Parr Instrument 1108 - Manual (Page 9)

1108 Oxygen Combustion Vessel
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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 fluffy materials such as vegetable fibers
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 Oxygen Combustion Vessel is
made of corrosion resistant alloys, repeated use
with high sulfur samples or with samples contain-
ing over 20 mg of chlorine may corrode the metal
surfaces and produce a dull film on the inner walls
of the bomb. Materials containing 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 electrodes to ignite and
burn. These corrosive attacks on the bomb can be
reduced by using smaller samples and by increas-
ing 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 pitting 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 loop of
the fuse should be positioned just slightly above the
surface of the sample. Some operators place one
end of a short piece of fine cotton thread over the
fuse loop, with the other end touching the liquid. In
any case, the wire itself should not be submerged in
the liquid.
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