Laboratory Methods of Organic Chemistry - Sciencemadness Dot Org

FILLING THE COMBUSTION TUBE 49
Filling the Combustion Tube.—First clean the combustion tube
with bichromate-sulphuric acid mixture, wash with distilled water,
and dry with gentle warming, drawing a current of air through by
means of a water-jet pump. Keep a stock of coarse wire-form
copper oxide (" for analysis ") and of more finely broken copper
oxide prepared from the coarse quality by powdering (not grinding)
in a mortar so that, after dust has been removed with a sieve, the
pieces are 1-2 mm. long. Ignite the copper oxide in a nickel basin
before use. Used copper oxide is ready for immediate re-use after
it has been sieved and heated to redness in air.
Care must be taken to avoid contamination of the copper oxide
with, alkaline solutions since such contamination results in nitrogen
values which, are always too low. The only remedy, should contamination
occur, is to boil with, acetic acid and re-ignite.
First fill the conical part of the tube with some silver wool.
Then push in some purified and ignited asbestos wool up to the
constriction, using a suitable glass rod smoothly rounded at the end
and pressing moderately hard, so that a pad 2-3 mm. thick is produced.
After the asbestos pour in a column of coarse copper oxide
12 cm. long, holding the tube vertically and striking it with the
palm of the hand so that a moderately compact column is formed.
In the same way, pour in now a 6 cm. column of the finely broken
copper oxide and, after that, a 10 cm. column of the coarse. Confine
this " permanent filling " in the tube by means of a second asbestos
pad, gently pressed and a few millimetres thick.
Now pass into the filled tube, from the wide end, a current of
hydrogen (washed with acid permanganate solution), reduce the
6 cm. column of finely broken copper oxide by heating at a moderately
high temperature with a Bunsen burner after completely
expelling the air, and allow to cool slowly in a current of hydrogen.
Then burn out the whole length of the newly prepared tube with its
" permanent filling " in the electric furnace at a high temperature
and leave to cool while filled with carbon dioxide at the pressure of
the Kipp. Always keep the tube when not in use connected to the
Kipp and full of carbon dioxide at this pressure.
The Half-micro-nitrometer.—The half-micro-nitrometer used to
collect the nitrogen has a capacity of 8—10 c.c, a volume which corresponds
to 20-30 mg. of substance. A completely satisfactory degree of
accuracy in measuring the gas is attained by graduating in sub-divisions
of 0-02 c.c.