Laboratory Methods of Organic Chemistry - Sciencemadness Dot Org

68 THE COMBUSTION
burner back again to the end of the tube and push the furnace so far
back that most of the layer of reduced copper extends outside the
furnace. Thus, for the most part, the reduced layer is preserved for
the next analysis. Now begin immediately to drive the products
of combustion completely into the absorption apparatus by passing
a further 180 c.c. of air through the tube. While the air is passing
through, again ignite the empty part of the tube for a short time,
beginning 1 cm. from the rubber stopper, using the burner and roll of
gauze. When 150 c.c. of water have been collected switch off the
current in the furnace. When the air current has been stopped close
the cock on the aspirator, then close the cocks of the absorption
apparatus, detach them from the combustion tube, close it with a
rubber cap, and let it cool, maintaining the pressure of air from the
gas holder. In this way the tube is ready for immediate use in the
next analysis. Weigh the absorption vessels after preparing them
suitably for the purpose (see below).
Liquids.—Weigh liquids in a small hard glass tube 1 about 4 cm.
long having glass supports (legs) at the open end and a ground glass
stopper. Sealed to the closed end is a small glass hook by means of
which the weighing tube can be moved in the combustion tube with
a bent wire. Clean the tube and drop in a small crystal of potassium
chlorate, melt the crystal over a small flame and let it solidify again.
After it has cooled, accurately weigh the prepared tube to 0-01 mg.
on a suitable wire frame and pour in from a fine capillary tube 20-30
mg. of the liquid to be analysed. Insert the stopper when weighing
volatile substances. For combustion push the tube with the substance,
open end towards the filling, into the combustion tube to
within 7-9 cm. of the furnace. If the tube is weighed stoppered,
loosen the stopper immediately before introducing into the combustion
tube, but leave the stopper in the tube. The stopper also
has a small glass hook sealed to it.
When carrying out the combustion of liquids heat somewhat
more carefully than is indicated above for solids, since liquids are
more volatile.
Weighing the Absorption Apparatus.—Prepare the absorption
apparatus for weighing and weigh, in exactly the same way as described
on p. 63.
While the combustion tube is cooling withdraw the boat with a
hooked platinum wire, ignite for a short time in the non-luminous flame
1 A. Friedrich, Z. angew. Chem., 1932, 45, 477.