The pinch cock at _d_ is then opened, and the apparatus allowed to
fill with carbon dioxide. When the pressure has become sufficient to
force the gas through the solution of permanganate, the pinch cock at
_f_ is removed. It should be opened only slightly and with great
caution at first, unless one is certain that the pressure is
sufficient. If the pressure is insufficient, the fact will be made
apparent by a rise of the permanganate in the small internal tube.
The flow of carbon dioxide is now reduced to a very slow current, or
entirely cut off. The contents of B are slowly heated, until the
decomposition of the nitrate is complete and the greater part of the
nitric oxide has been expelled, when the apparatus is again closed at
_f_ and _d_, and allowed to cool. The tube, _a_, is then washed out,
by the introduction through it into B of a few cubic centimeters of
strong hydrochloric acid.
The process of filling the apparatus with carbon dioxide, and of
heating the contents of B, is repeated. When it becomes apparent, from
the light color of the liquid in B, that all of the nitric oxide has
been expelled from it, the current of carbon dioxide is increased and
the heating discontinued. Care must be taken, however, not to admit
too strong a current of carbon dioxide, lest some of the nitric oxide
should be forced unabsorbed through the permanganate solution.
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