Parr Instrument 6300 - Manual (Page 72)

B
C
ALCULATIONS
P a r r I n s t r u m e n t C o m p a n y 72
T
HERMOCHEMICAL
C
ORRECTIONS
(C
ONTINUED
)
be calculated. In the 6300 Calorimeter this is a fixed
correction of 10 calories per test.
Cotton has a heat of combustion of 4000 calories per
gram. The actual thread being used should be weighed
to see how much is being burned. Ten centimeters of
a fine thread will weigh approximately 0.003 grams
which would release 12 calories as it burns. Heavier
threads weigh up to 0.010 grams per 10 centimeters
and increase this correction to 40 calories per test. The
finer the thread, the smaller errors will be if the thread
is not exactly ten centimeters in length. Polyester thread
is not recommended for use in the bomb because it has
a tendency to melt and fall away from the heating wire
before it ignites.
Using the fine thread mentioned above, the fuse
correction for the calorimeter would be the 10 calories
from electrical heating plus 12 calories from the
burning thread for a total of 22 calories per test. The
thread supplied by Parr has a mass of approximately
1 milligram per centimeter. This results in a total fuse
correction of 50 calories.
ASTM
AND
ISO M
ETHODS
D
IFFER
Current ASTM, ISO, and British Standard Methods
differ on their treatment of the nitric and sulfuric acid
thermochemical corrections. ASTM Methods call for
titrating the bomb washings to determine the total acid
present. This is assumed to be all nitric acid with a heat
of combustion of -14.1 Kcal per mole. The amount of
sulfur is then determined and converted to equivalents
of sulfuric acid. The difference between the heat of
formation of sulfuric acid (-72.2 Kcal per mole or -36.1
calories per milliequivalent) and nitric acid is then
subtracted as the sulfur correction.
Most other test methods treat nitric and sulfuric acid
corrections as entirely separate values instead of
combined values. This eliminates the requirement for
a total acid determination and permits the nitric acid
correction to be handled in a variety of ways, including
the assumption of a fixed nitric acid correction.
The 6300 Calorimeter can be set up to apply the acid
correction by either the ASTM or ISO convention, as
the user prefers. Care must be used to ensure the proper
corrections are applied, and the calculations made are
consistent with the procedure used.
Users may find it convenient to enter a fixed value for
the acid correction and avoid the need to determine
this correction for each test. Use of a fixed value for
the acid correction is highly recommended. Fixed acid
corrections can be entered when Acid Correction -
Thermochemical Corrections, is set to Fixed HNO
3
. A
correction of 8 calories is a good number for the fixed
nitric acid value. For most work, it is recommended to
set “Acid Value is Nitric Acid Only”, in Calculation
Factors to ON. Total errors of more than 3 calories will
seldom occur when using fixed nitric acid corrections.
Fixed sulfur corrections can be entered if a series of
samples contain a constant amount of sulfur. Fixed
sulfur corrections can be entered when Fixed Sulfur -
Thermochemical Corrections, is set to ON and then
enter percent sulfur as indicated on this line. Any errors
will be proportional to the difference between the actual
and assumed value for sulfur.
For ordinary work where benzoic acid is used, for
standardizing the calorimeter, the Fixed Sulfur
Correction, for Standardizations should be ON applying
a fixed value of 0.0 to all standardization tests. Benzoic
acid contains no sulfur.
Please note that the values entered into the test report
appear as entered in the report. Values for e
1
, e
2
and
e
3
are calculated and used as energy corrections in
accordance with the formulas and settings given above.
The formulas used above to arrive at e
1
or e
2
are not the
same as the formulas used for e
1
and e
2
which appear in
most ASTM bomb calorimetric procedures. However,
the sum of e
1
and e
2
, above, is equal to the sum of the
ASTM treatment of e
1
and e
2
.
Note:
Please review the following section on Acid and Sulfur
Corrections. Different standard test methods use different
values for the heat of formation of sulfuric acid.
These differences are generally insignificant. The 6300
Calorimeter uses the most recent, published values for all
thermochemical data.
Thermochemical Calculation Details
Traditionally, standard solutions and procedures have
been established to simplify the calculations related to