Handling Your Precious Metal Labware
INTRO.....................................................................Page Two
FUSIONS.................................................................Page Two
Sodium carbonate
Sodium carbonate mixed with sodium nitrate or nitrite
Sodium borate or sodium metaphosphate
Alkali bifluorides
Alkali chlorides or alkaline earth chlorides
Alkali bisulphate’s
IGNITIONS..............................................................Page Three
Ignition of filtered precipitates
Determination of ash in organic materials
Determination of volatile matter fuels
EVAPORATIONS......................................................Page Three
ELECTROLYTIC ANALYSIS......................................Page Four
CONDITIONS TO AVOID.........................................Page Four
Atmospheres
Liquids
Solids (inc fusions and vapours)
CARE OF PLATINUM LABWARE..............................Page Five
Cleaning of crucibles and dishes
Cleaning of electrodes
CORROSION...........................................................Page Seven
Oxides
Acids
Atmospheres
Alkali Metal Compounds
Elements
Miscellaneous
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INTRO
Pure platinum labware is suitable for numerous laboratory operations such as
fusions, ignitions and evaporations that are carried out in oxidising atmospheres.
Pure platinum also has exceptional qualities in the construction of electrodes by
providing an excellent combination of mechanical strength, electrical conductivity
and resistance to chemical and electrolytic attack. Both anode and cathode
remain constant in weight during electrolysis, and deposits can be easily
dissolved after weighing without damage to the electrodes.
When properly used, pure platinum labware provides many advantages. For
various applications the basic rules for use are outlined below:
FUSIONS
Fusions should always be done under oxidizing conditions using materials free
from organic substances. During fusion and cooling, the material should only
come in contact with platinum or a clean refractory.
As a general rule, fusions made in platinum-rhodium alloy labware are not
recommended due to the fact that rhodium is rendered partially soluble in certain
compounds, i.e., alkaline bisulphates and pyrosulphates. Generally used fusion
agents affect pure platinum in the following manner:
Sodium carbonate
The loss of weight of platinum amounts to a fraction of a milligram.
Sodium carbonate mixed with sodium nitrate or nitrite
Under ordinary laboratory conditions the loss of platinum is not likely to exceed
two milligrams.
Sodium borate or sodium metaphosphate
Atlow temperatures, pure platinum is unaffected by borax. Sodium
metaphosphate attacks the metal only at very high temperatures or under
reducing conditions.
Alkali bifluorides
The loss of weight of platinum is practically nil.
Alkali chlorides or alkaline earth chlorides
Both attack platinum above 1000°C in the presence of air, which liberates
chlorine from the fused salt in a neutral atmosphere, these chlorides are inert.
Alkali bisulphates
Platinum is attacked slightly above 700°C. This can be reduced by the addition of
ammonium sulphate. With the exception of alkali bifluorides, traces of platinum
should be sought when making an accurate analysis using any of the foregoing
fusion agents.
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IGNITIONS
Platinum is subject to attack during ignition by any easily reducible substance.
The risk of damage can be avoided by taking the proper precautions with the
materials involved.
Ignition of filtered precipitates
Platinum labware can be used in ignitions of the following precipitates: Barium
sulphate. Sulphates of metals not readily reducible.
•Alkaline earth carbonates, oxalates, etc.
•Oxides not readily reducible, such as Al2O3,MgO, BaO, CaO, SrO, TiO2,ZrO2,
ThO2,MoO3,WO3,Ta2O5,Cr2O3,Mn3O4,and in the absence of carbonaceous
matter: ZnO, CdO, NiO, CdO.
Dry the precipitate contained on the moist filter paper in a platinum dish by a
preliminary heating on a hot plate or similar source. After drying, transfer the
dish to the area in front of the hot open muffle of a furnace operating at 800°C.
Within 10 to 15 minutes, the paper will become completely charred. Next, place
the dish approximately 5cm inside the muffle, where it should remain until nearly
all carbonaceous matter has been burned off. Somewhat higher temperatures
may be required to remove the last traces of carbon and the decomposition of the
precipitate. This can be accomplished without detriment to the platinum providing
there is a free access of air. Precipitates such as MgNH4P04,MgNH4AsO4,and
CaWO4may be used without unnecessary danger if suitable care is exercised to
assure safety.
Determination of ash in organic materials
A shallow dish should be used and the contents heated well forward in a hot open
muffle until combustion is nearly complete. If the surface is disturbed from time
to time, combustion can be accelerated. After all carbonaceous matter has been
burned away, final combustion can be conducted in the hot zone of the muffle.
Determination of volatile matter in fuels
Particular care should be exercised in this operation. There is a danger in
contaminating the platinum dish because it is essential that a non-oxidizing
atmosphere be maintained in the crucible to prevent loss of free carbon. This is
particularly true if the fuel is rich in sulphur or phosphorous compounds.
EVAPORATIONS
Platinum labware can be used in evaporations containing the following:
•Sulphuric acid with or without hydrofluoric acid
•Hydrofluoric acid
•Hydrofluoric and nitric acid when other halides (especially chlorides) are present
•Hydrochloric acid in the absence of oxidising agents that yield nascent chlorine
•Alkali hydroxides or carbonates where contamination of the solution with silica
must be avoided
•Sodium peroxide solutions.
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ELECTROLYTIC ANALYSIS
Electrochemical methods of analysis are widely used because of their speed,
simplicity and economy. In multiple analysis of a routine nature, several solutions
can be electrolysed simultaneously. In single determinations, the analyst can
attend to other operations while electrolysis is in progress. Platinum electrodes
can be used to advantage in numerous applications: As anodes in most
electrolytes except strongly acid chloride solutions. As cathodes for the
deposition of metals from acid, alkaline or ammoniacal solutions. For the
deposition of zinc, gallium, and bismuth, the platinum should first be copperplated to prevent superficial alloying with the deposited metal.
CONDITIONS TO AVOID
When platinum is heated to high temperatures, it can be attacked by certain
atmospheres, solids, fusions and vapours. Under these circumstances, the
following should be avoided:
Atmospheres
1. Ammonia - darkens the surface,making it less lustrous. In time, it will develop
pores and create a crystalline appearance.
2. Sulphur dioxide - promotes the formation of sulphur trioxide,which results in
platinous sulphide coating.
3. Chlorine - converts the metal into a mass of crystals due to the alternate
formation and decomposition of platinous chloride.
4. Volatile chlorides - especially those that readily decompose.
5. Gases and vapours containing high contents of free carbon, because Platinum
disintegrates if it comes into contact with carbon. Therefore, platinum should
never be heated in the reducing zone of a gas flame.
Liquids
1.Aqua regia
2. Hydrochloric acid with oxidizing agents.
3. Concentrated sulphuric acid - harmful when extended periods are involved. In
most laboratory applications, the action is so slow that it is usually of negligible
importance.
4. Concentrated phosphoric acid - action is noticeable only after very prolonged
heating.
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Solids (including their fusions and vapours)
1.Sulphur, selenium and tellurium - action of sulphur is very slow, with
prolonged heating required to produce any serious effects. Selenium and
tellurium combine readily with platinum.
2. Phosphorus, arsenic and antimony - combine readily with platinum.
3. Magnesium pyrophosphate - harmful above 900°C.
4. Silica and borax - harmful at high temperatures.
5. Molten lead, zinc, tin, bismuth, silver, gold or copper - or mixtures that form
these metals by reduction.
6. Fused alkali oxides and peroxides - and, to a lesser degree, alkali hydroxides
inthe presence of air.
7. Fused nitrates - action intensified by the presence of alkali hydroxides or
carbonates, though not severe in any case.
8. Phosphorus pentoxide or phosphoric acid - action relatively slight.
9. Fused cyanides - platinocyanides are formed.
Iron oxide above 1200°C - oxygen is liberated and the iron combines with the
platinum. Silica, silicates, alumina and magnesia above 1600°C -no action occurs
at lower temperatures. Fused alkali and alkali chlorides - in the presence of air at
1000°C or above. Lead and bismuth oxides above 1250°C.
CARE OF PLATINUM LABWARE
The life of platinum labware can be extended by making it a practice to observe
some fundamental rules:
1.Avoid the unnecessary prolonged heating of platinum as it tends to promote
crystal growth that eventually produces cracks.
2. Place hot platinum vessels on a clean refractory material. Never place them on
acold metallic or dirty surface.
3. Heat platinum ware under oxidizing conditions whenever possible. When
heated over a Bunsen or Meker Burner, only the upper nonluminous cone should
be employed - not the inner cone. Avoid using a smoky flame because the free
carbon will embrittle the platinum.
4. Always use clean platinum-tipped tongs to handle hot platinum vessels. When
base metal tongs are the only implements available, allow the vessel to cool
before handling.
5. When dislodging melts from crucibles, exercise care to avoid distorting the
vessel. If distortion does occur, reshape the crucible.
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6. Use clean triangles (porcelain, alumina or preferably platinum) to support
crucibles when making fusions.
7. New platinum crucibles and dishes can exhibit adhesiveness at temperatures
higner than 1000°C. This is mainly a surface phenomenon which vanishes
gradually as the material becomes older. For this reason, it is advisable not to
place new crucibles and dishes in a furnace in such a way that they can come
into contact with each other. During the initial ignitions, a new crucible should be
covered with a previously used lid.
Cleaning of crucibles and dishes
Platinum vessels should always be cleaned after use. They can be immersed in a
chromic acid cleaning mixture to remove many impurities, particularly organic
matter. Boiling in hydrochloric acid may be required to remove insoluble
carbonates or metal oxides. This treatment can be followed by boiling in nitric
acid, but the article should first be thoroughly rinsed. Thorough rinsing avoids
the presence of hydrochloric acid in a nitric acid solution, which would attack the
platinum. The solvent action of fused potassium bisulphate can usually be
employed to remove adherent silica, silicates, metal and metal oxides. After
fusion, carefully run the molten bisulphate over the inner surface, allow to cool,
dissolve the melt in boiling water and rinse thoroughly. Boiling in hydrochloric
acid may be required to supplement this procedure. After cleaning, the platinum
crucible or dish may be polished by rubbing it gently with recommended mild
polish. Please contact GLC for their recommendation.
Cleaning of electrodes
A platinum electrode should be washed with water after removal from the
electrolyte. It should then be washed with alcohol and dried in a stream of warm
air at a temperature below 100°C. Electrodes should not be dried by vigorous
shaking as this action tends to bend the stem. Repeated bending and
straightening will eventually fracture the stem.
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CORROSION
OXYDES
REMARKS
iron, lead and bismuth oxide
reaction occurs above 1200°C
ZnO, CO3O4, NiO, CdO and
other reducible oxides
incineration must occur inder oxidizing
circumstances
more stabile oxides such as
Al2O3, MgO, TiO2 and ZnO2
incinerations possible without attack
SiO2 (and silicates)
attack under reducing circumstances
above 1220°C
ACIDS
REMARKS
concentrated phosphoric acid or reaction observable after prolonged
P2O5
heating
H2SO4, HF and HNO3
evaporations can be carried out
without much loss of platinum
HCl & oxidizing acid (e.g.
nitrohydrochloric acid)
fast attack
organic acids
no attack
ATMOSPHERES
REMARKS
ammonia, sulphur dioxide,
chlorine, bromine, fluorine and
volatile chlorides
these atmospheres attack platinum
air, nitrogen, oxygen, NOx,
iodine vapours, and CO2
negligible effect
hydrogen, carburetted gases
promote the reduction of a large group
and vapours,lighting gas flames
of substances and thus to be avoided
and reducing gases
ALKALI METAL COMPOUNDS REMARKS
Na2CO3 or K2CO3, mixtures of
them, alkali bifluorides KHSO4,
and BORAX
alkali nitrates and nitrates
only slight losses during normal fusions
1-2 mg loss of platinum per fusion
alkali chlorides and alkali earth attack above 1000°C, particularly at the
chlorides
metal line (3phases)
alkali bisulphates
alkali oxides, peroxides,
hydroxides, sulphides
attack occurs above 700°C
if liquid: all attack platinum
alkali phosphites
no attack at any temperature
sodium hypochlorite
no attack at 100°C
ELEMENTS
REMARKS
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Se, Te, As, Sb, Pb, Sn, Zn, Bi,
etc...
MISCELLANEOUS
magnesium pyrophosphate
in fact, all elements in their elementary
state have to be avoided
REMARKS
attack occurs above 900°C
molten cyanides
formation of platinum cyanides, severe
attack
organic incinerations, carbon
pre-heat carbonaceous materials to
very moderate temperatures under an
excess of oxygen
30% H2O2
attack at 100°C
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