Page 124 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

3 COMMON APPARATUS AND BASIC TECHNIQUES

and silicates in the presence of reducing agents; (d) sulphur (slight action),
selenium, and tellurium; (e) volatile halides (including iron(II1) chloride),
especially those which decompose readily; (f) al1 sulphides or mixtures
containing sulphur and a carbonate or hydroxide; and (g) substances of
unknown composition: (h) heating in an atmosphere containing chlorine,
sulphur dioxide, or ammonia, whereby the surface is rendered porous.
Solid carbon, however produced, presents a hazard. It rnay be burnt off at
low temperatures, with free access to air, without harm to the crucible, but it
should never be ignited strongly. Precipitates in filter paper should be treated
in a similar manner; strong ignition is only permissible after al1 the carbon has
been removed. Ashing in the presence of carbonaceous matter should not be
conducted in a platinum crucible, since metallic elements which rnay be present
will attack the platinum under reducing conditions.
Cleaning andpresevvation of platinum wave. Al1 platinum apparatus (crucibles,
dishes, etc.) should be kept clean, polished, and in proper shape. If, Say, a
platinum crucible becomes stained, a little sodium carbonate should be fused
in the crucible, the molten solid poured out on to a dry Stone or iron slab, the
residual solid dissolved out with water, and the vesse1 then digested with
concentrated hydrochloric acid: this treatment rnay be repeated, if necessary. If
fusion with sodium carbonate is without effect, potassium hydrogensulphate
rnay be substituted; a slight attack of the platinum will occur. Disodium
tetraborate rnay also be used. In some cases, the use of hydrofluoric acid or
potassium hydrogenfluoride rnay be necessary. Iron Stains rnay be removed by
heating the covered crucible with a gram or two of pure ammonium chloride
and applying the full heat of a burner for 2-3 minutes.
Al1 platinum vessels must be handled with care to prevent deformation and
denting. Platinum crucibles must on no account be squeezed with the object of
loosening the solidified cake after a fusion. Box-wood formers can be purchased
for crucibles and dishes; these are invaluable for re-shaping dented or deformed
platinum ware.
Platinum-clad stainless steel laboratory ware is available for the evaporation
of solutions of corrosive chemicals. These vessels have al1 the corrosion-resistance
properties of platinum up to about 550 OC. The main features are: (1) much
lower cost than similar apparatus of platinum; (2) the overall thickness is about
four times that of similar all-platinum apparatus, thus leading to greater
mechanical strength; and (3) less susceptible to damage by handling with
tongs, etc.

Silver apparatus. The chief uses of silver crucibles and dishes in the laboratory
are in the evaporation of alkaline solutions and for fusions with caustic alkalis;
in the latter case, the silver is slightly attacked. Gold vessels (m.p. 1050 OC) are
more resistant than silver to fused alkalis. Silver melts at 960 OC, and care should
therefore be taken when it is heated over a bare flame.
Nickel ware. Crucibles and dishes of nickel are employed for fusions with alkalis
and with sodium peroxide (CARE!). In the peroxide fusion a little nickel is
introduced, but this is usually not objectionable. No metal entirely withstands
the action offused sodium peroxide. Nickel oxidises in air, hence nickel apparatus
cannot be used for operations involving weighing.