The quantity of residual acid is
then determined by titrating with standard solution of alkali. This
method has been described under _Lime_.
GASOMETRIC METHOD.
This method is the quickest of all, and the least troublesome after the
apparatus has been once prepared. It yields fairly accurate results when
worked in the manner described below; but if greater precautions are
taken the results are exact. It depends on the measurement of the volume
of gas given off on treating the weighed sample with acid. The apparatus
described, page 52, is used. Weigh out a portion of the mineral which
shall contain not more than 0.15 gram of carbonic acid (or 0.4 gram of
carbonate of lime) and put it in the bottle. Put in the inner tube 10
c.c. of dilute hydrochloric acid (1--1), cork tightly, and read off the
level of the liquid in the burette after adjusting the pressure. Turn
the acid over on to the mineral. Run out the water so as to keep the
level in the two burettes the same. When effervescence has ceased,
rotate the contents of the bottle; finally, adjust the level in the
burettes and read off the volume. The increase in volume is due to the
evolved carbon dioxide. At the same time read off the "volume
corrector."
Some of the carbon dioxide remains dissolved in the acid in the
generating bottle, and the quantity thus dissolved will depend on the
amount of carbonate as well as on the amount of acid present.
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