A
series of four determinations gave differences, between the volumes of
permanganate calculated and used, of 0.05 to 0.15 c.c.
The principal objection to the method lies in the difficulty of
determining, in the presence of the brown oxide of manganese, the
exact point at which the oxidation is complete.
The carbon dioxide generator, A, was devised by us to take the place
of the ordinary generators, in which marble is used. We have found
that a submersion of twenty hours in boiling water does not suffice to
completely remove the air which, as is well known, is contained in
ordinary marble; hence some other substance must be employed as a
source of the gas. In the apparatus which we are about to describe,
the acid carbonate of sodium is used.
It consists of a long, narrow cylinder (450 x 60 mm.); a tightly
fitting rubber stopper, through which three tubes pass, as shown in
the figure; a small cylinder, F, containing mercury; and a sulphuric
acid reservoir, G.
The tube, _g_, is drawn out to a fine point at the end and curved, so
that the acid which is delivered into A falls upon and runs down the
outside of the tube. The tube, h, dips under the mercury in F. G and
_g_ are connected by means of a long piece of rubber tubing which is
supplied with a screw pinch cock.
The apparatus is made to give any required pressure by raising or
lowering G and F; but the elevation of G, as compared with that of F,
should always be such that the gas will force its way through h rather
than g.
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