Parr Instrument 6725 - Manual (Page 27)

6725
Semi-micro Calorimeter Instruction Manual
4-7
The calorimeter temperature should begin to significantly rise at this point, indicating
sample combustion. The calorimetric post-period will last for an additional 9-10 minutes
until the calorimeter temperature drift rate sufficiently stabilizes. At the conclusion of the
post-period the thermometer will turn off the stirrer and prepare a calorimetric test report
indicating the calorimeter temperature rise, fully corrected for all systemic heat leaks,
and whether a heat of combustion value or an energy equivalent (instrument calibration)
factor. Pressing the REPORT key, on the right hand side of the screen, and following
the indicated prompts, allows the user to view the test report.
At the conclusion of the test, open the cover, remove the bomb and Dewar. Remove the
bomb from the flask and open the valve to release the residual gas pressure before
attempting to remove the cap. This release should proceed slowly over a period of not
less than one minute to avoid entrainment losses.
After all pressure has been released, unscrew the cap; lift the head out of the cylinder
and place it on the support stand. Examine the interior of the bomb for soot or other
evidence of incomplete combustion. If such evidence is found, the test will have to be
discarded. Wash all interior surfaces of the bomb with a jet of distilled water and collect
the washings in a beaker.
Remove all unburned pieces of fuse wire from the bomb electrodes; straighten them and
measure their combined length in centimeters. Subtract this length from the initial length
of 10 centimeters and multiply this burned length by 2.3 calories per cm (for Parr 45C10
Fuse Wire) to obtain the fuse correction. The scale on the fuse wire card can be used to
obtain this value directly.
Titrate the bomb washings with a standard sodium carbonate solution using methyl
orange, red or purple indicator. A 0.0709N sodium carbonate solution is recommended
for this titration to simplify the calculation. This is prepared by dissolving 3.76 grams of
Na
2
CO
3
in the water and diluting to one liter. NaOH or KOH solutions of the same
normality may be used.
Analyze the bomb washings to determine the sulfur content of the sample if it
exceeds 0.1%. Methods for determining sulfur are discussed in Operating
Instructions No. 207M.