Parr Instrument 1108 - Manual (Page 8)

1108 Oxygen Combustion Vessel
P a r r I n s t r u m e n t C o m p a n y 8
Samples and Sample Holders
Particle Size and Moisture Content
Solid samples burn best in an oxygen bomb when
reduced to 60-mesh, or smaller, and compressed
into a pellet with a Parr Pellet Press. Particle size is
important because it influences the reaction rate.
Large particles may not burn completely and small
particles are easily swept out of the capsule by
turbulent gases during the rapid combustion. Com-
pression into a pellet is recommended since a pellet
burns less vigorously than a loose sample, resulting
in fewer incomplete combustions.
Materials such as coal burn well in the as-received
or air-dry condition, but do not burn bone-dry sam-
ples. A certain amount of moisture is desirable in
order to control the burning rate. Very dry samples
may burn so rapidly that a flame might reach the
seals or the soft valve seat in the bomb head, ignit-
ing these parts and possibly causing a serious burn-
out through the head. Moisture contents up to 20%
can be tolerated in many cases, but the optimum
moisture is best determined by trial combustions. If
moisture is to be added, drop water directly into a
loose sample or onto a pellet after the sample has
been weighed; then let the sample stand for awhile
to obtain uniform distribution.
Combustion Aids
Some samples may be difficult to ignite, or they
may burn so slowly that the particles become chilled
below the ignition point before complete combus-
tion is obtained. In such cases benzoic acid, white oil
or any other combustible material of known purity
can be mixed with the sample. Ethylene glycol,
butyl alcohol or decalin may also be used for this
purpose. It must be remembered, however, that a
combustion aid adds to the total energy released in
the bomb and the amount of sample may have to be
reduced to compensate for the added charge. If ben-
zoic acid is added as a combustion aid, it must be
added in a pellet form, .2 gm pellets. Never combust
benzoic acid in powder form.
Sample Pellets
One of the most useful techniques for handling
powdered samples is to compress the material into
a tablet or pellet before it is weighed. Pellets are
easier to handle than loose samples and they burn
slower in the bomb, thereby reducing the chances
for incomplete combustion. The Parr pellet press
offers a convenient means for preparing samples
in this manner. Pellets produced in this press are
ejected into a stainless steel receiver from which
they can be lifted and handled easily with a pair of
forceps. Most pellets for use in the 1108 Oxygen
Combustion Vessel are made in a one-half inch di-
ameter size, but smaller diameters can be produced
in the same press using interchangeable punch and
die sets. Complete pellet making instructions are
furnished with each press.
Combustion Capsules
Non-volatile samples to be tested in Parr Oxygen
Combustion Vessels are weighed and burned in
shallow capsules measuring approximately 1”
dia. and 7/16” deep. These are available in stain-
less steel, fused silica and platinum alloyed with
a 3-1/2% rhodium. Stainless steel capsules are
suitable for all tests except those in which a non-
metallic (fused silica) holder is desired or where
the superior corrosion resistance of a Pt-Rh cup is
required. Fused silica capsules should be used for
samples containing dissolved metals which can
ignite a stainless steel capsule and cause serious
bomb damage.
Stainless steel capsules will soon acquire a dull grey
finish after repeated use in an oxygen bomb due to
the formation of a hard, protective oxide film. This
dull finish not only protects the capsule but it also
promotes combustion and makes it easier to burn
the last traces of the sample. It is recommended,
therefore, that capsules be heated in a muffle fur-
nace at 500°C for 24 hours to develop this protective
coating uniformly on all surfaces. This treatment
should be performed after a capsule has been pol-
ished with an abrasive to remove any ash or other
surface deposits. Heating in a muffle is also a good
2811 Pellet Press