By J.W. PRATT, F.C.S.
It frequently happens in the laboratory that platinum vessels, after
long-continued use, begin to show signs of wear, and become perforated
with minute pinholes. When they have reached this stage, they are usually
accounted of no further utility, and are disposed of as scrap; not that
it is impossible to repair them--for with fine gold wire and an
oxyhydrogen jet this is easily feasible--but that the proper appliances
and skill are not in possession of all. Irrespective of the manipulation
of the hydrogen jet, it is rather difficult without long practice to hold
the end of the fine wire precisely over the aperture and to keep it in
position. It occurred to me that, if the gold in a finely divided
condition could be placed in very intimate contact with the platinum,
judging from the fusibility of gold-platinum alloys, union could be
effected at a lower temperature over the ordinary gas blowpipe. I tried
the experiment, and found the supposition correct. The substance I used
was auric chloride, AuCl_{3}, which, as is well known, splits up on
heating, first into aurous chloride, and at a higher temperature gives
off all its chlorine and leaves metallic gold. Operating on a perforated
platinum basin, in the first instance, I placed a few milligrammes of the
aurous chloride from a 15 grain tube precisely over the perforation, and
then gently heated to about 200° C. till the salt melted and ran through
the holes. A little further heating caused the reduced gold to solidify
on each side of the basin. The blowpipe was now brought to bear on the
bottom of the dish, right over the particular spots it was wished to
solder, and in a few moments, at a yellow-red heat (in daylight), the
gold was seen to "run." On the vessel being immediately withdrawn, a very
neat soldering was evident. The operation was repeated several times,
till in a few minutes the dish had been rendered quite tight and
serviceable.
Using the gold salt in this way, the principal difficulty experienced in
holding gold wire unflinchingly in the exact position vanishes, while
only a comparatively low temperature and small amount of gold is
necessary. Care must be taken to withdraw the platinum from the flame
just at the moment the gold is seen to run, for if the heat be continued
longer, the gold alloys with a larger surface of platinum, spreads, and
leaves the aperture empty. As in the case of all gold-soldered vessels,
the article cannot afterward be safely exposed to a temperature higher
than that at which the soldering was effected, and on this account it is
advisable to use as small an amount of auric chloride as possible. When
the perforations are of comparatively large size, the repairing is not so
easy, owing to the auric chloride, on fusing, refusing to fill them. I
find, however, that if some spongy platinum be mixed with a few
milligrammes of the gold salt, pressed into the perforation, and heat
applied as directed, a very good soldering can be effected. It is well to
hammer the surface of the platinum while hot, so as to secure perfect
union and welding of the two surfaces. This may be done in a few minutes
in such a manner as to render the repair indistinguishable. Strips of
platinum may be joined together in much the same way as already
described--a few crystals of auric chloride placed on each clean surface
and gently heated till nearly black, then bound together and further
heated for a few moments in the blowpipe flame. Rings and tubes can also
be formed on a mandrel, and soldered in the same fashion, and the chemist
thus enabled to build up small pieces of apparatus from sheet platinum in
the laboratory.--_Chem. News._
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