Laboratory Methods of Organic Chemistry - Sciencemadness Dot Org

THE COMBUSTION 65
ously been specially dried, or if it has been out of use for a long time,
heat the empty part of the tube, before beginning a series of analyses,
with a Bunsen burner after the furnace has reached steady temperature.
Employ a small roll of wire gauze and carefully shield the
rubber stopper with an asbestos plate, beginning to heat at a distance
of 1 cm. from the stopper. When heating with the burner
has extended to the furnace, remove the burner and slip the roll
of wire gauze back to the end of the tube so that the part of the
tube adjacent to the furnace cools again ready for the insertion of
the boat with the substance.
After the absorption apparatus has been weighed push the side
tube of the calcium chloride tube which leads to the water trap halfway
into the 1 -5 cm. connecting piece of rubber tubing; in the same
way attach the other end of the tube to the side tube of the sodium
hydroxide-asbestos tube which leads directly to sodium hydroxideasbestos,
using the 2 cm. connecting tube and taking care that the
ends of the tubes fit together as closely as possible, and attach the
absorption apparatus to the stand. Now rapidly check the rate of
flow of the bubbles, using a watch (number of bubbles in ten seconds)
and, if necessary, adjust the pressure regulator so that 9-10 c.c. of
air (or oxygen) pass per minute as determined by means of the
calibrated bubble-counter.
Failure of the bubble-counter is frequently due to the increase in
concentration of the alkali which results from long use. If such
"failure " occurs introduce fresh (but rather more dilute) alkali. Then
calibrate the bubble-counter anew. Another source of error is the sticking
together of the calcium chloride granules in the drying apparatus or
in the drying tube attached to the gas wash-bottle. Take the greatest
care to prevent the alkali flowing from the bubble-counter into the
rubber tube leading to the three-way cock.
Now fit the calcium chloride tube closely to the constricted part
of the combustion tube and connect the sodium hydroxide-asbestos
tube to the calcium chloride tube of the aspirator. Then remove
the rubber stopper from the combustion tube, push back the stand
carrying the drying apparatus so as to make more room, raise the
copper block with the boat to the opening of the combustion tube,
with the forceps insert the boat, push it inwards 4-5 cm. up to the
furnace with a clean glass rod of suitable size, taking care not to tip
the boat over, insert the rubber stopper loosely into the tube,
moistening the stopper if necessary with a very thin film of glycerol,