank as direct consumers of this acid are those that involve the following processes, namely: the bleaching of cotton goods; the removal of scale from iron in its various forms, such as castings, wire, etc.; the changing of corn starch into the variety of sugar commonly called glucose; the refining of bullion of gold and silver; the refining of petroleum oil; last, but not least, the manufacture of chemical fertilizers for agricultural use. Less directly, but still in enormous quantities, it is used in the manufacture of soda-ash, and bleaching powder already referred to as having reached an incredible consumption; in the manufacture of alum; in the manufacture of both of the great acids of commerce, chlorohydric acid and nitric acid, which must be said to come next to sulphuric acid in usefulness; and finally, in almost all the distinctly chemical industries.

Manufacture of Sulphuric Acid.

Notwithstanding the extremely low price of oil of vitriol and the immense quantity of it manufactured, its production implies a series of processes far more complicated than those involved in the preparation of any other well-known acid. Moreover, although the various intricate details of its preparation are matters of thorough _experimental_ knowledge to the producer, there are several steps which are not yet clearly comprehended even by the most eminent chemists of the age.

The process of manufacture, as at present conducted, is properly described as a continuous one. By this it is meant that the raw materials are steadily introduced at one end of the apparatus used, and the finished product is steadily drawn out at the other, the process meanwhile going on without interruption, night and day, for years. In order to a better comprehension of the process it is here described in four stages.

In the first stage, sulphur is burned in a current of air. The material employed is either partly refined Sicily sulphur, or what is largely used at the present day, some mineral compound of sulphur, like the iron and copper pyrites. In either case, sulphur dioxide (SO₂) is formed. This is the well-known choking gas given out by a burning sulphur match. As produced on a large scale the gas passes into a series of enormous leaden chambers. These are, in fact, rectangular rooms, often as large as one hundred and fifty feet long, twenty feet wide and fifteen feet high. Generally at least three chambers are in a series, connected by leaden pipes. Sulphur dioxide gas flows in a steady stream into the series of chambers and toward the high chimney of the works, whose draft produces the advance of gases through the whole apparatus.

The second stage is the most complicated one. It is the oxidizing of the sulphur dioxide (SO₂) into sulphur trioxide (SO₃). This is indeed accomplished by means of the oxygen of the air. But this oxygen is not capable of _directly_ changing SO₂ into SO₃. Certain gaseous oxides of nitrogen are forced into the chamber at this stage; and these have the remarkable power on the one hand of taking oxygen to themselves from the air, and on the other of imparting this oxygen to the compound SO₂ in such a way as to change it into the compound SO₃. Of course the air is impoverished by the operation, a fact which necessitates a fresh supply of it through the entire series of chambers.

The third stage is one whose principle has already been explained. At various parts of the chamber, jets of steam are blown in. While these aid mechanically in the progress of the gases through the entire series, their main purpose is to furnish water which shall combine with sulphuric anhydride to produce sulphuric acid.

Although this chemical change, represented by the following equation, has been given before, it may not be improper to repeat it here:

=SO₃= + =H₂O= = =H₂SO₄=

One molecule of One molecule of One molecule of
Sulphur trioxide, Water, Sulphuric acid.
80 18 98
parts by weight. parts by weight. parts by weight.
\____________________________________/ \_______________/
| |
98 98

The effect of the steam is to give rise to a steady rain of oil of vitriol in the chambers. Of course this liquid collects at the bottom. Thence it is drawn off, for treatment in a fourth stage. It is plain that up to this point the series of chemical reactions takes place in what we may characterize as a vast but irregular tube, open at both ends. This tube is enlarged here and there into great pockets which constitute the chambers. It is bent into a form appropriate to the conditions of the business. It is entered here and there by pipes for introducing the agents whose proper interaction gives rise to the product sought. It is also tapped for the purpose of drawing off the acid generated. This open tube has its final exit into the atmosphere through the tall chimney with which it is connected. It has its first connection with the atmosphere at the open throat, which swallows at once the vast volumes of sulphurous gas from the sulphur burned, and at the same time levies upon the air to contribute its oxygen to produce the substance which is the final purpose of the whole industry.

The fourth stage is the only one that may be properly said to be disconnected from the others. The continuous process already described cannot properly be made to produce acid of the strength demanded by commerce. In the fourth stage then, the acid from the chambers is boiled with a view of expelling some of the water in it, and thus of producing a more concentrated product. This evaporation is itself no inconsiderable portion of the business. It is conducted first in shallow tanks of lead, and finally in costly stills of platinum. When at length the acid in the platinum stills has attained the proper degree of concentration, it is drawn out by means of a siphon tube, and through a cooling tank of cold water, into the glass flasks called carboys, in which it makes its appearance in commerce.

Of course the account thus given is but a general sketch of this great industry. Associated with the apparatus and the processes here briefly described there are employed in actual working a multitude of other devices and operations. Indeed it might be anticipated that the successful conduct of a business of such magnitude and complexity would draw upon the inventive resources of some of the best minds that have been brought to bear upon chemical industries.

READING REFERENCES.

Sulphur Industry in Sicily.
=Barbaglia=, A.—Chem. News. xxxiv, 245; xxxv, 3, 28.
=Vincent=, C.—Am. Chem. Journal. vi, 63.
Sulphur, Extraction of.
=Sestini=, F.—Jour. of Chem. Soc. of London. xxviii, 335.

Sulphuric Acid.
=Affleck=, J.—Chem. News. xxxvii, 167, 192, 207.
=Hasenclever=, R.—Chem. News. xxxv, 48, 67, 88, 118, 183,
189, 214, 227.