By H. SCHUCHT.
Concerning the separations which take place at the positive pole, the
composition of the peroxides, and the manner of their determination,
relatively little has been done.
If solutions of the salts of lead, thallium, silver, bismuth, nickel,
and cobalt are decomposed by the current between platinum electrodes,
metal is deposited at the negative, and oxide at the positive
electrode. Manganese is precipitated only as peroxide. The formation
of peroxide is, of course, effected by the ozone found in the
electrolytic oxygen at the positive pole; the oxide existing in
solution is brought to a higher degree of oxidation, and is separated
out. Its formation may be decreased or entirely prevented by the
addition of readily oxidizible bodies, such as organic acids, lactose,
glycerine, and preferably by an excess of oxalic acid; but only until
the organic matter is transformed into carbonic acid. In this manner
Classen separates other metals from manganese in order to prevent the
saline solutions from being retained by the peroxide.
With solutions of silver, bismuth, nickel, and cobalt, it is often
practicable to prevent the separation of oxide by giving the current a
greater resistance--increasing the distance between the electrodes.
The proportion between the quantities of metal and of peroxide
deposited is not constant, and even if we disregard the concentration
of the solution, the strength of the current and secondary influences
(action of nascent hydrogen) is different in acid and in alkaline
solutions. In acid solutions much peroxide is formed; in alkaline
liquids, little or none. The reason of the difference is that ozone is
evolved principally in acid solutions, but appears in small quantities
only in alkaline liquids, or under certain circumstances not at all.
The quantity of peroxide deposited depends also on the temperature of
the saline solution; at ordinary temperatures the author obtained more
peroxide--the solution, the time, and the strength of current being
equal--than from a heated liquid. The cause is that ozone is destroyed
by heat and converted into ordinary oxygen. With the exception of lead
and thallium the quantity of metal deposited from an acid solution is
always greater than that of the peroxide.
_Lead._--Luckow has shown that from acid solutions--no matter what may
be the acid--lead is deposited at the anode as a mixture of anhydrous
and hydrated peroxide of variable composition. Only very strongly acid
solutions let all their lead fall down as peroxide; the precipitation
is rapid immediately on closing the circuit, and complete separation
is effected only in presence of at least 10 per cent. of free nitric
acid. As the current becomes stronger with the increase of free acid,
there is deposited upon the first compact layer a new stratum of
loosely adhering peroxide.
In presence of small quantities of other metals which are thrown down
by the current in the metallic state, such as copper, mercury, etc.,
peroxide alone is deposited from a solution of lead containing small
quantities only of free nitric acid.
The lead peroxide deposited is at first light brown or dark red, and
becomes constantly darker and finally taking a velvet-black. As its
stratification upon the platinum is unequal, it forms beautifully
colored rings.
Experiments show that the quantity of peroxide deposited depends on
the nature of the solution and the strength of the current. In case of
very feeble currents and slight acidity, its quantity is so small that
it does not need to be taken into consideration. If the lead solution
is very dilute scarcely any current is observed, lead solutions _per
se_ being very bad conductors of electricity.
Faintly acid concentrated lead solutions give loose peroxide along
with much spongy metallic lead. Free alkali decreases the separation
of peroxide; feebly alkaline solutions, concentrated and dilute, yield
relatively much peroxide along with metallic lead, while strongly
alkaline solutions deposit no peroxide.
Dried lead peroxide is so sparingly hygroscopic that it may be weighed
as such; its weight remains constant upon the balance for a long time.
In order to apply the peroxide for quantitative determinations, a
large surface must be exposed to action. As positive electrode a
platinum capsule is convenient, and a platinum disk as negative pole.
The capsule shape is necessary because the peroxide when deposited in
large quantities adheres only partially, and falls in part in thin
loose scales. It is necessary to siphon off the nitric solution,
since, like all peroxides, that of lead is not absolutely insoluble in
nitric acid. The methods of Riche and May give results which are
always too high, since portions of saline solution are retained by the
spongy deposit and can be but very imperfectly removed by washing.
This is especially the case in presence of free alkali.
The author has proceeded as follows: The lead peroxide is dried in the
capsule, and there is passed over it pure dry gaseous sulphurous acid
in a strong current from a rather narrow delivery tube. Lead sulphate
is formed with evolution of heat; it is let cool under the exsiccator,
and weighed as such. Or he ignites the peroxide along with finely
pulverized ammonium sulphite; the mass must have a pure white color.
After the conclusion of the reaction it is ignited for about 20
minutes. The results are too high. The proportion of actual lead
peroxide in the deposit ranges from 94 to 94.76 per cent. The peroxide
precipitated from a nitric solution may, under certain circumstances,
be anhydrous. This result is due to the secondary influences at the
positive pole, where the free acid gradually withdraws water from the
peroxide.
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
3½ 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 laminæ.
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.
_Selenium and Tellurium._--Both these bodies are readily and
completely reduced by the current either in acid or alkaline
solutions. Selenium is thrown down at first of a fine brownish red,
which gradually becomes darker. The deposit of tellurium is of a
bluish black color. If the current is feeble, the deposit of selenium
is moderately compact; that of tellurium is always loose, and it often
floats on the liquid. A strong current precipitates both as powders.
The positive pole is coated during electrolysis with a film of a dark
color in case of selenium, but of a lemon yellow with tellurium. As in
case of arsenic and antimony, the hydrogen evolved at the negative
pole combines with the reduced substances, forming hydrogen, selenide,
or telluride, which remain in part in solution in the liquid. The
reduced metal separates out at the anode in a friable
condition.--_Zeitschrift fur Analytische Chemie, and Chemical News._
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