Parr Instrument 6400 - Manual (Page 25)

6400
Calorimeter Instruction Manual
4-6
4. Set the cup in the capsule holder and arrange the auxiliary fuse so that it touches
the center of the tape disc.
5. Just before starting the test, prick the disc with a sharp needle to make a small
opening which is needed to prevent collapse of the disc when pressure is
applied.
6. Fill the bomb with the usual oxygen charging pressure.
7. The calorimeter will fire the bomb and complete the test in the usual manner.
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 combustion is obtained. In such
cases white oil or other suitable material of known purity can be mixed with the sample.
Ethylene glycol, butyl alcohol or decalin may be used for this purpose.
Note:
It must be remembered, 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.
When benzoic acid is combusted for standardization runs, it should be in the form of a
pellet to avoid possible damage to the bomb which might result from rapid combustion of
the loose powder.
Combustion Capsules
Non-volatile samples to be tested in Parr oxygen bombs are weighed and burned in
shallow capsules measuring approximately 1" diameter and 7/16" deep. These are
available in stainless steel, fused silica and platinum alloyed with 3-1/2% rhodium.
Stainless steel capsules (43AS) are furnished with each calorimeter. The stainless steel
capsules will acquire a dull gray 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. New capsules are heated in a muffle furnace at 500ºC for 24 hours to develop
this protective coating uniformly on all surfaces. This treatment should be repeated after
a capsule has been polished with an abrasive to remove any ash or other surface
deposits. Heating in a muffle is also a good way to destroy any traces of carbon or
combustible matter which might remain in the capsule from a previous test. Capsules
should be monitored for wear. Do not use the capsule if the wall or base thickness is
less than 0.025”.
Note:
After heating, place the capsules in a clean container and handle them
only with forceps when they are removed to be weighed on an analytical
balance.
When combusting samples that contain metal particles such as aluminum or
magnesium, the non-metallic (fused silica) 43A3 Capsule is required.