Laboratory Methods of Inorganic Chemistry, 2nd English Ed. 1928

SULPHURIC ACID MONOCHLORIDE. 203

chloric acids. Larger quantities react, after some time, suddenly

and very energetically, with a considerable evolution of heat.

Sulphuric Acid Monochloride.

(a) From Sulphuric Acid and Phosphorus Pentachloride.

The chlorination of sulphuric acid by phosphorus pentachloride takes

place according to the equation

SO 2 (OH) 2 + PC1 5 = SO 2 C1.OH + POCI3 + HC1.

The phosphorus oxychloride formed acts likewise as a chlorinating agent in

this case:

2H 2 SO 4 + POC1 3 = 2SO 2 C1.OH + HPO 3 + HC1.

First prepare pure sulphuric acid " monohydrate " by adding

fuming sulphuric acid to the ordinary concentrated acid until the

specific gravity at exactly 15° is 1.84.

To 200 g. of this acid in a liter flask add 150 g. of phosphorus

pentachloride in small portions from a glass spatula. The mix-

ture becomes somewhat heated, and large quantities of hydrogen

chloride escape. (The operation should be carried out under the

hood, or, better still, out-of-doors.) When all the phosphorus

pentachloride has been added, heat the flask on a Babo funnel

until the evolution of hydrogen chloride has ceased. Transfer

the liquid product to a distilling flask, or a tubulated retort, in
which a thermometer is inserted through a ring of asbestos cord

(not through a cork). Slip a glass tube, 40 cm. long and 1 cm. in

diameter, over the side arm of the distilling flask, or the neck

of the retort to serve as an air condenser; it is not necessary to

make the joint tight. Distil until the temperature has risen to
about 165°. To purify the crude product, redistil it from a

fractionating flask with a side arm condenser (Fig. 7, p. 6).

Boiling-point, 153°. Yield, 120 to 150 grams.

(6) From Sulphur Trioxide and Hydrogen Chloride. Melt some

commercial, 80% fuming sulphuric acid, which can be obtained

in small sealed flasks, by placing it for a short time in a warm

place. Add 200 g. of this acid to a large, gas-washing flask which
has ground glass joints; cool the flask and contents to the room

temperature and pass in a vigorous stream of gaseous hydrogen

chloride. As soon as the mixture becomes warm, cool it by

surrounding the flask with ice. When the hydrogen chloride

ceases to be absorbed, transfer the liquid to a distilling flask;