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.
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