A Manual of the Chemistry of the Carbon Compounds

16 THE CHEMISTRY OF
point of the tube broken off, and air drawn through, the whole apparatus
by means of an india-rubber tube fixed to the end of the potash
bulbs. This operation is necessary in order to pass the aqueous
vapour and carbonic dioxide filling the combustion tube into the
absorption apparatus. It now only remains to weigh the absorption
tubes again, the increase of weight giving respectively the quantities
of water and carbon dioxide produced.
If the solution be a liquid, it is weighed in a little sealed glass bulb
drawn out to a fine point. A little copper oxide is put into the tube
first, then the bulb with the point broken off, and afterwards, the tube
being filled with the oxide, the combustion is conducted as before.
Substances rich in carbon which are combustible only with difficulty,
are, by using this method, often incompletely burnt; the reduced
copper gets covered with carbon, -which, not being in contact
with the oxide, is not burned. In such a case the combustion must
be finished by passing a current of pure oxygen through the apparatus,
which is easily effected by placing a little fused potassium chlorate
at the far end of the combustion tube.
Instead of the method just described another is now generally
employed, which is to be recommended on account of its greater simplicity
and the more exact results obtained (Fig. 2). A combustion
tube is used which is open at both ends, one end being connected with
the absorption tubes and the other with a drying apparatus through
which either dry air or oxygen can be passed. The part of the
tube near the chloride of calcium tube is to two-thirds of its length
filled with granulated copper oxide, behind which the substance to be
analysed is placed in a platinum boat. In front of and in connection
with the absorption tubes is placed an aspirator, in order to ensure
the passage of the products of combustion through the absorption
tubes, and to prevent them by any chance from passing into the
drying apparatus. After the copper oxide has been heated to redness
the substance is gradually heated, a slow current of air being passed
at the same time through the apparatus in order to drive the products
of the combustion into the absorption tubes. As soon as the whole
tube is red-hot the current of air is changed for one of oxygen, by
which all the carbon left in the platinum boat is completely burned
and all the reduced copper re-oxidized.
This method fs very convenient, as after each combustion the apparatus
is exactly in the same state as it was before; and as soon as it is
cooled down a new combustion may be commenced.
If the body to be analysed contains nitrogen, a coil of copper is
placed in the fore part of the tube, and kept red-hot in order to
decompose any oxides of nitrogen which might be formed. Without
this precaution they -would be absorbed by the potash, and the weight
of the carbon be found too high.
Compounds containing chlorine, bromine, or iodine, yield on combustion
these elements in the free state. In order to prevent these
getting into the potash bulb, a coil of copper gauze, or, better still, a