80 HALOGEN COMPOUNDS.
From time to time raise the tubing through which the gas is flow-
ing but never let it reach above the top of the liquid chloride.
Purify the stannic chloride as described above.
This method of preparation corresponds to the method used
technically for the recovery of tin from waste tin plate.- Silicon Tetrachloride.
For preparing silicon tetrachloride from the elements, either commercial
silicon obtained in the electric furnace, or the crystallized product prepared
by the thermite process (No. 4), may be used.
Generate chlorine in a two-liter (or larger) flask from pyrolusite
and concentrated hydrochloric acid, and wash the gas once with
water and twice with concentrated sulphuric acid. Spread a layer
of about 10 g. finely powdered silicon loosely in a 40 cm, long com-
bustion tube which is placed over a row burner (Fig. 3, p. 2). Con-
nect one end of the combustion tube with the chlorine generator, and
draw out the other end to about the size of a lead pencil. Join this
narrow end with a gas wash-bottle, using a rubber connector and
pushing the ends of the glass tubes close together. Cool the wash
bottle by surrounding it with a mixture of ice and salt, and arrange
a glass tube to conduct the waste gases into the ventilating flue!
First of all, — and this is very important, — sweep the air com-
pletely out from the apparatus by passing a rapid stream of chlo-
rine gas for about half an hour. After that, heat the combustion
tube until the reaction begins and produces incandescence; the
flames beneath the tube may be turned quite low while the re-
action is progressing, and it is well to turn the tube from time to
time on its long axis. All of the silicon is acted upon, and only a
few flakes of silicon dioxide remain behind, while a trace of alu-
minium chloride condenses at the end of the tube.
Without using a thermometer, distil the impure product slowlyfrom a fractionating flask with side-arm condenser (Fig. 7, p. 6).
The greater part of the dissolved chlorine is thereby expelled, but
to remove the last of it, let the distillate stand about a day in con-
tact with mercury in a thick-walled bottle, stoppered with a cork
(not a glass stopper), shaking vigorously from time to time until
the liquid is decolorized. By again distilling, this time with a
thermometer, the compound is obtained pure. Boiling-point,
58° to 60°. Yield, 35 to 40 g.