Page 123 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

METAL APPARATUS 3.20

Apparatus can also be constructed from 'TRIM' which consists of palladium
coated with ZGS platinum; this permits the production of stouter apparatus
with the corrosion resistance of ZGS platinum at an appreciably cheaper price.
Platinum crucibles should be supported, when heated, upon a platinum
triangle. If the latter is not available, a silica triangle rnay be used. Nichrome
and other metal triangles should be avoided; pipe-clay triangles rnay contain
enough iron to damage the platinum. Hot platinum crucibles must always be
handled with platinum-tipped crucible tongs; unprotected brass or iron tongs
produce Stains on the crucible. Platinum vessels must not be exposed to a
luminous flame, nor rnay they be allowed to come into contact with the inner
cone of a gas flame; this rnay result in the disintegration of the surface of the
metal, causing it to become brittle, owing, probably, to the formation of a
carbide of platinum.
It must be appreciated that at high temperatures platinum permits the flame
gases to diffuse through it, and this rnay cause the reduction of some substances
not otherwise affected. Hence if a covered crucible is heated by a gas flame there
is a reducing atmosphere in the crucible: in an open crucible diffusion into the
air is so rapid that this effect is not appreciable. Thus if iron(II1) oxide is heated
in a covered crucible, it is partly reduced to metallic iron, which alloys with the
platinum; sodium sulphate is similarly partly reduced to the sulphide. It is,
advisable, therefore, in the ignition of iron compounds or sulphates to place the
crucible in a slanting position with free access of air.
Platinum apparatus rnay be used without significant loss for:
1. Fusions with (a) sodium carbonate or fusion mixture, (b) borax and lithium
metaborate, (c) alkali bifluorides, and (d) alkali hydrogensulphates (slight
attack in the last case above 700 OC, which is diminished by the addition of
ammonium sulphate).
2. Evaporations with (a) hydrofluoric acid, (b) hydrochloric acid in the absence
of oxidising agents which yield chlorine, and (c) concentrated sulphuric acid
(a slight attack rnay occur).
3. Ignition of (a) barium sulphate and sulphates of metals which are not readily
reducible, (b) the carbonates, oxalates, etc., of calcium, barium and strontium,
and (c) oxides which are not readily reducible, e.g. Cao, SrO, Al,O,, Cr,O,,
Mn,O,, TiO,, ZrO,, Tho,, MoO,, and WO,. (BaO, or compounds which
yield Ba0 on heating, attack platinum.)

Platinum is attacked under the following conditions, and such operations
must not be conducted in platinum vessels:
1. Heating with the following liquids: (a) aqua regia, (b) hydrochloric acid and
oxidising agents, (c) liquid mixtures which evolve bromine or iodine, and
(d) concentrated phosphoric acid (slight, but appreciable, action after
prolonged heating).
2. Heating with the following solids, their fusions, or vapours: (a) oxides,
peroxides, hydroxides, nitrates, nitrites, sulphides, cyanides, hexacyano-
ferrate(III), and hexacyanoferrate(I1) of the alkali and alkaline-earth metals
(except oxides and hydroxides of calcium and strontium); (b) molten lead,
silver, copper, zinc, bismuth, tin, or gold, or mixtures which form these metals
upon reduction; (c) phosphorus, arsenic, antimony, or Silicon, or mixtures
which form these elements upon reduction, particularly phosphates, arsenates,