Laboratory Methods of Organic Chemistry - Sciencemadness Dot Org

70 HALOGEN BY THE CAEIUS METHOD
mate-sulphuric acid mixture, wash out with water, connect to a
water-jet pump, and dry with gentle warming.
Weigh the substance in a small tube having a stem 8-10 cm.
long (Fig. 40) made in the laboratory from a suitable glass rod.
When weighing, lay the tube on a
small wire frame (Fig. 40). After
the approximate weight of the tube
has been determined, weigh out
40 accurately to 0-01 mg. 20-30 mg.
of substance for the halogen determination.
Hold the Carius tube horizontally, insert the weighing
tube as far as possible, bring into a vertical position, and let the
substance slip into the Carius tube. Carefully replace the weighing
tube on the balance and determine the exact weight of substance
taken by re-weighing.
According to the amount of substance taken add 60-90 mg. of
finely powdered silver nitrate (it is best to add one and a half times
the amount which corresponds to the expected halogen content).
Then, if the substance is only slowly attacked by nitric acid in the
cold, add 1 -0-1 -5 c.c. of red fuming nitric acid. In the case of substances
which react vigorously with nitric acid in the cold, hold the
Carius tube obliquely and add the acid in a small round-bottomed
tube (length 6 cm., width 0-8 cm.), cautiously allowing it to slide
to the bottom of the Carius tube and taking care that substance and
acid do not come into contact with each other.
Sealing the Tube.—In addition to compressed air use oxygen from a
cylinder connected to the blow-pipe when sealing the hard glass tube.
Grasp the tube in the middle with the left hand. Taking care that
the nitric acid does not escape from the small tube and reach the substance,
move the Carius tube into a position as near to horizontal as
possible and heat the end first with the luminous, then with the nonluminous
flame, and finally with the flame reinforced with some oxygen
until the glass softens, continually rotating the tube slowly throughout.
Now fuse a glass rod on to the inside of the tube, draw against the other
side, and, after this side also is attached to the rod, move the latter into
a position co-axial with the tube. Heat the tube now a little further
down, where it is still cylindrical, using first an almost completely
luminous flame and then, admitting a moderate amount of oxygen,
continue until the glass is soft. Keep the tube rotating continuously
and press very gently so that the walls collapse at this point; as soon as
the glass has thickened greatly, remove from the flame and slowly draw