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Page 34
The peroxide thrown down from alkaline solutions retains alkali so
obstinately that it cannot be removed by washing; the peroxide plays
here the part of an acid. The lead nitrate mechanically inclosed in
the peroxide is resolved by ignition into oxide, hyponitric acid, and
oxygen; this small proportion of lead oxide does not exert an
important influence on the final result. The quantity of matter
mechanically inclosed is relatively high, as in the precipitation of
much lead peroxide there is relatively more saline matter occluded
than when a few centigrammes are deposited. The peroxide incloses also
more foreign matter if it is thrown down upon a small surface than if
it is deposited in a thin layer over a broad surface. From numerous
analyses the author concludes that in presence of much free nitric
acid the proportion of water is increased; with free alkali the
reverse holds good.
_Thallium_ behaves similarly to lead. From a nitric acid solution it
is thrown down, according to the proportion of free acid, either as
sesquioxide only or in small quantities as silvery, metallic leaflets;
from alkaline solutions it is deposited as sesquioxide and metal, the
latter of a lead-gray color. Thallium solutions conduct the electric
current badly. Thallium oxide resembles lead peroxide in color; at a
strong heat it melts, becomes darker, and is converted into peroxide,
in which state it can be weighed.
_Silver._--All solutions of silver salts, except the nitrate, and
those containing a very large quantity of free nitric acid or
nitrates, deposit electrolytically merely metallic silver. In the
above mentioned exceptional cases there is formed a small quantity of
peroxide which adheres to the anode as a blackish-gray deposit. The
greatest quantity of peroxide is obtained on employing a concentrated,
strongly acid solution of the nitrate, and a strong current. If the
solution is very dilute we obtain no peroxide, or mere traces which
disappear again toward the end of the process. The peroxide is
deposited at first in small, dark, shining octahedral crystals;
subsequently, in an amorphous state. At 110� it evolves oxygen
suddenly, and is converted into metallic silver. It dissolves in
ammonia with a violent escape of nitrogen. In nitric acid it dissolves
without decomposition and with a red color.
The author uses a galvanic current for reducing silver residues,
consisting of sulphocyanide. The salt is mixed with sulphuric acid in
a roomy platinum capsule, and a fine platinum wire gauze is used as
positive electrode.
_Bismuth._--The current resolves bismuth solutions into metal and
bismutic acid. The latter is deposited at the positive pole, and in
thin layers appears of a golden-yellow, but in thick strata is darker,
approaching to red. Its formation is very gradual, and in time it
disappears again, owing to secondary actions of the current. On
ignition it becomes lemon yellow, and transitorily darker, even brown,
and passes into the sexquioxide.
_Nickel and Cobalt._--On the electrolysis of the ammonical solution
the sesquioxide appears at the positive pole. Its formation is
prevented by an excess of ammonia. The author never obtains more than
3� per cent. of the quantity of the metal. The sesquioxides dissolve
in ammonia without escape of nitrogen, and are usually anhydrous.
_Manganese._--Manganese is the only metal which is precipitated only
as peroxide. It is deposited at once on closing the circuit, and is at
first brown, then black and shining. Organic acids, ferrous oxide,
chromic oxide, ammonium salts, etc., prevent the formation of peroxide
and the red color produced by permanganic acid. In very dilute
strongly acid nitric solutions there is formed only permanganic acid,
which according to Riche is plainly visible in solutions containing
1/1000000 grm. manganese. On electrolyzing a manganiferous solution of
copper nitrate, red permanganic acid appeared in a stratum floating
above the platinum disk coated with brown peroxide. No manganese
peroxide was deposited. The peroxide adheres firmly to the platinum
when the proportion of free acid is small, not exceeding 3 per cent.,
and the current is not too strong. If the action of the current is
prolonged after the peroxide is thrown down, it falls off in lamin�.
According to Riche, in a nitric solution the manganese is deposited as
peroxide, also at the negative pole. This formation is not directly
due to the current, but is a precipitate occasioned by the production
of ammonia by the reduction of nitric acid. To determine the manganese
in peroxide electrolytically precipitated, it is heated to bright
redness in the platinum capsule until the weight becomes constant. The
results are too high.
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