QTAXTITA 77 I'A' IWI'Klt.}/1.\ .\TtO\X 77
CARBONATES
The Carbonate Radical, The determination of the carbonate
radical of solid carbonates can be made with accuracy only by
decomposing the carbonate with a .stronger acid, then purifying
the resulting carbon dioxide1 and absorbing it in Homo manner
in another reagent. This is the basis for both gravimetric and
volumetric methods. In the former class of methods (.he* earbon
dioxide* is absorbed in a basic substance* (usually potassium
hydroxide or soda lime) contained in a small apparatus that can
he weighed, the gain in weight serving as a measure of the
quantity of gas absorbed. Or it. is sometimes absorbed by a
solution of barium hydroxide, I lie barium carbonate so formed
being removed by filtration, dissolved in hydrochloric acid and
the barium reprcci pi fated as sulphate. Front the weight, of
barium sulphate the corresponding weight of carbon dioxide is
calculated.
In the volumetric modifications the absorbing reagent is a
standard solution of a base, such as barium hydroxide*, u measured
volume of this being titrated by a standard acid, solution after
the absorption in finished. Hither method will give accurate
results, if can* IK used.
Gravimetric Method.............Tin* necessary parts of the apparatus
are .shown in Fig. 20, In this figure, A represents a generating
flask in which the weighed sample of carbonate is placed. It
in a dropping funnel having a capacity of 50 ec and having the
lower end drawn out to a point find turned upward. Thin part
should extend to the* bottom of the flank. At the top of the
dropping funnel a drying tube (' is connected by means of u
rubber stopper and a bent- glass tube. The drying tube is filled
with soda lime* for the absorption of carbon dioxide from the air
that is later to be drawn through. Following the generating
flask is a short-, condenser I) (the lower end of which should be
beveled) and then I "-tubes #, /*' and (L The* first I'-fube is
omitted if sulphuric acid is to lie used for decomposing the car-
bonate or it is filled with nn absorbent for hydrochloric acid
vapors if thin add in to be used. The l>tubeĢ /'' mid <i are filled
with granular calcium chloride which absorbs mointure from the
gaw mixture, Following these in the apparatus // in which
potassium hydroxide In placed for the* absorption of carbon