Through the other opening passes a glass tube bent at an acute angle
and connected by a short rubber tube to an adjoining flask, two-thirds
filled with decinormal baryta solutions. These connections are all made
air tight. Sulphuric acid is allowed in small portions at a time to
flow into the mixture. Carbonic acid is evolved, and, passing into
the adjoining flask, is absorbed by the baryta, precipitating it as
carbonate. To prevent the precipitate forming around or choking up the
entrance tube, the flask must be agitated at short intervals to break it
off. The reaction so familiar to us in other determinations is expressed
thus:
MnO_{2}+KO,C_{2}O_{3}+2SO_{3} = MnO,SO_{3}+KO.SO_{3}+2CO_{2},
When no more carbonic acid is evolved, another tube from this last flask
is connected with the aspirator, the pinch-cock of the pipette open, and
air drawn through the apparatus for about half a minute, and thus all
the carbonic acid evolved absorbed, or the flasks may be slightly
heated. If danger of more carbonic acid being absorbed from the air is
feared, and always in very accurate analysis, a potassa tube may be
connected to the pipette before drawing the air through.
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