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Page 33
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.
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