Modern inorganic chemistry

300 GROUP VI

concentrated sulphuric acid, to which water is added at the required

rate:

SO 3 + H 2 SO 4 -> H 2 S 2 O 7

H 2 S 2 O 7 4- H 2 O -» 2H 2 SO 4

The 94 % acid from the sulphur dioxide drying towers (above) is

used here and its strength brought up to 98 %. This is "concentrated^1

sulphuric acid. Stronger acid up to "106%' may also be made. This

concentration is suitable for sulphonating in, for example, the deter-

gent industry.

Uses

The production of 'superphosphate' (calcium hydrogenphosphate

+ calcium sulphate) for fertilisers is the biggest use of sulphuric

acid. Second to this is the manufacture of ammonium sulphate

from ammonia (by the Haber process). This is also a fertiliser. Other

uses are: conversion of viscose to cellulose in the manufacture of

artificial silk, and so on; kpickling' (removal of oxide) of metals

before galvanising or electroplating; manufacture of explosives, pig-

ments and dyestuffs, as well as many other chemicals, for example

hydrochloric acid; refining of petroleum and sulphonation of oils

to make detergents; and in accumulators.

Properties

Pure sulphuric acid is a colourless, viscous and rather heavy liquid

(density 1.84 g cm"^3 ). On heating, it decomposes near its boiling

point, forming sulphur trioxide and a constant boiling (603 K)

mixture of water and sulphuric acid containing 98 % of the latter.

This is 'concentrated' sulphuric acid, which is usually used. Further

heating gives complete dissociation into water and sulphur trioxide.

Affinity for water

Concentrated sulphuric acid has a strong affinity for water and
great heat is evolved on mixing; hence the acid must be added to water
to dilute it. Because of this affinity, the acid can be used to dry
gases with which it does not react, for example oxygen, chlorine,
sulphur dioxide, and is used in desiccators. It will remove water of
crystallisation from some compounds, for example