HYDROBROMIC ACID. 71
tube, and heat this plug with a wide flame. Any iodine, distilling
as such without combining with the hydrogen, will be deposited
in the wide part of the tube. Lead the escaping gas mixture into
a test tube containing a little water and from thence into two
successive small flasks each containing some water. Before
starting the experiment, make sure that the entire apparatus is
filled with hydrogen, so that there will be no danger of an explosion
when the contents of the flask and the platinized asbestos are
heated.
(b) By the Intermediate Formation of Phosphorus Bromide.Provide a 500 c.c. flask with a stopper and dropping funnel and
connect it by means of delivery tubing with two U-tubes placed
in series. From the last U-tube carry a bent delivery tube
through a tight-fitting cork well towards the bottom of a flask
which is to serve as a receiving vessel. Provide this flask with an
exit tube starting from just inside the cork and bending down-
ward externally so as to nearly reach the surface of some water
placed in another flask. Do not, during the process which follows,
allow the tube entering either of the last-mentioned flasks to dip
beneath the surface of the solution contained in them.
Place in the evolution flask, first a layer of 25 g. sand, then upon
this a mixture of 100 g. sand and 25 g. red phosphorus, and moisten
the whole with 45 c.c. water. Fill the first U-tube with glass
beads, the second with glass beads mixed with moist red phos-
phorus, and in the receiving flask place 80 c.c. of water. The
delivery tube which enters the receiver must reach only to a
point just above the surface of the liquid, since if it dipped into
the solution, the latter might be sucked back into the evolution
flask.
Wrap the evolution flask in a towel, because it sometimes breaksat the beginning of the experiment; cool the first U-tube with a
mixture of salt and ice, and the receiver with ice. Introduce
from the funnel 200 g. bromine, drop by drop, into the mixture of
sand and phosphorus. At the beginning of the reaction cool the
evolution flask by placing it in a dish containing cold water.
Yield, about 250 g. of concentrated hydrobromic acid, which is
collected in the first receiving flask. Determine its density by
means of a Westphal balance. Hydrobromic acid can also be
prepared from bromine and hydrogen sulphide. (Cf. A. Recoura,