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

ANHYDROUS CHROMIUM TRICHLORIDE. 77D

Larger amounts of hydrogen chloride may be obtained by

treating 200 g. common salt (sodium chloride) with a cooled
mixture of 320 g. sulphuric acid and 80 g. water in a round-

bottomed flask, which may be conveniently heated upon a Babo

boiling funnel. On cooling, the flow of the gas slackens, but it

can be started again by renewed heating. When the mixture has
become exhausted, it should be poured out of the flask while

still warm, since on cooling it becomes solid, and is then difficult

to remove. But it must be poured into a dry receptacle to avoid

an explosive formation of steam.

The following method, which depends upon the fact that hydro-
gen chloride is but slightly soluble in concentrated sulphuric

acid, is also to be recommended.^1 Allow concentrated sulphuric

acid to drop from a dropping funnel into a large suction flask

containing concentrated hydrochloric acid, to which a handful of
common salt may with advantage be added. It is advisable to

let the sulphuric acid fall first into a small test-tube so that by

its flowing uniformly over the edge of the latter a steady evolu-

tion of gas is produced. The rate of flow of gas from the flask
is governed by the stopcock which regulates the dropping of the

sulphuric acid.

The reverse process of adding hydrochloric acid to concen-

trated sulphuric acid may also be used, but in order to make the

lighter hydrochloric acid solution mix thoroughly with the heavier
sulphuric acid it is necessary to draw out the stem of the dropping

funnel into a capillary which reaches nearly to the bottom of the

generating flask.

44. Anhydrous Chromium Trichloride.

Place 10 to 20 g. of coarsely powdered, metallic chromium

(No. 3) in an apparatus constructed like that used in the last

preparation. Expel the air completely by means of a stream of

dry chlorine, which will take at least half an hour, and then heat

the metal in a current of perfectly dry chlorine for from thirty
minutes to an hour at as high a temperature as possible. After
cooling, replace the chlorine by carbon dioxide and shake the
preparation out of the tube. Beautiful, glistening, violet flakes

(^1) The same method may be used for the production of sulphur dioxide if
concentrated sulphuric acid is allowed to drop into a solution of commercial
sodium bisulphite.