Laboratory Methods of Organic Chemistry - Sciencemadness Dot Org

48 THE DUMAS METHOD
globe through a glass tube shaped like a hook with the point upwards.
This tube is attached by means of a short piece of rubber tubing to the
inner end of the tube carrying a glass stop-cock which is fitted into the
side opening of the globe. A rubber stopper, greased with a little
vaseline, holds the stop-cock in place in this opening. Now pour dilute
hydrochloric acid into the apparatus until the lowest globe and half of
the uppermost globe are filled (as in Fig. 32) and drop two small pieces
FIG. 32
of marble into the stem of the funnel globe so that they are trapped and
remove the air dissolved in the acid by producing a vigorous evolution
of carbon dioxide. Accelerate this removal by opening and closing
the cock repeatedly.
As a rule a newly charged Kipp supplies sufficiently pure carbon
dioxide only after two or three days' standing. This time is required for
the air adsorbed on the surface of the glass and rubber to be given up
to the carbon dioxide atmosphere. When the " micro-bubbles " rising
in the nitrometer, often overtaking each other and combining—when
these bubbles ascend at a uniform rate, the gas may be regarded as
satisfactory. The diameter of the bubbles (examined with a lens)
should not exceed one-fifth of the distance between two graduation
marks (about 0-2 mm.).
The Kipp is attached to the combustion tube by means of a Z-shaped
glass tube one end of which is drawn out to form a thick-walled, roughly
conical capillary. This is pushed through the hole in the rubber stopper
fixed in the combustion tube. At the other end is attached a short
glass tube filled with asbestos wool to trap acid fog. This tube fits over
the horizontal delivery tube from the stop-cock of the Kipp, the wellfitting
glass-to-glass joint being made as tight as possible by means of a
piece of rubber tubing moistened with a little glycerol (see Fig. 32).'