Manual

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9
1108P Oxygen Combustion Vessels
ished with an abrasive to remove any ash or other
surface deposits. Heating in a muffle is also a good
way to destroy any trace of carbon or combustible
matter which might remain in the capsule from a
previous test. After heating, place the capsules in a
clean container and handle them only with forceps
when they are removed to be weighed on an ana-
lytical balance.
Capsules should be monitored for wear. Do not use
the capsule if the wall or base thickness is less than
0.025”.
Foodstuffs and Cellulosic Materials
Fibrous and uffy materials such as vegetable bers
may have to be packed into the combustion capsule
and moistened to slow the burning rate, but food-
stuffs and cellulosic samples generally burn with
little difficulty. Partial drying may be necessary if the
moisture content is too high to obtain ignition. But
if the sample is heat sensitive and cannot be dried,
a water soluble combustion aid such as ethylene
glycol can be added to promote ignition.
Coarse Samples
In most cases it may be necessary to burn coarse
samples without size reduction since grinding or
drying may introduce unwanted changes. There is
no objection to this if the coarse sample will ig-
nite and burn completely. Whole wheat grains and
coarse charcoal chunks are typical of materials
which will burn satisfactorily without grinding and
with no additives or special procedure.
Corrosive Samples
Although the Parr bomb is made of corrosion resis-
tant alloys, repeated use with high sulfur samples or
with samples containing over 20 mg of chlorine may
corrode the metal surfaces and produce a dull film
on the inner walls of the bomb. Materials contain-
ing appreciable amounts of caustic; such as dried
black liquor from a pulp mill, may also damage the
bomb, with the caustic attacking the alloy capsule
and causing the metal capsule and the bomb elec-
trodes to ignite and burn. These corrosive attacks on
the bomb can be reduced by using smaller samples
and by increasing the amount of liquid placed in the
bottom of the bomb. If a corrosive film develops on
the bomb surfaces it should be removed by proper
polishing before it grows to a point where deep pit-
ting occurs.
Liquid Samples
Non-volatile samples are treated in the same man-
ner as solid materials. Oils and other liquids which
are not volatile at room temperature can be weighed
directly into open combustion capsules. The cotton
thread should be positioned just slightly above the
surface of the sample or just touching the surface.
Gelatin Capsules
Volatile liquid samples to be burned in an oxygen
bomb can be handled conveniently in Parr 3601
Gelatin Capsules. These 0.9 mL capsules consist of
two cups which telescope together with a friction fit
adequate to retain most liquids. Corrections must
be made for the heat of combustion of gelatin (ap-
proximately. 4600 cal/g) if the capsules are used for
43A6 Combustion Capsule with
Adhesive Tape Seal
3601 Gelatin Capsules
43AS Combustion Capsules