This appears to be caused by the iron precipitate as it
were inclosing, and mechanically drawing down the light manganese
precipitate, provided a weak chemical union between the two precipitates
does not even take place, depending on the tendency of peroxide of
manganese to behave toward bases, as, for instance, hydrate of lime
as an acid. Hence it thus follows that it ought to be arranged that
a sufficient quantity of iron[1] (at least the same quantity as of
manganese) be present in the liquid at titration, also that time be
given for the precipitate to fall, so that the color of the solution may
be observed between every addition of chameleon solution.
[Footnote 1: For this in case of need a solution of perchloride of iron
free of manganese may be employed.]
When the content of manganese is large, it is sometimes rather long
before the solution is ready for titration. The reason of this appears
to be that a part of the manganese is first precipitated as hydrated
sesquioxide, which is afterward oxidized to hydrated peroxide, for the
upper portion of the liquid may sometimes be colored by chameleon, while
the lower portion, which is in closer contact with the precipitate,
is less colored or absolutely colorless.
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