[Illustration: FIG. 79.]
[Illustration: FIG. 80.]
~Direct Gravimetric Method.~--Fit up the apparatus shown in the diagram
(fig. 79). The various tubes are supported by a fixed rod with nails and
wire loops, and connected by short lengths of rubber-tubing. The first
tube contains soda-lime. The small flask is fitted with a rubber-stopper
perforated with two holes, through one of which passes the tube of a
pipette holding 25 or 30 c.c. This pipette is to contain the acid. The
substance to be determined is weighed out into the flask. The second
tube contains strong sulphuric acid; the third, pumice stone, saturated
with copper sulphate solution, and dried until nearly white (at 200° C.);
the fourth contains recently fused calcium chloride; and the fifth,
which is the weighed tube in which the carbonic acid is absorbed,
contains calcium chloride and soda-lime,[120] as shown in fig. 80. The
sixth also contains calcium chloride and soda-lime; its object is to
prevent the access of moisture and carbonic acid to the weighed tube
from this direction; it is connected with an aspirator.
Having weighed the ~U~-tube and got the apparatus in order, weigh up 1,
2, or 5 grams of the substance and place in the flask. Fill the pipette
with dilute acid, close the clamp, and cork the flask. Then see that the
apparatus is tight.
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