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Explanation:
Sulphuric acid
Sulphuric acid is probably the most important of all chemicals,
because of its extensive use in a very large number of manufacturing operations. Of the immense quantities made yearly, the greater part does not come upon the market; for, being expensive and difficult to ship, consumers of large amounts generally make their own acid.
The commercial grades of acid have special names. A moderately strong acid (50°_55° He.), such as condenses in the lead chambers, is known as "chamber acid." It contains from 62 to 70 per cent of H2S04, and is strong enough for use in the manufacture of fertilizer, and for other purposes requiring a dilute acid. By concentrating this chamber acid, an acid of 60° Be. is obtained, containing about 78 per cent of H2S04> which is sufficiently strong for most technical uses. Further evaporation in platinum or iron pans yields an acid of 66° Be., containing 93.5 per cent of H2S04, and known as oil of vitriol. Faming or Nordhausen acid, which is still more concentrated, is prepared by special means. It is essentially a solution of sulphuric anhydride (S03) in sulphuric acid. This is the acid which was prepared by the alchemists in the Middle Ages.
In about the year 1740, Ward, an Englishman, began to make sulphuric acid on a moderately large scale. He burned sulphur and nitre (KNO3) together, and condensed the vapors in glass vessels containing a little water. The dilute acid so formed was then concentrated in glass alembics or retorts. In this way all acid was produced at a lower price than the fuming acid could be made. and the industry was soon established on [t commercial scale. The reactions involved ill Ward's process are the bases of the method now in use; this consists in bringing together, under suitable conditions, sulphur dioxide, oxygen, and water as steam, in the presence of certain oxides of nitrogen. The latter probably act as carriers of the oxygen, causing it to unite with the sulphur dioxide and water to form the acid. The apparent reaction is;-
S02 + H20 + 0 = H2S04-
Hut this does not represent the actual proc
First there is a union of sulphur dioxide, nitrous anhydride,
oxygen, and water, to form nitrosylsulphuric acid, which probably separates as part of the mist or fog seen in the lead chambers. But in the presence of water vapor or of dilute sulphuric acid, this nitrosylsulphuric acid is at once decomposed, according to reaction (2), sulphuric acid being formed, and nitrous anhydride regenerated; or if sulphur dioxide and oxygen are concerned in the process, then reaction (3) occurs. This cycle of reactions repeats an indefinite number of times. But in the first lead chamber, where the temperature is rather high and an excess of water vapor is usually present, the following secondary reactions probably occur
to a greater or less extent:-
4) 2 S02.(OH).(ONO) + S02 + 2 H20 = 3 H2S04 + 2 NO,
this reaction being only momentary.
Since there is usually an excess of oxygen present, however, the nitric oxide here formed is at once brought into action again, thus:-
5) 2 S02 + 2 NO + 30 + H20 = 2 S02.(OH).(0N0).
If there is a deficiency of oxygen, the nitric oxide is not returned to the process, but passes through the several chambers and, since it is not absorbed by the concentrated acid in the Gay-Lussac tower, it escapes into the atmosphere and is lost.
The nitrogen oxides are derived from nitric acid, or by the action of sulphuric acid on sodium nitrate in the nitre pots. When nitric acid is used, it must be introduced in the form of vapor, or at least as a very fine spray, whereupon it reacts as follows:-
6) 2 S02 + 2 HNO2 + H20 = 2 H2S04 + N203,
The first proposal to use pyrites originated with an Englishman named Hill, who took out a patent for the process in 1818. But it was not until 1838, when the Sicilian government sold the monopoly of the sulphur export to a French firm which nearly trebled the price of crude brimstone, that pyrites began to find favor with acid makers, At the present time, because it is cheap and easily obtained, pyrites has almost completely replaced sulphur for acid making. The product from pyrites is usually contaminated with arsenic, and often with zinc, copper, and selenium.
By the oxidation of pyrites in a suitable furnace, the sulphur is converted to dioxide, and iron oxide remains. The reaction may
be written as follows:-
2 FeS2 + 11 0 =4 S02+Fe203
This is not exact, however, as some sulphur remains in the ore
and some sulphur trioxide is formed. The proper regulation of the pyrites burners is one of the problems of the manufacturer. If the ore contains over 35 per cent of sulphur, the burning, once started, generates sufficient heat to maintain the combustion, and no fuel is necessary. But zinc sulphide and the" mattes" from metallurgical processes must be heated by fuel.