Laboratory Methods of Organic Chemistry - Sciencemadness Dot Org

50 THE DUMAS METHOD
The gas inlet tube of the nitrometer carries a fused-in glass stop-cock
having a tap with a long arm. In order further to increase the degree
of fine adjustment two fine grooves pointed at the outer ends are cut in
the barrel of the tap (Fig. 33) by means of a sharp triangular
file. The positions of these grooves are such
that the long arm must be turned upwards in order to
allow the gas to pass. The gas inlet tube of the nitrometer
is connected to the combustion tube by means of
an obtuse-angled capillary tube and a piece of thickwalled
rubber tubing covering the well-fitting glass-toglass
joint. The external diameter of the capillary
tube at the joint exactly corresponds with that of the
tube from the stop-cock. The horizontal arm of the
FIG. 33 capillary tube is drawn out to a roughly conical tip
having external diameter exactly the same as that of
the constricted part of the combustion tube. A piece of narrowbored
pressure tubing, 2-5-3 cm. long, moistened with a little glycerol,
serves to cover the carefully fitted glass-to-glass joint between combustion
tube and capillary tube. When the apparatus is taken
apart the capillary tube is always left attached to the nitrometer.
Before it is filled the nitrometer is cleaned with bichromate-sulphuric
acid mixture. The rubber tube connecting the levelling bulb and the
nitrometer is made fast with wire binding. Pure mercury is run in
from the bulb until the level is 1-2 mm. above the highest point of the
orifice of the inlet tube. The stop-cocks are greased slightly with
vaseline, the grooves being kept clear. The liquid for filling the nitrometer
is 50 per cent potassium hydroxide solution (from pure " stick
potash ") which has been made completely free from froth by shaking
with finely powdered baryta (2 g. per 200 g. of solution) and filtering
through a dry filter.
The levelling bulb is closed with a rubber stopper carrying a short
glass tube drawn out to a capillary.
Preparing the Substance.—Burn solid substances either after drying
in air or immediately after drying to constant weight in an evacuated
desiccator over sulphuric acid. It is not advisable first to powder the
substance finely and so to increase its surface area unnecessarily. Such
procedure adds greatly to the difficulty of weighing hygroscopic substances.
If the substance retains some of the solvent dry at an increased
temperature in a vacuum in a so-called drying pistol or, more
conveniently, in a copper block desiccator (Pregl) which can easily be
finely adjusted to any desired temperature by regulating the microflame
which heats the block (Fig. 34). Weigh hygroscopic substances
in a small weighing tube.