At the Royal Powder Works at Spandau, Prussia, frequent ignition of the
powder at a certain stage of the process led to an examination of the
machinery, when it was found that where, at certain parts, bronze pieces
which were soldered were in constant contact with the moist powder, the
solder was much corroded and in part entirely destroyed, and that in the
joints had collected a substance which, on being scraped out with a
chisel, exploded with emission of sparks. It was suspected that the
formation of this explosive material was in some way connected with the
corrosion of the solder, and the subject was referred for investigation
to Rudolph Weber, of the School of Technology, at Berlin. The main
results of his investigation are here given.
The explosive properties of the substance indicated a probable
nitro-compound of one of the solder metals (tin and lead), and as the
lead salts are more stable and better understood than those of tin, it
was resolved to investigate the latter, in hope of obtaining a similar
explosive compound. Experiments on the action of moist potassium nitrate
on pure tin led to no result, as no explosive body was formed. Stannous
nitrate, Sn(NO_{3})_{2}, formed by the action of dilute nitric acid on
tin, has long been known, but only in solution, as it is decomposed on
evaporating. By adding freshly precipitated moist brown stannous oxide
to cool nitric acid of sp. gr. 1.20, as long as solution occurred, and
then cooling the solution to -20 deg., Weber obtained an abundance of
crystals of the composition Sn(NO_{3})_{2} + 20H_{2}O. They resemble
crystals of potassium chlorate. They cannot be kept, as they liquefy at
ordinary temperatures. An insoluble _basic_ salt was obtained by
digesting an excess of moist stannous oxide in solution of stannous
nitrate, or by adding to a solution of stannous nitrate by degrees, with
constant stirring, a quantity of sodium carbonate solution insufficient
for complete precipitation. Thus obtained, the basic salt, which has the
composition Sn_{2}N_{2}O_{7}, is a snow-white crystalline powder, which
is partially decomposed by water, and slowly oxidized by long exposure
to the air, or by heating to 100 deg.. By rapid heating to a higher
temperature, as well as by percussion and friction, it explodes
violently, giving off a shower of sparks. This compound is also formed
when a fine spray of nitric acid (sp. gr. 1.20) is thrown upon a surface
of tin or solder. It is also formed when tin or solder is exposed to the
action of a solution of copper nitrate, and thus formed presents the
properties already described.
In this, then, we have a probable cause of the explosions occurring in
the powder works; but the explanation of the formation of the substance
is wanting, as potassium nitrate was shown not to give an explosive
substance with tin. A thin layer of a mixture of sulphur and potassium
nitrate was placed between sheets of tin and copper foil, and allowed to
stand, being kept constantly moist. After a time the copper was found to
have become coated with sulphide, while the tin was largely converted
into the explosive basic nitrate. The conditions are obviously the same
as those found in the powder machinery, where bronze and tin solder are
constantly in contact with moist gunpowder. The chemical action is
probably this: the sulphur of the powder forms, with the copper of the
bronze, copper sulphide; this is oxidized to sulphate, which reacts with
the niter of the powder, forming potassium sulphate and copper nitrate;
the latter, as shown above, then forms with the tin of the solder the
explosive basic nitrate, which, being insoluble, gradually collects in
the joints, and finally leads to an explosion.--_Journal fuer Praktische
Chemie._
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