77G HALOGEN COMPOUNDS.
Phosphorus Trichloride. Use the same apparatus as in the
preparation of sulphur chloride (No. 45), except that the heating
bath under the retort is in this case superfluous. Cut 31 g. of
yellow phosphorus under water in a porcelain mortar into pieces
which are not too small. Take the pieces out with pincers, dry
them by rapidly pressing between filter papers and introduce them
through the tubulus into the retort which has previously been
filled with carbon dioxide gas. Avoid handling the phosphorus
with the fingers, because it takes fire easily, and phosphorus burns
are severe and frequently dangerous. Any residue of phosphorus
left in the mortar should be wiped out with moist filter paper, and
the paper immediately burned.
On conducting chlorine into the retort, the phosphorus ignites,
melts, and burns with a pale flame to phosphorus trichloride.
Outside heating of the retort is unnecessary, but it is important
to have the stream of chlorine pass rapidly and steadily. Regu-
late the reaction from time to time, as necessary, by raising or
lowering the tube through which the chlorine enters the retort;
this tube should be fitted so that it can be moved readily.
Lower the tube if a white sublimate of phosphorus pentachloride
forms in the upper part of the neck of the retort; raise it a
little if a yellowish-red sublimate begins to make the neck of
the retort opaque. This regulation, which is necessary for the
proper carrying out of the experiment, does not involve any
difficulty.
Purify the crude product by distilling it from a flask with a
side-arm condenser (Fig. 7, p. 6). Boiling-point, 76°. Yield, 100
to 120 g.
Phosphorus Pentachloride. Close a wide-mouthed liter bottle
by means of a three-holed cork. Through one hole introduce the
stem of a dropping funnel so that it reaches just inside the
stopper; through the second hole insert a glass tube of 1 cm.
bore which reaches to the middle of the bottle and serves for the
introduction of chlorine; through the third hole insert another
tube which ends just below the cork and serves for the escape of
the excess of chlorine. Provide the 1 cm. wide vertical tube with a
side arm through which the chlorine is to be introduced, and close
the upper end with a cork stopper. Then when the lower part of
the tube becomes clogged with phosphorus pentachloride, it is